all: merge dev.typealias into master

[gostls13.git] / src / reflect / value.go

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package reflect

import (
        "math"
        "runtime"
        "unsafe"
)

const ptrSize = 4 << (^uintptr(0) >> 63) // unsafe.Sizeof(uintptr(0)) but an ideal const

// Value is the reflection interface to a Go value.
//
// Not all methods apply to all kinds of values. Restrictions,
// if any, are noted in the documentation for each method.
// Use the Kind method to find out the kind of value before
// calling kind-specific methods. Calling a method
// inappropriate to the kind of type causes a run time panic.
//
// The zero Value represents no value.
// Its IsValid method returns false, its Kind method returns Invalid,
// its String method returns "<invalid Value>", and all other methods panic.
// Most functions and methods never return an invalid value.
// If one does, its documentation states the conditions explicitly.
//
// A Value can be used concurrently by multiple goroutines provided that
// the underlying Go value can be used concurrently for the equivalent
// direct operations.
//
// Using == on two Values does not compare the underlying values
// they represent, but rather the contents of the Value structs.
// To compare two Values, compare the results of the Interface method.
type Value struct {
        // typ holds the type of the value represented by a Value.
        typ *rtype

        // Pointer-valued data or, if flagIndir is set, pointer to data.
        // Valid when either flagIndir is set or typ.pointers() is true.
        ptr unsafe.Pointer

        // flag holds metadata about the value.
        // The lowest bits are flag bits:
        //      - flagStickyRO: obtained via unexported not embedded field, so read-only
        //      - flagEmbedRO: obtained via unexported embedded field, so read-only
        //      - flagIndir: val holds a pointer to the data
        //      - flagAddr: v.CanAddr is true (implies flagIndir)
        //      - flagMethod: v is a method value.
        // The next five bits give the Kind of the value.
        // This repeats typ.Kind() except for method values.
        // The remaining 23+ bits give a method number for method values.
        // If flag.kind() != Func, code can assume that flagMethod is unset.
        // If ifaceIndir(typ), code can assume that flagIndir is set.
        flag

        // A method value represents a curried method invocation
        // like r.Read for some receiver r. The typ+val+flag bits describe
        // the receiver r, but the flag's Kind bits say Func (methods are
        // functions), and the top bits of the flag give the method number
        // in r's type's method table.
}

type flag uintptr

const (
        flagKindWidth        = 5 // there are 27 kinds
        flagKindMask    flag = 1<<flagKindWidth - 1
        flagStickyRO    flag = 1 << 5
        flagEmbedRO     flag = 1 << 6
        flagIndir       flag = 1 << 7
        flagAddr        flag = 1 << 8
        flagMethod      flag = 1 << 9
        flagMethodShift      = 10
        flagRO          flag = flagStickyRO | flagEmbedRO
)

func (f flag) kind() Kind {
        return Kind(f & flagKindMask)
}

// pointer returns the underlying pointer represented by v.
// v.Kind() must be Ptr, Map, Chan, Func, or UnsafePointer
func (v Value) pointer() unsafe.Pointer {
        if v.typ.size != ptrSize || !v.typ.pointers() {
                panic("can't call pointer on a non-pointer Value")
        }
        if v.flag&flagIndir != 0 {
                return *(*unsafe.Pointer)(v.ptr)
        }
        return v.ptr
}

// packEface converts v to the empty interface.
func packEface(v Value) interface{} {
        t := v.typ
        var i interface{}
        e := (*emptyInterface)(unsafe.Pointer(&i))
        // First, fill in the data portion of the interface.
        switch {
        case ifaceIndir(t):
                if v.flag&flagIndir == 0 {
                        panic("bad indir")
                }
                // Value is indirect, and so is the interface we're making.
                ptr := v.ptr
                if v.flag&flagAddr != 0 {
                        // TODO: pass safe boolean from valueInterface so
                        // we don't need to copy if safe==true?
                        c := unsafe_New(t)
                        typedmemmove(t, c, ptr)
                        ptr = c
                }
                e.word = ptr
        case v.flag&flagIndir != 0:
                // Value is indirect, but interface is direct. We need
                // to load the data at v.ptr into the interface data word.
                e.word = *(*unsafe.Pointer)(v.ptr)
        default:
                // Value is direct, and so is the interface.
                e.word = v.ptr
        }
        // Now, fill in the type portion. We're very careful here not
        // to have any operation between the e.word and e.typ assignments
        // that would let the garbage collector observe the partially-built
        // interface value.
        e.typ = t
        return i
}

// unpackEface converts the empty interface i to a Value.
func unpackEface(i interface{}) Value {
        e := (*emptyInterface)(unsafe.Pointer(&i))
        // NOTE: don't read e.word until we know whether it is really a pointer or not.
        t := e.typ
        if t == nil {
                return Value{}
        }
        f := flag(t.Kind())
        if ifaceIndir(t) {
                f |= flagIndir
        }
        return Value{t, e.word, f}
}

// A ValueError occurs when a Value method is invoked on
// a Value that does not support it. Such cases are documented
// in the description of each method.
type ValueError struct {
        Method string
        Kind   Kind
}

func (e *ValueError) Error() string {
        if e.Kind == 0 {
                return "reflect: call of " + e.Method + " on zero Value"
        }
        return "reflect: call of " + e.Method + " on " + e.Kind.String() + " Value"
}

// methodName returns the name of the calling method,
// assumed to be two stack frames above.
func methodName() string {
        pc, _, _, _ := runtime.Caller(2)
        f := runtime.FuncForPC(pc)
        if f == nil {
                return "unknown method"
        }
        return f.Name()
}

// emptyInterface is the header for an interface{} value.
type emptyInterface struct {
        typ  *rtype
        word unsafe.Pointer
}

// nonEmptyInterface is the header for a interface value with methods.
type nonEmptyInterface struct {
        // see ../runtime/iface.go:/Itab
        itab *struct {
                ityp   *rtype // static interface type
                typ    *rtype // dynamic concrete type
                link   unsafe.Pointer
                bad    int32
                unused int32
                fun    [100000]unsafe.Pointer // method table
        }
        word unsafe.Pointer
}

// mustBe panics if f's kind is not expected.
// Making this a method on flag instead of on Value
// (and embedding flag in Value) means that we can write
// the very clear v.mustBe(Bool) and have it compile into
// v.flag.mustBe(Bool), which will only bother to copy the
// single important word for the receiver.
func (f flag) mustBe(expected Kind) {
        if f.kind() != expected {
                panic(&ValueError{methodName(), f.kind()})
        }
}

// mustBeExported panics if f records that the value was obtained using
// an unexported field.
func (f flag) mustBeExported() {
        if f == 0 {
                panic(&ValueError{methodName(), 0})
        }
        if f&flagRO != 0 {
                panic("reflect: " + methodName() + " using value obtained using unexported field")
        }
}

// mustBeAssignable panics if f records that the value is not assignable,
// which is to say that either it was obtained using an unexported field
// or it is not addressable.
func (f flag) mustBeAssignable() {
        if f == 0 {
                panic(&ValueError{methodName(), Invalid})
        }
        // Assignable if addressable and not read-only.
        if f&flagRO != 0 {
                panic("reflect: " + methodName() + " using value obtained using unexported field")
        }
        if f&flagAddr == 0 {
                panic("reflect: " + methodName() + " using unaddressable value")
        }
}

// Addr returns a pointer value representing the address of v.
// It panics if CanAddr() returns false.
// Addr is typically used to obtain a pointer to a struct field
// or slice element in order to call a method that requires a
// pointer receiver.
func (v Value) Addr() Value {
        if v.flag&flagAddr == 0 {
                panic("reflect.Value.Addr of unaddressable value")
        }
        return Value{v.typ.ptrTo(), v.ptr, (v.flag & flagRO) | flag(Ptr)}
}

// Bool returns v's underlying value.
// It panics if v's kind is not Bool.
func (v Value) Bool() bool {
        v.mustBe(Bool)
        return *(*bool)(v.ptr)
}

// Bytes returns v's underlying value.
// It panics if v's underlying value is not a slice of bytes.
func (v Value) Bytes() []byte {
        v.mustBe(Slice)
        if v.typ.Elem().Kind() != Uint8 {
                panic("reflect.Value.Bytes of non-byte slice")
        }
        // Slice is always bigger than a word; assume flagIndir.
        return *(*[]byte)(v.ptr)
}

// runes returns v's underlying value.
// It panics if v's underlying value is not a slice of runes (int32s).
func (v Value) runes() []rune {
        v.mustBe(Slice)
        if v.typ.Elem().Kind() != Int32 {
                panic("reflect.Value.Bytes of non-rune slice")
        }
        // Slice is always bigger than a word; assume flagIndir.
        return *(*[]rune)(v.ptr)
}

// CanAddr reports whether the value's address can be obtained with Addr.
// Such values are called addressable. A value is addressable if it is
// an element of a slice, an element of an addressable array,
// a field of an addressable struct, or the result of dereferencing a pointer.
// If CanAddr returns false, calling Addr will panic.
func (v Value) CanAddr() bool {
        return v.flag&flagAddr != 0
}

// CanSet reports whether the value of v can be changed.
// A Value can be changed only if it is addressable and was not
// obtained by the use of unexported struct fields.
// If CanSet returns false, calling Set or any type-specific
// setter (e.g., SetBool, SetInt) will panic.
func (v Value) CanSet() bool {
        return v.flag&(flagAddr|flagRO) == flagAddr
}

// Call calls the function v with the input arguments in.
// For example, if len(in) == 3, v.Call(in) represents the Go call v(in[0], in[1], in[2]).
// Call panics if v's Kind is not Func.
// It returns the output results as Values.
// As in Go, each input argument must be assignable to the
// type of the function's corresponding input parameter.
// If v is a variadic function, Call creates the variadic slice parameter
// itself, copying in the corresponding values.
func (v Value) Call(in []Value) []Value {
        v.mustBe(Func)
        v.mustBeExported()
        return v.call("Call", in)
}

