go/types, types2: implement Alias proposal (export API)

[gostls13.git] / src / cmd / compile / internal / types2 / named.go

// Copyright 2011 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 types2

import (
        "cmd/compile/internal/syntax"
        "sync"
        "sync/atomic"
)

// Type-checking Named types is subtle, because they may be recursively
// defined, and because their full details may be spread across multiple
// declarations (via methods). For this reason they are type-checked lazily,
// to avoid information being accessed before it is complete.
//
// Conceptually, it is helpful to think of named types as having two distinct
// sets of information:
//  - "LHS" information, defining their identity: Obj() and TypeArgs()
//  - "RHS" information, defining their details: TypeParams(), Underlying(),
//    and methods.
//
// In this taxonomy, LHS information is available immediately, but RHS
// information is lazy. Specifically, a named type N may be constructed in any
// of the following ways:
//  1. type-checked from the source
//  2. loaded eagerly from export data
//  3. loaded lazily from export data (when using unified IR)
//  4. instantiated from a generic type
//
// In cases 1, 3, and 4, it is possible that the underlying type or methods of
// N may not be immediately available.
//  - During type-checking, we allocate N before type-checking its underlying
//    type or methods, so that we may resolve recursive references.
//  - When loading from export data, we may load its methods and underlying
//    type lazily using a provided load function.
//  - After instantiating, we lazily expand the underlying type and methods
//    (note that instances may be created while still in the process of
//    type-checking the original type declaration).
//
// In cases 3 and 4 this lazy construction may also occur concurrently, due to
// concurrent use of the type checker API (after type checking or importing has
// finished). It is critical that we keep track of state, so that Named types
// are constructed exactly once and so that we do not access their details too
// soon.
//
// We achieve this by tracking state with an atomic state variable, and
// guarding potentially concurrent calculations with a mutex. At any point in
// time this state variable determines which data on N may be accessed. As
// state monotonically progresses, any data available at state M may be
// accessed without acquiring the mutex at state N, provided N >= M.
//
// GLOSSARY: Here are a few terms used in this file to describe Named types:
//  - We say that a Named type is "instantiated" if it has been constructed by
//    instantiating a generic named type with type arguments.
//  - We say that a Named type is "declared" if it corresponds to a type
//    declaration in the source. Instantiated named types correspond to a type
//    instantiation in the source, not a declaration. But their Origin type is
//    a declared type.
//  - We say that a Named type is "resolved" if its RHS information has been
//    loaded or fully type-checked. For Named types constructed from export
//    data, this may involve invoking a loader function to extract information
//    from export data. For instantiated named types this involves reading
//    information from their origin.
//  - We say that a Named type is "expanded" if it is an instantiated type and
//    type parameters in its underlying type and methods have been substituted
//    with the type arguments from the instantiation. A type may be partially
//    expanded if some but not all of these details have been substituted.
//    Similarly, we refer to these individual details (underlying type or
//    method) as being "expanded".
//  - When all information is known for a named type, we say it is "complete".
//
// Some invariants to keep in mind: each declared Named type has a single
// corresponding object, and that object's type is the (possibly generic) Named
// type. Declared Named types are identical if and only if their pointers are
// identical. On the other hand, multiple instantiated Named types may be
// identical even though their pointers are not identical. One has to use
// Identical to compare them. For instantiated named types, their obj is a
// synthetic placeholder that records their position of the corresponding
// instantiation in the source (if they were constructed during type checking).
//
// To prevent infinite expansion of named instances that are created outside of
// type-checking, instances share a Context with other instances created during
// their expansion. Via the pidgeonhole principle, this guarantees that in the
// presence of a cycle of named types, expansion will eventually find an
// existing instance in the Context and short-circuit the expansion.
//
// Once an instance is complete, we can nil out this shared Context to unpin
// memory, though this Context may still be held by other incomplete instances
// in its "lineage".