// CallSlice calls the variadic function v with the input arguments in,
// assigning the slice in[len(in)-1] to v's final variadic argument.
// For example, if len(in) == 3, v.CallSlice(in) represents the Go call v(in[0], in[1], in[2]...).
// CallSlice panics if v's Kind is not Func or if v is not variadic.
// It returns the output results as Values.
// As in Go, each input argument must be assignable to the
// type of the function's corresponding input parameter.
func (v Value) CallSlice(in []Value) []Value {
        v.mustBe(Func)
        v.mustBeExported()
        return v.call("CallSlice", in)
}

var callGC bool // for testing; see TestCallMethodJump

func (v Value) call(op string, in []Value) []Value {
        // Get function pointer, type.
        t := v.typ
        var (
                fn       unsafe.Pointer
                rcvr     Value
                rcvrtype *rtype
        )
        if v.flag&flagMethod != 0 {
                rcvr = v
                rcvrtype, t, fn = methodReceiver(op, v, int(v.flag)>>flagMethodShift)
        } else if v.flag&flagIndir != 0 {
                fn = *(*unsafe.Pointer)(v.ptr)
        } else {
                fn = v.ptr
        }

        if fn == nil {
                panic("reflect.Value.Call: call of nil function")
        }

        isSlice := op == "CallSlice"
        n := t.NumIn()
        if isSlice {
                if !t.IsVariadic() {
                        panic("reflect: CallSlice of non-variadic function")
                }
                if len(in) < n {
                        panic("reflect: CallSlice with too few input arguments")
                }
                if len(in) > n {
                        panic("reflect: CallSlice with too many input arguments")
                }
        } else {
                if t.IsVariadic() {
                        n--
                }
                if len(in) < n {
                        panic("reflect: Call with too few input arguments")
                }
                if !t.IsVariadic() && len(in) > n {
                        panic("reflect: Call with too many input arguments")
                }
        }
        for _, x := range in {
                if x.Kind() == Invalid {
                        panic("reflect: " + op + " using zero Value argument")
                }
        }
        for i := 0; i < n; i++ {
                if xt, targ := in[i].Type(), t.In(i); !xt.AssignableTo(targ) {
                        panic("reflect: " + op + " using " + xt.String() + " as type " + targ.String())
                }
        }
        if !isSlice && t.IsVariadic() {
                // prepare slice for remaining values
                m := len(in) - n
                slice := MakeSlice(t.In(n), m, m)
                elem := t.In(n).Elem()
                for i := 0; i < m; i++ {
                        x := in[n+i]
                        if xt := x.Type(); !xt.AssignableTo(elem) {
                                panic("reflect: cannot use " + xt.String() + " as type " + elem.String() + " in " + op)
                        }
                        slice.Index(i).Set(x)
                }
                origIn := in
                in = make([]Value, n+1)
                copy(in[:n], origIn)
                in[n] = slice
        }

        nin := len(in)
        if nin != t.NumIn() {
                panic("reflect.Value.Call: wrong argument count")
        }
        nout := t.NumOut()

        // Compute frame type.
        frametype, _, retOffset, _, framePool := funcLayout(t, rcvrtype)

        // Allocate a chunk of memory for frame.
        var args unsafe.Pointer
        if nout == 0 {
                args = framePool.Get().(unsafe.Pointer)
        } else {
                // Can't use pool if the function has return values.
                // We will leak pointer to args in ret, so its lifetime is not scoped.
                args = unsafe_New(frametype)
        }
        off := uintptr(0)

        // Copy inputs into args.
        if rcvrtype != nil {
                storeRcvr(rcvr, args)
                off = ptrSize
        }
        for i, v := range in {
                v.mustBeExported()
                targ := t.In(i).(*rtype)
                a := uintptr(targ.align)
                off = (off + a - 1) &^ (a - 1)
                n := targ.size
                addr := unsafe.Pointer(uintptr(args) + off)
                v = v.assignTo("reflect.Value.Call", targ, addr)
                if v.flag&flagIndir != 0 {
                        typedmemmove(targ, addr, v.ptr)
                } else {
                        *(*unsafe.Pointer)(addr) = v.ptr
                }
                off += n
        }

        // Call.
        call(frametype, fn, args, uint32(frametype.size), uint32(retOffset))

        // For testing; see TestCallMethodJump.
        if callGC {
                runtime.GC()
        }

        var ret []Value
        if nout == 0 {
                // This is untyped because the frame is really a
                // stack, even though it's a heap object.
                memclrNoHeapPointers(args, frametype.size)
                framePool.Put(args)
        } else {
                // Zero the now unused input area of args,
                // because the Values returned by this function contain pointers to the args object,
                // and will thus keep the args object alive indefinitely.
                memclrNoHeapPointers(args, retOffset)
                // Wrap Values around return values in args.
                ret = make([]Value, nout)
                off = retOffset
                for i := 0; i < nout; i++ {
                        tv := t.Out(i)
                        a := uintptr(tv.Align())
                        off = (off + a - 1) &^ (a - 1)
                        fl := flagIndir | flag(tv.Kind())
                        ret[i] = Value{tv.common(), unsafe.Pointer(uintptr(args) + off), fl}
                        off += tv.Size()
                }
        }

        return ret
}

// callReflect is the call implementation used by a function
// returned by MakeFunc. In many ways it is the opposite of the
// method Value.call above. The method above converts a call using Values
// into a call of a function with a concrete argument frame, while
// callReflect converts a call of a function with a concrete argument
// frame into a call using Values.
// It is in this file so that it can be next to the call method above.
// The remainder of the MakeFunc implementation is in makefunc.go.
//
// NOTE: This function must be marked as a "wrapper" in the generated code,
// so that the linker can make it work correctly for panic and recover.
// The gc compilers know to do that for the name "reflect.callReflect".
func callReflect(ctxt *makeFuncImpl, frame unsafe.Pointer) {
        ftyp := ctxt.typ
        f := ctxt.fn

        // Copy argument frame into Values.
        ptr := frame
        off := uintptr(0)
        in := make([]Value, 0, int(ftyp.inCount))
        for _, typ := range ftyp.in() {
                off += -off & uintptr(typ.align-1)
                addr := unsafe.Pointer(uintptr(ptr) + off)
                v := Value{typ, nil, flag(typ.Kind())}
                if ifaceIndir(typ) {
                        // value cannot be inlined in interface data.
                        // Must make a copy, because f might keep a reference to it,
                        // and we cannot let f keep a reference to the stack frame
                        // after this function returns, not even a read-only reference.
                        v.ptr = unsafe_New(typ)
                        typedmemmove(typ, v.ptr, addr)
                        v.flag |= flagIndir
                } else {
                        v.ptr = *(*unsafe.Pointer)(addr)
                }
                in = append(in, v)
                off += typ.size
        }

        // Call underlying function.
        out := f(in)
        numOut := ftyp.NumOut()
        if len(out) != numOut {
                panic("reflect: wrong return count from function created by MakeFunc")
        }

        // Copy results back into argument frame.
        if numOut > 0 {
                off += -off & (ptrSize - 1)
                if runtime.GOARCH == "amd64p32" {
                        off = align(off, 8)
                }
                for i, typ := range ftyp.out() {
                        v := out[i]
                        if v.typ != typ {
                                panic("reflect: function created by MakeFunc using " + funcName(f) +
                                        " returned wrong type: have " +
                                        out[i].typ.String() + " for " + typ.String())
                        }
                        if v.flag&flagRO != 0 {
                                panic("reflect: function created by MakeFunc using " + funcName(f) +
                                        " returned value obtained from unexported field")
                        }
                        off += -off & uintptr(typ.align-1)
                        addr := unsafe.Pointer(uintptr(ptr) + off)
                        if v.flag&flagIndir != 0 {
                                typedmemmove(typ, addr, v.ptr)
                        } else {
                                *(*unsafe.Pointer)(addr) = v.ptr
                        }
                        off += typ.size
                }
        }

        // runtime.getArgInfo expects to be able to find ctxt on the
        // stack when it finds our caller, makeFuncStub. Make sure it
        // doesn't get garbage collected.
        runtime.KeepAlive(ctxt)
}

// methodReceiver returns information about the receiver
// described by v. The Value v may or may not have the
// flagMethod bit set, so the kind cached in v.flag should
// not be used.
// The return value rcvrtype gives the method's actual receiver type.
// The return value t gives the method type signature (without the receiver).
// The return value fn is a pointer to the method code.
func methodReceiver(op string, v Value, methodIndex int) (rcvrtype, t *rtype, fn unsafe.Pointer) {
        i := methodIndex
        if v.typ.Kind() == Interface {
                tt := (*interfaceType)(unsafe.Pointer(v.typ))
                if uint(i) >= uint(len(tt.methods)) {
                        panic("reflect: internal error: invalid method index")
                }
                m := &tt.methods[i]
                if !tt.nameOff(m.name).isExported() {
                        panic("reflect: " + op + " of unexported method")
                }
                iface := (*nonEmptyInterface)(v.ptr)
                if iface.itab == nil {
                        panic("reflect: " + op + " of method on nil interface value")
                }
                rcvrtype = iface.itab.typ
                fn = unsafe.Pointer(&iface.itab.fun[i])
                t = tt.typeOff(m.typ)
        } else {
                rcvrtype = v.typ
                ut := v.typ.uncommon()
                if ut == nil || uint(i) >= uint(ut.mcount) {
                        panic("reflect: internal error: invalid method index")
                }
                m := ut.methods()[i]
                if !v.typ.nameOff(m.name).isExported() {
                        panic("reflect: " + op + " of unexported method")
                }
                ifn := v.typ.textOff(m.ifn)
                fn = unsafe.Pointer(&ifn)
                t = v.typ.typeOff(m.mtyp)
        }
        return
}

// v is a method receiver. Store at p the word which is used to
// encode that receiver at the start of the argument list.
// Reflect uses the "interface" calling convention for
// methods, which always uses one word to record the receiver.
func storeRcvr(v Value, p unsafe.Pointer) {
        t := v.typ
        if t.Kind() == Interface {
                // the interface data word becomes the receiver word
                iface := (*nonEmptyInterface)(v.ptr)
                *(*unsafe.Pointer)(p) = iface.word
        } else if v.flag&flagIndir != 0 && !ifaceIndir(t) {
                *(*unsafe.Pointer)(p) = *(*unsafe.Pointer)(v.ptr)
        } else {
                *(*unsafe.Pointer)(p) = v.ptr
        }
}

// align returns the result of rounding x up to a multiple of n.
// n must be a power of two.
func align(x, n uintptr) uintptr {
        return (x + n - 1) &^ (n - 1)
}

// callMethod is the call implementation used by a function returned
// by makeMethodValue (used by v.Method(i).Interface()).
// It is a streamlined version of the usual reflect call: the caller has
// already laid out the argument frame for us, so we don't have
// to deal with individual Values for each argument.
// It is in this file so that it can be next to the two similar functions above.
// The remainder of the makeMethodValue implementation is in makefunc.go.
//
// NOTE: This function must be marked as a "wrapper" in the generated code,
// so that the linker can make it work correctly for panic and recover.
// The gc compilers know to do that for the name "reflect.callMethod".
func callMethod(ctxt *methodValue, frame unsafe.Pointer) {
        rcvr := ctxt.rcvr
        rcvrtype, t, fn := methodReceiver("call", rcvr, ctxt.method)
        frametype, argSize, retOffset, _, framePool := funcLayout(t, rcvrtype)