// A Named represents a named (defined) type.
type Named struct {
        check *Checker  // non-nil during type-checking; nil otherwise
        obj   *TypeName // corresponding declared object for declared types; see above for instantiated types

        // fromRHS holds the type (on RHS of declaration) this *Named type is derived
        // from (for cycle reporting). Only used by validType, and therefore does not
        // require synchronization.
        fromRHS Type

        // information for instantiated types; nil otherwise
        inst *instance

        mu         sync.Mutex     // guards all fields below
        state_     uint32         // the current state of this type; must only be accessed atomically
        underlying Type           // possibly a *Named during setup; never a *Named once set up completely
        tparams    *TypeParamList // type parameters, or nil

        // methods declared for this type (not the method set of this type)
        // Signatures are type-checked lazily.
        // For non-instantiated types, this is a fully populated list of methods. For
        // instantiated types, methods are individually expanded when they are first
        // accessed.
        methods []*Func

        // loader may be provided to lazily load type parameters, underlying type, and methods.
        loader func(*Named) (tparams []*TypeParam, underlying Type, methods []*Func)
}

// instance holds information that is only necessary for instantiated named
// types.
type instance struct {
        orig            *Named    // original, uninstantiated type
        targs           *TypeList // type arguments
        expandedMethods int       // number of expanded methods; expandedMethods <= len(orig.methods)
        ctxt            *Context  // local Context; set to nil after full expansion
}

// namedState represents the possible states that a named type may assume.
type namedState uint32

const (
        unresolved namedState = iota // tparams, underlying type and methods might be unavailable
        resolved                     // resolve has run; methods might be incomplete (for instances)
        complete                     // all data is known
)

// NewNamed returns a new named type for the given type name, underlying type, and associated methods.
// If the given type name obj doesn't have a type yet, its type is set to the returned named type.
// The underlying type must not be a *Named.
func NewNamed(obj *TypeName, underlying Type, methods []*Func) *Named {
        if asNamed(underlying) != nil {
                panic("underlying type must not be *Named")
        }
        return (*Checker)(nil).newNamed(obj, underlying, methods)
}

// resolve resolves the type parameters, methods, and underlying type of n.
// This information may be loaded from a provided loader function, or computed
// from an origin type (in the case of instances).
//
// After resolution, the type parameters, methods, and underlying type of n are
// accessible; but if n is an instantiated type, its methods may still be
// unexpanded.
func (n *Named) resolve() *Named {
        if n.state() >= resolved { // avoid locking below
                return n
        }

        // TODO(rfindley): if n.check is non-nil we can avoid locking here, since
        // type-checking is not concurrent. Evaluate if this is worth doing.
        n.mu.Lock()
        defer n.mu.Unlock()

        if n.state() >= resolved {
                return n
        }

        if n.inst != nil {
                assert(n.underlying == nil) // n is an unresolved instance
                assert(n.loader == nil)     // instances are created by instantiation, in which case n.loader is nil

                orig := n.inst.orig
                orig.resolve()
                underlying := n.expandUnderlying()

                n.tparams = orig.tparams
                n.underlying = underlying
                n.fromRHS = orig.fromRHS // for cycle detection

                if len(orig.methods) == 0 {
                        n.setState(complete) // nothing further to do
                        n.inst.ctxt = nil
                } else {
                        n.setState(resolved)
                }
                return n
        }

        // TODO(mdempsky): Since we're passing n to the loader anyway
        // (necessary because types2 expects the receiver type for methods
        // on defined interface types to be the Named rather than the
        // underlying Interface), maybe it should just handle calling
        // SetTypeParams, SetUnderlying, and AddMethod instead?  Those
        // methods would need to support reentrant calls though. It would
        // also make the API more future-proof towards further extensions.
        if n.loader != nil {
                assert(n.underlying == nil)
                assert(n.TypeArgs().Len() == 0) // instances are created by instantiation, in which case n.loader is nil

                tparams, underlying, methods := n.loader(n)

                n.tparams = bindTParams(tparams)
                n.underlying = underlying
                n.fromRHS = underlying // for cycle detection
                n.methods = methods
                n.loader = nil
        }

        n.setState(complete)
        return n
}

// state atomically accesses the current state of the receiver.
func (n *Named) state() namedState {
        return namedState(atomic.LoadUint32(&n.state_))
}

// setState atomically stores the given state for n.
// Must only be called while holding n.mu.
func (n *Named) setState(state namedState) {
        atomic.StoreUint32(&n.state_, uint32(state))
}

// newNamed is like NewNamed but with a *Checker receiver.
func (check *Checker) newNamed(obj *TypeName, underlying Type, methods []*Func) *Named {
        typ := &Named{check: check, obj: obj, fromRHS: underlying, underlying: underlying, methods: methods}
        if obj.typ == nil {
                obj.typ = typ
        }
        // Ensure that typ is always sanity-checked.
        if check != nil {
                check.needsCleanup(typ)
        }
        return typ
}

// newNamedInstance creates a new named instance for the given origin and type
// arguments, recording pos as the position of its synthetic object (for error
// reporting).
//
// If set, expanding is the named type instance currently being expanded, that
// led to the creation of this instance.
func (check *Checker) newNamedInstance(pos syntax.Pos, orig *Named, targs []Type, expanding *Named) *Named {
        assert(len(targs) > 0)

        obj := NewTypeName(pos, orig.obj.pkg, orig.obj.name, nil)
        inst := &instance{orig: orig, targs: newTypeList(targs)}

        // Only pass the expanding context to the new instance if their packages
        // match. Since type reference cycles are only possible within a single
        // package, this is sufficient for the purposes of short-circuiting cycles.
        // Avoiding passing the context in other cases prevents unnecessary coupling
        // of types across packages.
        if expanding != nil && expanding.Obj().pkg == obj.pkg {
                inst.ctxt = expanding.inst.ctxt
        }
        typ := &Named{check: check, obj: obj, inst: inst}
        obj.typ = typ
        // Ensure that typ is always sanity-checked.
        if check != nil {
                check.needsCleanup(typ)
        }
        return typ
}

func (t *Named) cleanup() {
        assert(t.inst == nil || t.inst.orig.inst == nil)
        // Ensure that every defined type created in the course of type-checking has
        // either non-*Named underlying type, or is unexpanded.
        //
        // This guarantees that we don't leak any types whose underlying type is
        // *Named, because any unexpanded instances will lazily compute their
        // underlying type by substituting in the underlying type of their origin.
        // The origin must have either been imported or type-checked and expanded
        // here, and in either case its underlying type will be fully expanded.
        switch t.underlying.(type) {
        case nil:
                if t.TypeArgs().Len() == 0 {
                        panic("nil underlying")
                }
        case *Named:
                t.under() // t.under may add entries to check.cleaners
        }
        t.check = nil
}

// Obj returns the type name for the declaration defining the named type t. For
// instantiated types, this is same as the type name of the origin type.
func (t *Named) Obj() *TypeName {
        if t.inst == nil {
                return t.obj
        }
        return t.inst.orig.obj
}

// Origin returns the generic type from which the named type t is
// instantiated. If t is not an instantiated type, the result is t.
func (t *Named) Origin() *Named {
        if t.inst == nil {
                return t
        }
        return t.inst.orig
}

// TypeParams returns the type parameters of the named type t, or nil.
// The result is non-nil for an (originally) generic type even if it is instantiated.
func (t *Named) TypeParams() *TypeParamList { return t.resolve().tparams }

// SetTypeParams sets the type parameters of the named type t.
// t must not have type arguments.
func (t *Named) SetTypeParams(tparams []*TypeParam) {
        assert(t.inst == nil)
        t.resolve().tparams = bindTParams(tparams)
}

// TypeArgs returns the type arguments used to instantiate the named type t.
func (t *Named) TypeArgs() *TypeList {
        if t.inst == nil {
                return nil
        }
        return t.inst.targs
}

// NumMethods returns the number of explicit methods defined for t.
func (t *Named) NumMethods() int {
        return len(t.Origin().resolve().methods)
}

// Method returns the i'th method of named type t for 0 <= i < t.NumMethods().
//
// For an ordinary or instantiated type t, the receiver base type of this
// method is the named type t. For an uninstantiated generic type t, each
// method receiver is instantiated with its receiver type parameters.
func (t *Named) Method(i int) *Func {
        t.resolve()

        if t.state() >= complete {
                return t.methods[i]
        }

        assert(t.inst != nil) // only instances should have incomplete methods
        orig := t.inst.orig

        t.mu.Lock()
        defer t.mu.Unlock()

        if len(t.methods) != len(orig.methods) {
                assert(len(t.methods) == 0)
                t.methods = make([]*Func, len(orig.methods))
        }

        if t.methods[i] == nil {
                assert(t.inst.ctxt != nil) // we should still have a context remaining from the resolution phase
                t.methods[i] = t.expandMethod(i)
                t.inst.expandedMethods++