        // Make a new frame that is one word bigger so we can store the receiver.
        args := framePool.Get().(unsafe.Pointer)

        // Copy in receiver and rest of args.
        storeRcvr(rcvr, args)
        typedmemmovepartial(frametype, unsafe.Pointer(uintptr(args)+ptrSize), frame, ptrSize, argSize-ptrSize)

        // Call.
        call(frametype, fn, args, uint32(frametype.size), uint32(retOffset))

        // Copy return values. On amd64p32, the beginning of return values
        // is 64-bit aligned, so the caller's frame layout (which doesn't have
        // a receiver) is different from the layout of the fn call, which has
        // a receiver.
        // Ignore any changes to args and just copy return values.
        callerRetOffset := retOffset - ptrSize
        if runtime.GOARCH == "amd64p32" {
                callerRetOffset = align(argSize-ptrSize, 8)
        }
        typedmemmovepartial(frametype,
                unsafe.Pointer(uintptr(frame)+callerRetOffset),
                unsafe.Pointer(uintptr(args)+retOffset),
                retOffset,
                frametype.size-retOffset)

        // This is untyped because the frame is really a stack, even
        // though it's a heap object.
        memclrNoHeapPointers(args, frametype.size)
        framePool.Put(args)

        // See the comment in callReflect.
        runtime.KeepAlive(ctxt)
}

// funcName returns the name of f, for use in error messages.
func funcName(f func([]Value) []Value) string {
        pc := *(*uintptr)(unsafe.Pointer(&f))
        rf := runtime.FuncForPC(pc)
        if rf != nil {
                return rf.Name()
        }
        return "closure"
}

// Cap returns v's capacity.
// It panics if v's Kind is not Array, Chan, or Slice.
func (v Value) Cap() int {
        k := v.kind()
        switch k {
        case Array:
                return v.typ.Len()
        case Chan:
                return chancap(v.pointer())
        case Slice:
                // Slice is always bigger than a word; assume flagIndir.
                return (*sliceHeader)(v.ptr).Cap
        }
        panic(&ValueError{"reflect.Value.Cap", v.kind()})
}

// Close closes the channel v.
// It panics if v's Kind is not Chan.
func (v Value) Close() {
        v.mustBe(Chan)
        v.mustBeExported()
        chanclose(v.pointer())
}

// Complex returns v's underlying value, as a complex128.
// It panics if v's Kind is not Complex64 or Complex128
func (v Value) Complex() complex128 {
        k := v.kind()
        switch k {
        case Complex64:
                return complex128(*(*complex64)(v.ptr))
        case Complex128:
                return *(*complex128)(v.ptr)
        }
        panic(&ValueError{"reflect.Value.Complex", v.kind()})
}

// Elem returns the value that the interface v contains
// or that the pointer v points to.
// It panics if v's Kind is not Interface or Ptr.
// It returns the zero Value if v is nil.
func (v Value) Elem() Value {
        k := v.kind()
        switch k {
        case Interface:
                var eface interface{}
                if v.typ.NumMethod() == 0 {
                        eface = *(*interface{})(v.ptr)
                } else {
                        eface = (interface{})(*(*interface {
                                M()
                        })(v.ptr))
                }
                x := unpackEface(eface)
                if x.flag != 0 {
                        x.flag |= v.flag & flagRO
                }
                return x
        case Ptr:
                ptr := v.ptr
                if v.flag&flagIndir != 0 {
                        ptr = *(*unsafe.Pointer)(ptr)
                }
                // The returned value's address is v's value.
                if ptr == nil {
                        return Value{}
                }
                tt := (*ptrType)(unsafe.Pointer(v.typ))
                typ := tt.elem
                fl := v.flag&flagRO | flagIndir | flagAddr
                fl |= flag(typ.Kind())
                return Value{typ, ptr, fl}
        }
        panic(&ValueError{"reflect.Value.Elem", v.kind()})
}

// Field returns the i'th field of the struct v.
// It panics if v's Kind is not Struct or i is out of range.
func (v Value) Field(i int) Value {
        if v.kind() != Struct {
                panic(&ValueError{"reflect.Value.Field", v.kind()})
        }
        tt := (*structType)(unsafe.Pointer(v.typ))
        if uint(i) >= uint(len(tt.fields)) {
                panic("reflect: Field index out of range")
        }
        field := &tt.fields[i]
        typ := field.typ

        // Inherit permission bits from v, but clear flagEmbedRO.
        fl := v.flag&(flagStickyRO|flagIndir|flagAddr) | flag(typ.Kind())
        // Using an unexported field forces flagRO.
        if !field.name.isExported() {
                if field.anon() {
                        fl |= flagEmbedRO
                } else {
                        fl |= flagStickyRO
                }
        }
        // Either flagIndir is set and v.ptr points at struct,
        // or flagIndir is not set and v.ptr is the actual struct data.
        // In the former case, we want v.ptr + offset.
        // In the latter case, we must have field.offset = 0,
        // so v.ptr + field.offset is still okay.
        ptr := unsafe.Pointer(uintptr(v.ptr) + field.offset())
        return Value{typ, ptr, fl}
}

// FieldByIndex returns the nested field corresponding to index.
// It panics if v's Kind is not struct.
func (v Value) FieldByIndex(index []int) Value {
        if len(index) == 1 {
                return v.Field(index[0])
        }
        v.mustBe(Struct)
        for i, x := range index {
                if i > 0 {
                        if v.Kind() == Ptr && v.typ.Elem().Kind() == Struct {
                                if v.IsNil() {
                                        panic("reflect: indirection through nil pointer to embedded struct")
                                }
                                v = v.Elem()
                        }
                }
                v = v.Field(x)
        }
        return v
}

// FieldByName returns the struct field with the given name.
// It returns the zero Value if no field was found.
// It panics if v's Kind is not struct.
func (v Value) FieldByName(name string) Value {
        v.mustBe(Struct)
        if f, ok := v.typ.FieldByName(name); ok {
                return v.FieldByIndex(f.Index)
        }
        return Value{}
}

// FieldByNameFunc returns the struct field with a name
// that satisfies the match function.
// It panics if v's Kind is not struct.
// It returns the zero Value if no field was found.
func (v Value) FieldByNameFunc(match func(string) bool) Value {
        if f, ok := v.typ.FieldByNameFunc(match); ok {
                return v.FieldByIndex(f.Index)
        }
        return Value{}
}

// Float returns v's underlying value, as a float64.
// It panics if v's Kind is not Float32 or Float64
func (v Value) Float() float64 {
        k := v.kind()
        switch k {
        case Float32:
                return float64(*(*float32)(v.ptr))
        case Float64:
                return *(*float64)(v.ptr)
        }
        panic(&ValueError{"reflect.Value.Float", v.kind()})
}

var uint8Type = TypeOf(uint8(0)).(*rtype)

// Index returns v's i'th element.
// It panics if v's Kind is not Array, Slice, or String or i is out of range.
func (v Value) Index(i int) Value {
        switch v.kind() {
        case Array:
                tt := (*arrayType)(unsafe.Pointer(v.typ))
                if uint(i) >= uint(tt.len) {
                        panic("reflect: array index out of range")
                }
                typ := tt.elem
                offset := uintptr(i) * typ.size

                // Either flagIndir is set and v.ptr points at array,
                // or flagIndir is not set and v.ptr is the actual array data.
                // In the former case, we want v.ptr + offset.
                // In the latter case, we must be doing Index(0), so offset = 0,
                // so v.ptr + offset is still okay.
                val := unsafe.Pointer(uintptr(v.ptr) + offset)
                fl := v.flag&(flagRO|flagIndir|flagAddr) | flag(typ.Kind()) // bits same as overall array
                return Value{typ, val, fl}

        case Slice:
                // Element flag same as Elem of Ptr.
                // Addressable, indirect, possibly read-only.
                s := (*sliceHeader)(v.ptr)
                if uint(i) >= uint(s.Len) {
                        panic("reflect: slice index out of range")
                }
                tt := (*sliceType)(unsafe.Pointer(v.typ))
                typ := tt.elem
                val := arrayAt(s.Data, i, typ.size)
                fl := flagAddr | flagIndir | v.flag&flagRO | flag(typ.Kind())
                return Value{typ, val, fl}

        case String:
                s := (*stringHeader)(v.ptr)
                if uint(i) >= uint(s.Len) {
                        panic("reflect: string index out of range")
                }
                p := arrayAt(s.Data, i, 1)
                fl := v.flag&flagRO | flag(Uint8) | flagIndir
                return Value{uint8Type, p, fl}
        }
        panic(&ValueError{"reflect.Value.Index", v.kind()})
}

// Int returns v's underlying value, as an int64.
// It panics if v's Kind is not Int, Int8, Int16, Int32, or Int64.
func (v Value) Int() int64 {
        k := v.kind()
        p := v.ptr
        switch k {
        case Int:
                return int64(*(*int)(p))
        case Int8:
                return int64(*(*int8)(p))
        case Int16:
                return int64(*(*int16)(p))
        case Int32:
                return int64(*(*int32)(p))
        case Int64:
                return *(*int64)(p)
        }
        panic(&ValueError{"reflect.Value.Int", v.kind()})
}

// CanInterface reports whether Interface can be used without panicking.
func (v Value) CanInterface() bool {
        if v.flag == 0 {
                panic(&ValueError{"reflect.Value.CanInterface", Invalid})
        }
        return v.flag&flagRO == 0
}

// Interface returns v's current value as an interface{}.
// It is equivalent to:
//      var i interface{} = (v's underlying value)
// It panics if the Value was obtained by accessing
// unexported struct fields.
func (v Value) Interface() (i interface{}) {
        return valueInterface(v, true)
}

func valueInterface(v Value, safe bool) interface{} {
        if v.flag == 0 {
                panic(&ValueError{"reflect.Value.Interface", 0})
        }
        if safe && v.flag&flagRO != 0 {
                // Do not allow access to unexported values via Interface,
                // because they might be pointers that should not be
                // writable or methods or function that should not be callable.
                panic("reflect.Value.Interface: cannot return value obtained from unexported field or method")
        }
        if v.flag&flagMethod != 0 {
                v = makeMethodValue("Interface", v)
        }

        if v.kind() == Interface {
                // Special case: return the element inside the interface.
                // Empty interface has one layout, all interfaces with
                // methods have a second layout.
                if v.NumMethod() == 0 {
                        return *(*interface{})(v.ptr)
                }
                return *(*interface {
                        M()
                })(v.ptr)
        }

        // TODO: pass safe to packEface so we don't need to copy if safe==true?
        return packEface(v)
}

// InterfaceData returns the interface v's value as a uintptr pair.
// It panics if v's Kind is not Interface.
func (v Value) InterfaceData() [2]uintptr {
        // TODO: deprecate this
        v.mustBe(Interface)
        // We treat this as a read operation, so we allow
        // it even for unexported data, because the caller
        // has to import "unsafe" to turn it into something
        // that can be abused.
        // Interface value is always bigger than a word; assume flagIndir.
        return *(*[2]uintptr)(v.ptr)
}

// IsNil reports whether its argument v is nil. The argument must be
// a chan, func, interface, map, pointer, or slice value; if it is
// not, IsNil panics. Note that IsNil is not always equivalent to a
// regular comparison with nil in Go. For example, if v was created
// by calling ValueOf with an uninitialized interface variable i,
// i==nil will be true but v.IsNil will panic as v will be the zero
// Value.
func (v Value) IsNil() bool {
        k := v.kind()
        switch k {
        case Chan, Func, Map, Ptr:
                if v.flag&flagMethod != 0 {
                        return false
                }
                ptr := v.ptr
                if v.flag&flagIndir != 0 {
                        ptr = *(*unsafe.Pointer)(ptr)
                }
                return ptr == nil
        case Interface, Slice:
                // Both interface and slice are nil if first word is 0.
                // Both are always bigger than a word; assume flagIndir.
                return *(*unsafe.Pointer)(v.ptr) == nil
        }
        panic(&ValueError{"reflect.Value.IsNil", v.kind()})
}

// IsValid reports whether v represents a value.
// It returns false if v is the zero Value.
// If IsValid returns false, all other methods except String panic.
// Most functions and methods never return an invalid value.
// If one does, its documentation states the conditions explicitly.
func (v Value) IsValid() bool {
        return v.flag != 0
}

// Kind returns v's Kind.
// If v is the zero Value (IsValid returns false), Kind returns Invalid.
func (v Value) Kind() Kind {
        return v.kind()
}

// Len returns v's length.
// It panics if v's Kind is not Array, Chan, Map, Slice, or String.
func (v Value) Len() int {
        k := v.kind()
        switch k {
        case Array:
                tt := (*arrayType)(unsafe.Pointer(v.typ))
                return int(tt.len)
        case Chan:
                return chanlen(v.pointer())
        case Map:
                return maplen(v.pointer())
        case Slice:
                // Slice is bigger than a word; assume flagIndir.
                return (*sliceHeader)(v.ptr).Len
        case String:
                // String is bigger than a word; assume flagIndir.
                return (*stringHeader)(v.ptr).Len
        }
        panic(&ValueError{"reflect.Value.Len", v.kind()})
}

// MapIndex returns the value associated with key in the map v.
// It panics if v's Kind is not Map.
// It returns the zero Value if key is not found in the map or if v represents a nil map.
// As in Go, the key's value must be assignable to the map's key type.
func (v Value) MapIndex(key Value) Value {
        v.mustBe(Map)
        tt := (*mapType)(unsafe.Pointer(v.typ))

        // Do not require key to be exported, so that DeepEqual
        // and other programs can use all the keys returned by
        // MapKeys as arguments to MapIndex. If either the map
        // or the key is unexported, though, the result will be
        // considered unexported. This is consistent with the
        // behavior for structs, which allow read but not write
        // of unexported fields.
        key = key.assignTo("reflect.Value.MapIndex", tt.key, nil)

        var k unsafe.Pointer
        if key.flag&flagIndir != 0 {
                k = key.ptr
        } else {
                k = unsafe.Pointer(&key.ptr)
        }
        e := mapaccess(v.typ, v.pointer(), k)
        if e == nil {
                return Value{}
        }
        typ := tt.elem
        fl := (v.flag | key.flag) & flagRO
        fl |= flag(typ.Kind())
        if ifaceIndir(typ) {
                // Copy result so future changes to the map
                // won't change the underlying value.
                c := unsafe_New(typ)
                typedmemmove(typ, c, e)
                return Value{typ, c, fl | flagIndir}
        } else {
                return Value{typ, *(*unsafe.Pointer)(e), fl}
        }
}

// MapKeys returns a slice containing all the keys present in the map,
// in unspecified order.
// It panics if v's Kind is not Map.
// It returns an empty slice if v represents a nil map.
func (v Value) MapKeys() []Value {
        v.mustBe(Map)
        tt := (*mapType)(unsafe.Pointer(v.typ))
        keyType := tt.key

        fl := v.flag&flagRO | flag(keyType.Kind())

        m := v.pointer()
        mlen := int(0)
        if m != nil {
                mlen = maplen(m)
        }
        it := mapiterinit(v.typ, m)
        a := make([]Value, mlen)
        var i int
        for i = 0; i < len(a); i++ {
                key := mapiterkey(it)
                if key == nil {
                        // Someone deleted an entry from the map since we
                        // called maplen above. It's a data race, but nothing
                        // we can do about it.
                        break
                }
                if ifaceIndir(keyType) {
                        // Copy result so future changes to the map
                        // won't change the underlying value.
                        c := unsafe_New(keyType)
                        typedmemmove(keyType, c, key)
                        a[i] = Value{keyType, c, fl | flagIndir}
                } else {
                        a[i] = Value{keyType, *(*unsafe.Pointer)(key), fl}
                }
                mapiternext(it)
        }
        return a[:i]
}

// Method returns a function value corresponding to v's i'th method.
// The arguments to a Call on the returned function should not include
// a receiver; the returned function will always use v as the receiver.
// Method panics if i is out of range or if v is a nil interface value.
func (v Value) Method(i int) Value {
        if v.typ == nil {
                panic(&ValueError{"reflect.Value.Method", Invalid})
        }
        if v.flag&flagMethod != 0 || uint(i) >= uint(v.typ.NumMethod()) {
                panic("reflect: Method index out of range")
        }
        if v.typ.Kind() == Interface && v.IsNil() {
                panic("reflect: Method on nil interface value")
        }
        fl := v.flag & (flagStickyRO | flagIndir) // Clear flagEmbedRO
        fl |= flag(Func)
        fl |= flag(i)<<flagMethodShift | flagMethod
        return Value{v.typ, v.ptr, fl}
}

// NumMethod returns the number of methods in the value's method set.
func (v Value) NumMethod() int {
        if v.typ == nil {
                panic(&ValueError{"reflect.Value.NumMethod", Invalid})
        }
        if v.flag&flagMethod != 0 {
                return 0
        }
        return v.typ.NumMethod()
}

// MethodByName returns a function value corresponding to the method
// of v with the given name.
// The arguments to a Call on the returned function should not include
// a receiver; the returned function will always use v as the receiver.
// It returns the zero Value if no method was found.
func (v Value) MethodByName(name string) Value {
        if v.typ == nil {
                panic(&ValueError{"reflect.Value.MethodByName", Invalid})
        }
        if v.flag&flagMethod != 0 {
                return Value{}
        }
        m, ok := v.typ.MethodByName(name)
        if !ok {
                return Value{}
        }
        return v.Method(m.Index)
}

// NumField returns the number of fields in the struct v.
// It panics if v's Kind is not Struct.
func (v Value) NumField() int {
        v.mustBe(Struct)
        tt := (*structType)(unsafe.Pointer(v.typ))
        return len(tt.fields)
}

// OverflowComplex reports whether the complex128 x cannot be represented by v's type.
// It panics if v's Kind is not Complex64 or Complex128.
func (v Value) OverflowComplex(x complex128) bool {
        k := v.kind()
        switch k {
        case Complex64:
                return overflowFloat32(real(x)) || overflowFloat32(imag(x))
        case Complex128:
                return false
        }
        panic(&ValueError{"reflect.Value.OverflowComplex", v.kind()})
}

// OverflowFloat reports whether the float64 x cannot be represented by v's type.
// It panics if v's Kind is not Float32 or Float64.
func (v Value) OverflowFloat(x float64) bool {
        k := v.kind()
        switch k {
        case Float32:
                return overflowFloat32(x)
        case Float64:
                return false
        }
        panic(&ValueError{"reflect.Value.OverflowFloat", v.kind()})
}

func overflowFloat32(x float64) bool {
        if x < 0 {
                x = -x
        }
        return math.MaxFloat32 < x && x <= math.MaxFloat64
}

// OverflowInt reports whether the int64 x cannot be represented by v's type.
// It panics if v's Kind is not Int, Int8, int16, Int32, or Int64.
func (v Value) OverflowInt(x int64) bool {
        k := v.kind()
        switch k {
        case Int, Int8, Int16, Int32, Int64:
                bitSize := v.typ.size * 8
                trunc := (x << (64 - bitSize)) >> (64 - bitSize)
                return x != trunc
        }
        panic(&ValueError{"reflect.Value.OverflowInt", v.kind()})
}

// OverflowUint reports whether the uint64 x cannot be represented by v's type.
// It panics if v's Kind is not Uint, Uintptr, Uint8, Uint16, Uint32, or Uint64.
func (v Value) OverflowUint(x uint64) bool {
        k := v.kind()
        switch k {
        case Uint, Uintptr, Uint8, Uint16, Uint32, Uint64:
                bitSize := v.typ.size * 8
                trunc := (x << (64 - bitSize)) >> (64 - bitSize)
                return x != trunc
        }
        panic(&ValueError{"reflect.Value.OverflowUint", v.kind()})
}

// Pointer returns v's value as a uintptr.
// It returns uintptr instead of unsafe.Pointer so that
// code using reflect cannot obtain unsafe.Pointers
// without importing the unsafe package explicitly.
// It panics if v's Kind is not Chan, Func, Map, Ptr, Slice, or UnsafePointer.
//
// If v's Kind is Func, the returned pointer is an underlying
// code pointer, but not necessarily enough to identify a
// single function uniquely. The only guarantee is that the
// result is zero if and only if v is a nil func Value.
//
// If v's Kind is Slice, the returned pointer is to the first
// element of the slice. If the slice is nil the returned value
// is 0.  If the slice is empty but non-nil the return value is non-zero.
func (v Value) Pointer() uintptr {
        // TODO: deprecate
        k := v.kind()
        switch k {
        case Chan, Map, Ptr, UnsafePointer:
                return uintptr(v.pointer())
        case Func:
                if v.flag&flagMethod != 0 {
                        // As the doc comment says, the returned pointer is an
                        // underlying code pointer but not necessarily enough to
                        // identify a single function uniquely. All method expressions
                        // created via reflect have the same underlying code pointer,
                        // so their Pointers are equal. The function used here must
                        // match the one used in makeMethodValue.
                        f := methodValueCall
                        return **(**uintptr)(unsafe.Pointer(&f))
                }
                p := v.pointer()
                // Non-nil func value points at data block.
                // First word of data block is actual code.
                if p != nil {
                        p = *(*unsafe.Pointer)(p)
                }
                return uintptr(p)

        case Slice:
                return (*SliceHeader)(v.ptr).Data
        }
        panic(&ValueError{"reflect.Value.Pointer", v.kind()})
}

// Recv receives and returns a value from the channel v.
// It panics if v's Kind is not Chan.
// The receive blocks until a value is ready.
// The boolean value ok is true if the value x corresponds to a send
// on the channel, false if it is a zero value received because the channel is closed.
func (v Value) Recv() (x Value, ok bool) {
        v.mustBe(Chan)
        v.mustBeExported()
        return v.recv(false)
}

// internal recv, possibly non-blocking (nb).
// v is known to be a channel.
func (v Value) recv(nb bool) (val Value, ok bool) {
        tt := (*chanType)(unsafe.Pointer(v.typ))
        if ChanDir(tt.dir)&RecvDir == 0 {
                panic("reflect: recv on send-only channel")
        }
        t := tt.elem
        val = Value{t, nil, flag(t.Kind())}
        var p unsafe.Pointer
        if ifaceIndir(t) {
                p = unsafe_New(t)
                val.ptr = p
                val.flag |= flagIndir
        } else {
                p = unsafe.Pointer(&val.ptr)
        }
        selected, ok := chanrecv(v.typ, v.pointer(), nb, p)
        if !selected {
                val = Value{}
        }
        return
}

// Send sends x on the channel v.
// It panics if v's kind is not Chan or if x's type is not the same type as v's element type.
// As in Go, x's value must be assignable to the channel's element type.
func (v Value) Send(x Value) {
        v.mustBe(Chan)
        v.mustBeExported()
        v.send(x, false)
}

// internal send, possibly non-blocking.
// v is known to be a channel.
func (v Value) send(x Value, nb bool) (selected bool) {
        tt := (*chanType)(unsafe.Pointer(v.typ))
        if ChanDir(tt.dir)&SendDir == 0 {
                panic("reflect: send on recv-only channel")
        }
        x.mustBeExported()
        x = x.assignTo("reflect.Value.Send", tt.elem, nil)
        var p unsafe.Pointer
        if x.flag&flagIndir != 0 {
                p = x.ptr
        } else {
                p = unsafe.Pointer(&x.ptr)
        }
        return chansend(v.typ, v.pointer(), p, nb)
}

// Set assigns x to the value v.
// It panics if CanSet returns false.
// As in Go, x's value must be assignable to v's type.
func (v Value) Set(x Value) {
        v.mustBeAssignable()
        x.mustBeExported() // do not let unexported x leak
        var target unsafe.Pointer
        if v.kind() == Interface {
                target = v.ptr
        }
        x = x.assignTo("reflect.Set", v.typ, target)
        if x.flag&flagIndir != 0 {
                typedmemmove(v.typ, v.ptr, x.ptr)
        } else {
                *(*unsafe.Pointer)(v.ptr) = x.ptr
        }
}

// SetBool sets v's underlying value.
// It panics if v's Kind is not Bool or if CanSet() is false.
func (v Value) SetBool(x bool) {
        v.mustBeAssignable()
        v.mustBe(Bool)
        *(*bool)(v.ptr) = x
}

// SetBytes sets v's underlying value.
// It panics if v's underlying value is not a slice of bytes.
func (v Value) SetBytes(x []byte) {
        v.mustBeAssignable()
        v.mustBe(Slice)
        if v.typ.Elem().Kind() != Uint8 {
                panic("reflect.Value.SetBytes of non-byte slice")
        }
        *(*[]byte)(v.ptr) = x
}

// setRunes sets v's underlying value.
// It panics if v's underlying value is not a slice of runes (int32s).
func (v Value) setRunes(x []rune) {
        v.mustBeAssignable()
        v.mustBe(Slice)
        if v.typ.Elem().Kind() != Int32 {
                panic("reflect.Value.setRunes of non-rune slice")
        }
        *(*[]rune)(v.ptr) = x
}

// SetComplex sets v's underlying value to x.
// It panics if v's Kind is not Complex64 or Complex128, or if CanSet() is false.
func (v Value) SetComplex(x complex128) {
        v.mustBeAssignable()
        switch k := v.kind(); k {
        default:
                panic(&ValueError{"reflect.Value.SetComplex", v.kind()})
        case Complex64:
                *(*complex64)(v.ptr) = complex64(x)
        case Complex128:
                *(*complex128)(v.ptr) = x
        }
}

// SetFloat sets v's underlying value to x.
// It panics if v's Kind is not Float32 or Float64, or if CanSet() is false.
func (v Value) SetFloat(x float64) {
        v.mustBeAssignable()
        switch k := v.kind(); k {
        default:
                panic(&ValueError{"reflect.Value.SetFloat", v.kind()})
        case Float32:
                *(*float32)(v.ptr) = float32(x)
        case Float64:
                *(*float64)(v.ptr) = x
        }
}

// SetInt sets v's underlying value to x.
// It panics if v's Kind is not Int, Int8, Int16, Int32, or Int64, or if CanSet() is false.
func (v Value) SetInt(x int64) {
        v.mustBeAssignable()
        switch k := v.kind(); k {
        default:
                panic(&ValueError{"reflect.Value.SetInt", v.kind()})
        case Int:
                *(*int)(v.ptr) = int(x)
        case Int8:
                *(*int8)(v.ptr) = int8(x)
        case Int16:
                *(*int16)(v.ptr) = int16(x)
        case Int32:
                *(*int32)(v.ptr) = int32(x)
        case Int64:
                *(*int64)(v.ptr) = x
        }
}

// SetLen sets v's length to n.
// It panics if v's Kind is not Slice or if n is negative or
// greater than the capacity of the slice.
func (v Value) SetLen(n int) {
        v.mustBeAssignable()
        v.mustBe(Slice)
        s := (*sliceHeader)(v.ptr)
        if uint(n) > uint(s.Cap) {
                panic("reflect: slice length out of range in SetLen")
        }
        s.Len = n
}

// SetCap sets v's capacity to n.
// It panics if v's Kind is not Slice or if n is smaller than the length or
// greater than the capacity of the slice.
func (v Value) SetCap(n int) {
        v.mustBeAssignable()
        v.mustBe(Slice)
        s := (*sliceHeader)(v.ptr)
        if n < s.Len || n > s.Cap {
                panic("reflect: slice capacity out of range in SetCap")
        }
        s.Cap = n
}

// SetMapIndex sets the value associated with key in the map v to val.
// It panics if v's Kind is not Map.
// If val is the zero Value, SetMapIndex deletes the key from the map.
// Otherwise if v holds a nil map, SetMapIndex will panic.
// As in Go, key's value must be assignable to the map's key type,
// and val's value must be assignable to the map's value type.
func (v Value) SetMapIndex(key, val Value) {
        v.mustBe(Map)
        v.mustBeExported()
        key.mustBeExported()
        tt := (*mapType)(unsafe.Pointer(v.typ))
        key = key.assignTo("reflect.Value.SetMapIndex", tt.key, nil)
        var k unsafe.Pointer
        if key.flag&flagIndir != 0 {
                k = key.ptr
        } else {
                k = unsafe.Pointer(&key.ptr)
        }
        if val.typ == nil {
                mapdelete(v.typ, v.pointer(), k)
                return
        }
        val.mustBeExported()
        val = val.assignTo("reflect.Value.SetMapIndex", tt.elem, nil)
        var e unsafe.Pointer
        if val.flag&flagIndir != 0 {
                e = val.ptr
        } else {
                e = unsafe.Pointer(&val.ptr)
        }
        mapassign(v.typ, v.pointer(), k, e)
}

// SetUint sets v's underlying value to x.
// It panics if v's Kind is not Uint, Uintptr, Uint8, Uint16, Uint32, or Uint64, or if CanSet() is false.
func (v Value) SetUint(x uint64) {
        v.mustBeAssignable()
        switch k := v.kind(); k {
        default:
                panic(&ValueError{"reflect.Value.SetUint", v.kind()})
        case Uint:
                *(*uint)(v.ptr) = uint(x)
        case Uint8:
                *(*uint8)(v.ptr) = uint8(x)
        case Uint16:
                *(*uint16)(v.ptr) = uint16(x)
        case Uint32:
                *(*uint32)(v.ptr) = uint32(x)
        case Uint64:
                *(*uint64)(v.ptr) = x
        case Uintptr:
                *(*uintptr)(v.ptr) = uintptr(x)
        }
}

// SetPointer sets the unsafe.Pointer value v to x.
// It panics if v's Kind is not UnsafePointer.
func (v Value) SetPointer(x unsafe.Pointer) {
        v.mustBeAssignable()
        v.mustBe(UnsafePointer)
        *(*unsafe.Pointer)(v.ptr) = x
}

// SetString sets v's underlying value to x.
// It panics if v's Kind is not String or if CanSet() is false.
func (v Value) SetString(x string) {
        v.mustBeAssignable()
        v.mustBe(String)
        *(*string)(v.ptr) = x
}

// Slice returns v[i:j].
// It panics if v's Kind is not Array, Slice or String, or if v is an unaddressable array,
// or if the indexes are out of bounds.
func (v Value) Slice(i, j int) Value {
        var (
                cap  int
                typ  *sliceType
                base unsafe.Pointer
        )
        switch kind := v.kind(); kind {
        default:
                panic(&ValueError{"reflect.Value.Slice", v.kind()})

        case Array:
                if v.flag&flagAddr == 0 {
                        panic("reflect.Value.Slice: slice of unaddressable array")
                }
                tt := (*arrayType)(unsafe.Pointer(v.typ))
                cap = int(tt.len)
                typ = (*sliceType)(unsafe.Pointer(tt.slice))
                base = v.ptr

        case Slice:
                typ = (*sliceType)(unsafe.Pointer(v.typ))
                s := (*sliceHeader)(v.ptr)
                base = s.Data
                cap = s.Cap

        case String:
                s := (*stringHeader)(v.ptr)
                if i < 0 || j < i || j > s.Len {
                        panic("reflect.Value.Slice: string slice index out of bounds")
                }
                t := stringHeader{arrayAt(s.Data, i, 1), j - i}
                return Value{v.typ, unsafe.Pointer(&t), v.flag}
        }

        if i < 0 || j < i || j > cap {
                panic("reflect.Value.Slice: slice index out of bounds")
        }

        // Declare slice so that gc can see the base pointer in it.
        var x []unsafe.Pointer

        // Reinterpret as *sliceHeader to edit.
        s := (*sliceHeader)(unsafe.Pointer(&x))
        s.Len = j - i
        s.Cap = cap - i
        if cap-i > 0 {
                s.Data = arrayAt(base, i, typ.elem.Size())
        } else {
                // do not advance pointer, to avoid pointing beyond end of slice
                s.Data = base
        }

        fl := v.flag&flagRO | flagIndir | flag(Slice)
        return Value{typ.common(), unsafe.Pointer(&x), fl}
}

// Slice3 is the 3-index form of the slice operation: it returns v[i:j:k].
// It panics if v's Kind is not Array or Slice, or if v is an unaddressable array,
// or if the indexes are out of bounds.
func (v Value) Slice3(i, j, k int) Value {
        var (
                cap  int
                typ  *sliceType
                base unsafe.Pointer
        )
        switch kind := v.kind(); kind {
        default:
                panic(&ValueError{"reflect.Value.Slice3", v.kind()})

        case Array:
                if v.flag&flagAddr == 0 {
                        panic("reflect.Value.Slice3: slice of unaddressable array")
                }
                tt := (*arrayType)(unsafe.Pointer(v.typ))
                cap = int(tt.len)
                typ = (*sliceType)(unsafe.Pointer(tt.slice))
                base = v.ptr

        case Slice:
                typ = (*sliceType)(unsafe.Pointer(v.typ))
                s := (*sliceHeader)(v.ptr)
                base = s.Data
                cap = s.Cap
        }

        if i < 0 || j < i || k < j || k > cap {
                panic("reflect.Value.Slice3: slice index out of bounds")
        }

        // Declare slice so that the garbage collector
        // can see the base pointer in it.
        var x []unsafe.Pointer

        // Reinterpret as *sliceHeader to edit.
        s := (*sliceHeader)(unsafe.Pointer(&x))
        s.Len = j - i
        s.Cap = k - i
        if k-i > 0 {
                s.Data = arrayAt(base, i, typ.elem.Size())
        } else {
                // do not advance pointer, to avoid pointing beyond end of slice
                s.Data = base
        }

        fl := v.flag&flagRO | flagIndir | flag(Slice)
        return Value{typ.common(), unsafe.Pointer(&x), fl}
}

// String returns the string v's underlying value, as a string.
// String is a special case because of Go's String method convention.
// Unlike the other getters, it does not panic if v's Kind is not String.
// Instead, it returns a string of the form "<T value>" where T is v's type.
// The fmt package treats Values specially. It does not call their String
// method implicitly but instead prints the concrete values they hold.
func (v Value) String() string {
        switch k := v.kind(); k {
        case Invalid:
                return "<invalid Value>"
        case String:
                return *(*string)(v.ptr)
        }
        // If you call String on a reflect.Value of other type, it's better to
        // print something than to panic. Useful in debugging.
        return "<" + v.Type().String() + " Value>"
}

// TryRecv attempts to receive a value from the channel v but will not block.
// It panics if v's Kind is not Chan.
// If the receive delivers a value, x is the transferred value and ok is true.
// If the receive cannot finish without blocking, x is the zero Value and ok is false.
// If the channel is closed, x is the zero value for the channel's element type and ok is false.
func (v Value) TryRecv() (x Value, ok bool) {
        v.mustBe(Chan)
        v.mustBeExported()
        return v.recv(true)
}

// TrySend attempts to send x on the channel v but will not block.
// It panics if v's Kind is not Chan.
// It reports whether the value was sent.
// As in Go, x's value must be assignable to the channel's element type.
func (v Value) TrySend(x Value) bool {
        v.mustBe(Chan)
        v.mustBeExported()
        return v.send(x, true)
}

// Type returns v's type.
func (v Value) Type() Type {
        f := v.flag
        if f == 0 {
                panic(&ValueError{"reflect.Value.Type", Invalid})
        }
        if f&flagMethod == 0 {
                // Easy case
                return v.typ
        }

        // Method value.
        // v.typ describes the receiver, not the method type.
        i := int(v.flag) >> flagMethodShift
        if v.typ.Kind() == Interface {
                // Method on interface.
                tt := (*interfaceType)(unsafe.Pointer(v.typ))
                if uint(i) >= uint(len(tt.methods)) {
                        panic("reflect: internal error: invalid method index")
                }
                m := &tt.methods[i]
                return v.typ.typeOff(m.typ)
        }
        // Method on concrete type.
        ut := v.typ.uncommon()
        if ut == nil || uint(i) >= uint(ut.mcount) {
                panic("reflect: internal error: invalid method index")
        }
        m := ut.methods()[i]
        return v.typ.typeOff(m.mtyp)
}

// Uint returns v's underlying value, as a uint64.
// It panics if v's Kind is not Uint, Uintptr, Uint8, Uint16, Uint32, or Uint64.
func (v Value) Uint() uint64 {
        k := v.kind()
        p := v.ptr
        switch k {
        case Uint:
                return uint64(*(*uint)(p))
        case Uint8:
                return uint64(*(*uint8)(p))
        case Uint16:
                return uint64(*(*uint16)(p))
        case Uint32:
                return uint64(*(*uint32)(p))
        case Uint64:
                return *(*uint64)(p)
        case Uintptr:
                return uint64(*(*uintptr)(p))
        }
        panic(&ValueError{"reflect.Value.Uint", v.kind()})
}

// UnsafeAddr returns a pointer to v's data.
// It is for advanced clients that also import the "unsafe" package.
// It panics if v is not addressable.
func (v Value) UnsafeAddr() uintptr {
        // TODO: deprecate
        if v.typ == nil {
                panic(&ValueError{"reflect.Value.UnsafeAddr", Invalid})
        }
        if v.flag&flagAddr == 0 {
                panic("reflect.Value.UnsafeAddr of unaddressable value")
        }
        return uintptr(v.ptr)
}

// StringHeader is the runtime representation of a string.
// It cannot be used safely or portably and its representation may
// change in a later release.
// Moreover, the Data field is not sufficient to guarantee the data
// it references will not be garbage collected, so programs must keep
// a separate, correctly typed pointer to the underlying data.
type StringHeader struct {
        Data uintptr
        Len  int
}

// stringHeader is a safe version of StringHeader used within this package.
type stringHeader struct {
        Data unsafe.Pointer
        Len  int
}

// SliceHeader is the runtime representation of a slice.
// It cannot be used safely or portably and its representation may
// change in a later release.
// Moreover, the Data field is not sufficient to guarantee the data
// it references will not be garbage collected, so programs must keep
// a separate, correctly typed pointer to the underlying data.
type SliceHeader struct {
        Data uintptr
        Len  int
        Cap  int
}

// sliceHeader is a safe version of SliceHeader used within this package.
type sliceHeader struct {
        Data unsafe.Pointer
        Len  int
        Cap  int
}

func typesMustMatch(what string, t1, t2 Type) {
        if t1 != t2 {
                panic(what + ": " + t1.String() + " != " + t2.String())
        }
}

// arrayAt returns the i-th element of p, a C-array whose elements are
// eltSize wide (in bytes).
func arrayAt(p unsafe.Pointer, i int, eltSize uintptr) unsafe.Pointer {
        return unsafe.Pointer(uintptr(p) + uintptr(i)*eltSize)
}

// grow grows the slice s so that it can hold extra more values, allocating
// more capacity if needed. It also returns the old and new slice lengths.
func grow(s Value, extra int) (Value, int, int) {
        i0 := s.Len()
        i1 := i0 + extra
        if i1 < i0 {
                panic("reflect.Append: slice overflow")
        }
        m := s.Cap()
        if i1 <= m {
                return s.Slice(0, i1), i0, i1
        }
        if m == 0 {
                m = extra
        } else {
                for m < i1 {
                        if i0 < 1024 {
                                m += m
                        } else {
                                m += m / 4
                        }
                }
        }
        t := MakeSlice(s.Type(), i1, m)
        Copy(t, s)
        return t, i0, i1
}

// Append appends the values x to a slice s and returns the resulting slice.
// As in Go, each x's value must be assignable to the slice's element type.
func Append(s Value, x ...Value) Value {
        s.mustBe(Slice)
        s, i0, i1 := grow(s, len(x))
        for i, j := i0, 0; i < i1; i, j = i+1, j+1 {
                s.Index(i).Set(x[j])
        }
        return s
}

// AppendSlice appends a slice t to a slice s and returns the resulting slice.
// The slices s and t must have the same element type.
func AppendSlice(s, t Value) Value {
        s.mustBe(Slice)
        t.mustBe(Slice)
        typesMustMatch("reflect.AppendSlice", s.Type().Elem(), t.Type().Elem())
        s, i0, i1 := grow(s, t.Len())
        Copy(s.Slice(i0, i1), t)
        return s
}

// Copy copies the contents of src into dst until either
// dst has been filled or src has been exhausted.
// It returns the number of elements copied.
// Dst and src each must have kind Slice or Array, and
// dst and src must have the same element type.
func Copy(dst, src Value) int {
        dk := dst.kind()
        if dk != Array && dk != Slice {
                panic(&ValueError{"reflect.Copy", dk})
        }
        if dk == Array {
                dst.mustBeAssignable()
        }
        dst.mustBeExported()

        sk := src.kind()
        if sk != Array && sk != Slice {
                panic(&ValueError{"reflect.Copy", sk})
        }
        src.mustBeExported()

        de := dst.typ.Elem()
        se := src.typ.Elem()
        typesMustMatch("reflect.Copy", de, se)

        var ds, ss sliceHeader
        if dk == Array {
                ds.Data = dst.ptr
                ds.Len = dst.Len()
                ds.Cap = ds.Len
        } else {
                ds = *(*sliceHeader)(dst.ptr)
        }
        if sk == Array {
                ss.Data = src.ptr
                ss.Len = src.Len()
                ss.Cap = ss.Len
        } else {
                ss = *(*sliceHeader)(src.ptr)
        }

        return typedslicecopy(de.common(), ds, ss)
}

// A runtimeSelect is a single case passed to rselect.
// This must match ../runtime/select.go:/runtimeSelect
type runtimeSelect struct {
        dir SelectDir      // SelectSend, SelectRecv or SelectDefault
        typ *rtype         // channel type
        ch  unsafe.Pointer // channel
        val unsafe.Pointer // ptr to data (SendDir) or ptr to receive buffer (RecvDir)
}

// rselect runs a select. It returns the index of the chosen case.
// If the case was a receive, val is filled in with the received value.
// The conventional OK bool indicates whether the receive corresponds
// to a sent value.
//go:noescape
func rselect([]runtimeSelect) (chosen int, recvOK bool)

// A SelectDir describes the communication direction of a select case.
type SelectDir int

// NOTE: These values must match ../runtime/select.go:/selectDir.

const (
        _             SelectDir = iota
        SelectSend              // case Chan <- Send
        SelectRecv              // case <-Chan:
        SelectDefault           // default
)

// A SelectCase describes a single case in a select operation.
// The kind of case depends on Dir, the communication direction.
//
// If Dir is SelectDefault, the case represents a default case.
// Chan and Send must be zero Values.
//
// If Dir is SelectSend, the case represents a send operation.
// Normally Chan's underlying value must be a channel, and Send's underlying value must be
// assignable to the channel's element type. As a special case, if Chan is a zero Value,
// then the case is ignored, and the field Send will also be ignored and may be either zero
// or non-zero.
//
// If Dir is SelectRecv, the case represents a receive operation.
// Normally Chan's underlying value must be a channel and Send must be a zero Value.
// If Chan is a zero Value, then the case is ignored, but Send must still be a zero Value.
// When a receive operation is selected, the received Value is returned by Select.
//
type SelectCase struct {
        Dir  SelectDir // direction of case
        Chan Value     // channel to use (for send or receive)
        Send Value     // value to send (for send)
}

// Select executes a select operation described by the list of cases.
// Like the Go select statement, it blocks until at least one of the cases
// can proceed, makes a uniform pseudo-random choice,
// and then executes that case. It returns the index of the chosen case
// and, if that case was a receive operation, the value received and a
// boolean indicating whether the value corresponds to a send on the channel
// (as opposed to a zero value received because the channel is closed).
func Select(cases []SelectCase) (chosen int, recv Value, recvOK bool) {
        // NOTE: Do not trust that caller is not modifying cases data underfoot.
        // The range is safe because the caller cannot modify our copy of the len
        // and each iteration makes its own copy of the value c.
        runcases := make([]runtimeSelect, len(cases))
        haveDefault := false
        for i, c := range cases {
                rc := &runcases[i]
                rc.dir = c.Dir
                switch c.Dir {
                default:
                        panic("reflect.Select: invalid Dir")

                case SelectDefault: // default
                        if haveDefault {
                                panic("reflect.Select: multiple default cases")
                        }
                        haveDefault = true
                        if c.Chan.IsValid() {
                                panic("reflect.Select: default case has Chan value")
                        }
                        if c.Send.IsValid() {
                                panic("reflect.Select: default case has Send value")
                        }

                case SelectSend:
                        ch := c.Chan
                        if !ch.IsValid() {
                                break
                        }
                        ch.mustBe(Chan)
                        ch.mustBeExported()
                        tt := (*chanType)(unsafe.Pointer(ch.typ))
                        if ChanDir(tt.dir)&SendDir == 0 {
                                panic("reflect.Select: SendDir case using recv-only channel")
                        }
                        rc.ch = ch.pointer()
                        rc.typ = &tt.rtype
                        v := c.Send
                        if !v.IsValid() {
                                panic("reflect.Select: SendDir case missing Send value")
                        }
                        v.mustBeExported()
                        v = v.assignTo("reflect.Select", tt.elem, nil)
                        if v.flag&flagIndir != 0 {
                                rc.val = v.ptr
                        } else {
                                rc.val = unsafe.Pointer(&v.ptr)
                        }

                case SelectRecv:
                        if c.Send.IsValid() {
                                panic("reflect.Select: RecvDir case has Send value")
                        }
                        ch := c.Chan
                        if !ch.IsValid() {
                                break
                        }
                        ch.mustBe(Chan)
                        ch.mustBeExported()
                        tt := (*chanType)(unsafe.Pointer(ch.typ))
                        if ChanDir(tt.dir)&RecvDir == 0 {
                                panic("reflect.Select: RecvDir case using send-only channel")
                        }
                        rc.ch = ch.pointer()
                        rc.typ = &tt.rtype
                        rc.val = unsafe_New(tt.elem)
                }
        }

        chosen, recvOK = rselect(runcases)
        if runcases[chosen].dir == SelectRecv {
                tt := (*chanType)(unsafe.Pointer(runcases[chosen].typ))
                t := tt.elem
                p := runcases[chosen].val
                fl := flag(t.Kind())
                if ifaceIndir(t) {
                        recv = Value{t, p, fl | flagIndir}
                } else {
                        recv = Value{t, *(*unsafe.Pointer)(p), fl}
                }
        }
        return chosen, recv, recvOK
}

/*
 * constructors
 */

// implemented in package runtime
func unsafe_New(*rtype) unsafe.Pointer
func unsafe_NewArray(*rtype, int) unsafe.Pointer

// MakeSlice creates a new zero-initialized slice value
// for the specified slice type, length, and capacity.
func MakeSlice(typ Type, len, cap int) Value {
        if typ.Kind() != Slice {
                panic("reflect.MakeSlice of non-slice type")
        }
        if len < 0 {
                panic("reflect.MakeSlice: negative len")
        }
        if cap < 0 {
                panic("reflect.MakeSlice: negative cap")
        }
        if len > cap {
                panic("reflect.MakeSlice: len > cap")
        }

        s := sliceHeader{unsafe_NewArray(typ.Elem().(*rtype), cap), len, cap}
        return Value{typ.common(), unsafe.Pointer(&s), flagIndir | flag(Slice)}
}

// MakeChan creates a new channel with the specified type and buffer size.
func MakeChan(typ Type, buffer int) Value {
        if typ.Kind() != Chan {
                panic("reflect.MakeChan of non-chan type")
        }
        if buffer < 0 {
                panic("reflect.MakeChan: negative buffer size")
        }
        if typ.ChanDir() != BothDir {
                panic("reflect.MakeChan: unidirectional channel type")
        }
        ch := makechan(typ.(*rtype), uint64(buffer))
        return Value{typ.common(), ch, flag(Chan)}
}

// MakeMap creates a new map of the specified type.
func MakeMap(typ Type) Value {
        if typ.Kind() != Map {
                panic("reflect.MakeMap of non-map type")
        }
        m := makemap(typ.(*rtype))
        return Value{typ.common(), m, flag(Map)}
}

// Indirect returns the value that v points to.
// If v is a nil pointer, Indirect returns a zero Value.
// If v is not a pointer, Indirect returns v.
func Indirect(v Value) Value {
        if v.Kind() != Ptr {
                return v
        }
        return v.Elem()
}

// ValueOf returns a new Value initialized to the concrete value
// stored in the interface i. ValueOf(nil) returns the zero Value.
func ValueOf(i interface{}) Value {
        if i == nil {
                return Value{}
        }

        // TODO: Maybe allow contents of a Value to live on the stack.
        // For now we make the contents always escape to the heap. It
        // makes life easier in a few places (see chanrecv/mapassign
        // comment below).
        escapes(i)

        return unpackEface(i)
}

// Zero returns a Value representing the zero value for the specified type.
// The result is different from the zero value of the Value struct,
// which represents no value at all.
// For example, Zero(TypeOf(42)) returns a Value with Kind Int and value 0.
// The returned value is neither addressable nor settable.
func Zero(typ Type) Value {
        if typ == nil {
                panic("reflect: Zero(nil)")
        }
        t := typ.common()
        fl := flag(t.Kind())
        if ifaceIndir(t) {
                return Value{t, unsafe_New(typ.(*rtype)), fl | flagIndir}
        }
        return Value{t, nil, fl}
}

// New returns a Value representing a pointer to a new zero value
// for the specified type. That is, the returned Value's Type is PtrTo(typ).
func New(typ Type) Value {
        if typ == nil {
                panic("reflect: New(nil)")
        }
        ptr := unsafe_New(typ.(*rtype))
        fl := flag(Ptr)
        return Value{typ.common().ptrTo(), ptr, fl}
}

// NewAt returns a Value representing a pointer to a value of the
// specified type, using p as that pointer.
func NewAt(typ Type, p unsafe.Pointer) Value {
        fl := flag(Ptr)
        return Value{typ.common().ptrTo(), p, fl}
}

// assignTo returns a value v that can be assigned directly to typ.
// It panics if v is not assignable to typ.
// For a conversion to an interface type, target is a suggested scratch space to use.
func (v Value) assignTo(context string, dst *rtype, target unsafe.Pointer) Value {
        if v.flag&flagMethod != 0 {
                v = makeMethodValue(context, v)
        }

        switch {
        case directlyAssignable(dst, v.typ):
                // Overwrite type so that they match.
                // Same memory layout, so no harm done.
                v.typ = dst
                fl := v.flag & (flagRO | flagAddr | flagIndir)
                fl |= flag(dst.Kind())
                return Value{dst, v.ptr, fl}

        case implements(dst, v.typ):
                if target == nil {
                        target = unsafe_New(dst)
                }
                x := valueInterface(v, false)
                if dst.NumMethod() == 0 {
                        *(*interface{})(target) = x
                } else {
                        ifaceE2I(dst, x, target)
                }
                return Value{dst, target, flagIndir | flag(Interface)}
        }

        // Failed.
        panic(context + ": value of type " + v.typ.String() + " is not assignable to type " + dst.String())
}

// Convert returns the value v converted to type t.
// If the usual Go conversion rules do not allow conversion
// of the value v to type t, Convert panics.
func (v Value) Convert(t Type) Value {
        if v.flag&flagMethod != 0 {
                v = makeMethodValue("Convert", v)
        }
        op := convertOp(t.common(), v.typ)
        if op == nil {
                panic("reflect.Value.Convert: value of type " + v.typ.String() + " cannot be converted to type " + t.String())
        }
        return op(v, t)
}

// convertOp returns the function to convert a value of type src
// to a value of type dst. If the conversion is illegal, convertOp returns nil.
func convertOp(dst, src *rtype) func(Value, Type) Value {
        switch src.Kind() {
        case Int, Int8, Int16, Int32, Int64:
                switch dst.Kind() {
                case Int, Int8, Int16, Int32, Int64, Uint, Uint8, Uint16, Uint32, Uint64, Uintptr:
                        return cvtInt
                case Float32, Float64:
                        return cvtIntFloat
                case String:
                        return cvtIntString
                }

        case Uint, Uint8, Uint16, Uint32, Uint64, Uintptr:
                switch dst.Kind() {
                case Int, Int8, Int16, Int32, Int64, Uint, Uint8, Uint16, Uint32, Uint64, Uintptr:
                        return cvtUint
                case Float32, Float64:
                        return cvtUintFloat
                case String:
                        return cvtUintString
                }

        case Float32, Float64:
                switch dst.Kind() {
                case Int, Int8, Int16, Int32, Int64:
                        return cvtFloatInt
                case Uint, Uint8, Uint16, Uint32, Uint64, Uintptr:
                        return cvtFloatUint
                case Float32, Float64:
                        return cvtFloat
                }

        case Complex64, Complex128:
                switch dst.Kind() {
                case Complex64, Complex128:
                        return cvtComplex
                }

        case String:
                if dst.Kind() == Slice && dst.Elem().PkgPath() == "" {
                        switch dst.Elem().Kind() {
                        case Uint8:
                                return cvtStringBytes
                        case Int32:
                                return cvtStringRunes
                        }
                }

        case Slice:
                if dst.Kind() == String && src.Elem().PkgPath() == "" {
                        switch src.Elem().Kind() {
                        case Uint8:
                                return cvtBytesString
                        case Int32:
                                return cvtRunesString
                        }
                }
        }

        // dst and src have same underlying type.
        if haveIdenticalUnderlyingType(dst, src, false) {
                return cvtDirect
        }

        // dst and src are unnamed pointer types with same underlying base type.
        if dst.Kind() == Ptr && dst.Name() == "" &&
                src.Kind() == Ptr && src.Name() == "" &&
                haveIdenticalUnderlyingType(dst.Elem().common(), src.Elem().common(), false) {
                return cvtDirect
        }

        if implements(dst, src) {
                if src.Kind() == Interface {
                        return cvtI2I
                }
                return cvtT2I
        }

        return nil
}

// makeInt returns a Value of type t equal to bits (possibly truncated),
// where t is a signed or unsigned int type.
func makeInt(f flag, bits uint64, t Type) Value {
        typ := t.common()
        ptr := unsafe_New(typ)
        switch typ.size {
        case 1:
                *(*uint8)(ptr) = uint8(bits)
        case 2:
                *(*uint16)(ptr) = uint16(bits)
        case 4:
                *(*uint32)(ptr) = uint32(bits)
        case 8:
                *(*uint64)(ptr) = bits
        }
        return Value{typ, ptr, f | flagIndir | flag(typ.Kind())}
}

// makeFloat returns a Value of type t equal to v (possibly truncated to float32),
// where t is a float32 or float64 type.
func makeFloat(f flag, v float64, t Type) Value {
        typ := t.common()
        ptr := unsafe_New(typ)
        switch typ.size {
        case 4:
                *(*float32)(ptr) = float32(v)
        case 8:
                *(*float64)(ptr) = v
        }
        return Value{typ, ptr, f | flagIndir | flag(typ.Kind())}
}

// makeComplex returns a Value of type t equal to v (possibly truncated to complex64),
// where t is a complex64 or complex128 type.
func makeComplex(f flag, v complex128, t Type) Value {
        typ := t.common()
        ptr := unsafe_New(typ)
        switch typ.size {
        case 8:
                *(*complex64)(ptr) = complex64(v)
        case 16:
                *(*complex128)(ptr) = v
        }
        return Value{typ, ptr, f | flagIndir | flag(typ.Kind())}
}

func makeString(f flag, v string, t Type) Value {
        ret := New(t).Elem()
        ret.SetString(v)
        ret.flag = ret.flag&^flagAddr | f
        return ret
}

func makeBytes(f flag, v []byte, t Type) Value {
        ret := New(t).Elem()
        ret.SetBytes(v)
        ret.flag = ret.flag&^flagAddr | f
        return ret
}

func makeRunes(f flag, v []rune, t Type) Value {
        ret := New(t).Elem()
        ret.setRunes(v)
        ret.flag = ret.flag&^flagAddr | f
        return ret
}

// These conversion functions are returned by convertOp
// for classes of conversions. For example, the first function, cvtInt,
// takes any value v of signed int type and returns the value converted
// to type t, where t is any signed or unsigned int type.

// convertOp: intXX -> [u]intXX
func cvtInt(v Value, t Type) Value {
        return makeInt(v.flag&flagRO, uint64(v.Int()), t)
}

// convertOp: uintXX -> [u]intXX
func cvtUint(v Value, t Type) Value {
        return makeInt(v.flag&flagRO, v.Uint(), t)
}

// convertOp: floatXX -> intXX
func cvtFloatInt(v Value, t Type) Value {
        return makeInt(v.flag&flagRO, uint64(int64(v.Float())), t)
}

// convertOp: floatXX -> uintXX
func cvtFloatUint(v Value, t Type) Value {
        return makeInt(v.flag&flagRO, uint64(v.Float()), t)
}

// convertOp: intXX -> floatXX
func cvtIntFloat(v Value, t Type) Value {
        return makeFloat(v.flag&flagRO, float64(v.Int()), t)
}

// convertOp: uintXX -> floatXX
func cvtUintFloat(v Value, t Type) Value {
        return makeFloat(v.flag&flagRO, float64(v.Uint()), t)
}

// convertOp: floatXX -> floatXX
func cvtFloat(v Value, t Type) Value {
        return makeFloat(v.flag&flagRO, v.Float(), t)
}

// convertOp: complexXX -> complexXX
func cvtComplex(v Value, t Type) Value {
        return makeComplex(v.flag&flagRO, v.Complex(), t)
}

// convertOp: intXX -> string
func cvtIntString(v Value, t Type) Value {
        return makeString(v.flag&flagRO, string(v.Int()), t)
}

// convertOp: uintXX -> string
func cvtUintString(v Value, t Type) Value {
        return makeString(v.flag&flagRO, string(v.Uint()), t)
}

// convertOp: []byte -> string
func cvtBytesString(v Value, t Type) Value {
        return makeString(v.flag&flagRO, string(v.Bytes()), t)
}

// convertOp: string -> []byte
func cvtStringBytes(v Value, t Type) Value {
        return makeBytes(v.flag&flagRO, []byte(v.String()), t)
}

// convertOp: []rune -> string
func cvtRunesString(v Value, t Type) Value {
        return makeString(v.flag&flagRO, string(v.runes()), t)
}

// convertOp: string -> []rune
func cvtStringRunes(v Value, t Type) Value {
        return makeRunes(v.flag&flagRO, []rune(v.String()), t)
}

// convertOp: direct copy
func cvtDirect(v Value, typ Type) Value {
        f := v.flag
        t := typ.common()
        ptr := v.ptr
        if f&flagAddr != 0 {
                // indirect, mutable word - make a copy
                c := unsafe_New(t)
                typedmemmove(t, c, ptr)
                ptr = c
                f &^= flagAddr
        }
        return Value{t, ptr, v.flag&flagRO | f} // v.flag&flagRO|f == f?
}

// convertOp: concrete -> interface
func cvtT2I(v Value, typ Type) Value {
        target := unsafe_New(typ.common())
        x := valueInterface(v, false)
        if typ.NumMethod() == 0 {
                *(*interface{})(target) = x
        } else {
                ifaceE2I(typ.(*rtype), x, target)
        }
        return Value{typ.common(), target, v.flag&flagRO | flagIndir | flag(Interface)}
}

// convertOp: interface -> interface
func cvtI2I(v Value, typ Type) Value {
        if v.IsNil() {
                ret := Zero(typ)
                ret.flag |= v.flag & flagRO
                return ret
        }
        return cvtT2I(v.Elem(), typ)
}

// implemented in ../runtime
func chancap(ch unsafe.Pointer) int
func chanclose(ch unsafe.Pointer)
func chanlen(ch unsafe.Pointer) int

// Note: some of the noescape annotations below are technically a lie,
// but safe in the context of this package. Functions like chansend
// and mapassign don't escape the referent, but may escape anything
// the referent points to (they do shallow copies of the referent).
// It is safe in this package because the referent may only point
// to something a Value may point to, and that is always in the heap
// (due to the escapes() call in ValueOf).

//go:noescape
func chanrecv(t *rtype, ch unsafe.Pointer, nb bool, val unsafe.Pointer) (selected, received bool)

//go:noescape
func chansend(t *rtype, ch unsafe.Pointer, val unsafe.Pointer, nb bool) bool

func makechan(typ *rtype, size uint64) (ch unsafe.Pointer)
func makemap(t *rtype) (m unsafe.Pointer)

//go:noescape
func mapaccess(t *rtype, m unsafe.Pointer, key unsafe.Pointer) (val unsafe.Pointer)

//go:noescape
func mapassign(t *rtype, m unsafe.Pointer, key, val unsafe.Pointer)

//go:noescape
func mapdelete(t *rtype, m unsafe.Pointer, key unsafe.Pointer)

// m escapes into the return value, but the caller of mapiterinit
// doesn't let the return value escape.
//go:noescape
func mapiterinit(t *rtype, m unsafe.Pointer) unsafe.Pointer

//go:noescape
func mapiterkey(it unsafe.Pointer) (key unsafe.Pointer)

//go:noescape
func mapiternext(it unsafe.Pointer)

//go:noescape
func maplen(m unsafe.Pointer) int

// call calls fn with a copy of the n argument bytes pointed at by arg.
// After fn returns, reflectcall copies n-retoffset result bytes
// back into arg+retoffset before returning. If copying result bytes back,
// the caller must pass the argument frame type as argtype, so that
// call can execute appropriate write barriers during the copy.
func call(argtype *rtype, fn, arg unsafe.Pointer, n uint32, retoffset uint32)

func ifaceE2I(t *rtype, src interface{}, dst unsafe.Pointer)

// typedmemmove copies a value of type t to dst from src.
//go:noescape
func typedmemmove(t *rtype, dst, src unsafe.Pointer)

// typedmemmovepartial is like typedmemmove but assumes that
// dst and src point off bytes into the value and only copies size bytes.
//go:noescape
func typedmemmovepartial(t *rtype, dst, src unsafe.Pointer, off, size uintptr)

// typedslicecopy copies a slice of elemType values from src to dst,
// returning the number of elements copied.
//go:noescape
func typedslicecopy(elemType *rtype, dst, src sliceHeader) int

//go:noescape
func memclrNoHeapPointers(ptr unsafe.Pointer, n uintptr)

// Dummy annotation marking that the value x escapes,
// for use in cases where the reflect code is so clever that
// the compiler cannot follow.
func escapes(x interface{}) {
        if dummy.b {
                dummy.x = x
        }
}

var dummy struct {
        b bool
        x interface{}
}