                // Check if we've created all methods at this point. If we have, mark the
                // type as fully expanded.
                if t.inst.expandedMethods == len(orig.methods) {
                        t.setState(complete)
                        t.inst.ctxt = nil // no need for a context anymore
                }
        }

        return t.methods[i]
}

// expandMethod substitutes type arguments in the i'th method for an
// instantiated receiver.
func (t *Named) expandMethod(i int) *Func {
        // t.orig.methods is not lazy. origm is the method instantiated with its
        // receiver type parameters (the "origin" method).
        origm := t.inst.orig.Method(i)
        assert(origm != nil)

        check := t.check
        // Ensure that the original method is type-checked.
        if check != nil {
                check.objDecl(origm, nil)
        }

        origSig := origm.typ.(*Signature)
        rbase, _ := deref(origSig.Recv().Type())

        // If rbase is t, then origm is already the instantiated method we're looking
        // for. In this case, we return origm to preserve the invariant that
        // traversing Method->Receiver Type->Method should get back to the same
        // method.
        //
        // This occurs if t is instantiated with the receiver type parameters, as in
        // the use of m in func (r T[_]) m() { r.m() }.
        if rbase == t {
                return origm
        }

        sig := origSig
        // We can only substitute if we have a correspondence between type arguments
        // and type parameters. This check is necessary in the presence of invalid
        // code.
        if origSig.RecvTypeParams().Len() == t.inst.targs.Len() {
                smap := makeSubstMap(origSig.RecvTypeParams().list(), t.inst.targs.list())
                var ctxt *Context
                if check != nil {
                        ctxt = check.context()
                }
                sig = check.subst(origm.pos, origSig, smap, t, ctxt).(*Signature)
        }

        if sig == origSig {
                // No substitution occurred, but we still need to create a new signature to
                // hold the instantiated receiver.
                copy := *origSig
                sig = &copy
        }

        var rtyp Type
        if origm.hasPtrRecv() {
                rtyp = NewPointer(t)
        } else {
                rtyp = t
        }

        sig.recv = substVar(origSig.recv, rtyp)
        return substFunc(origm, sig)
}

// SetUnderlying sets the underlying type and marks t as complete.
// t must not have type arguments.
func (t *Named) SetUnderlying(underlying Type) {
        assert(t.inst == nil)
        if underlying == nil {
                panic("underlying type must not be nil")
        }
        if asNamed(underlying) != nil {
                panic("underlying type must not be *Named")
        }
        t.resolve().underlying = underlying
        if t.fromRHS == nil {
                t.fromRHS = underlying // for cycle detection
        }
}

// AddMethod adds method m unless it is already in the method list.
// t must not have type arguments.
func (t *Named) AddMethod(m *Func) {
        assert(t.inst == nil)
        t.resolve()
        if i, _ := lookupMethod(t.methods, m.pkg, m.name, false); i < 0 {
                t.methods = append(t.methods, m)
        }
}

// TODO(gri) Investigate if Unalias can be moved to where underlying is set.
func (t *Named) Underlying() Type { return Unalias(t.resolve().underlying) }
func (t *Named) String() string   { return TypeString(t, nil) }

// ----------------------------------------------------------------------------
// Implementation
//
// TODO(rfindley): reorganize the loading and expansion methods under this
// heading.

// under returns the expanded underlying type of n0; possibly by following
// forward chains of named types. If an underlying type is found, resolve
// the chain by setting the underlying type for each defined type in the
// chain before returning it. If no underlying type is found or a cycle
// is detected, the result is Typ[Invalid]. If a cycle is detected and
// n0.check != nil, the cycle is reported.
//
// This is necessary because the underlying type of named may be itself a
// named type that is incomplete:
//
//      type (
//              A B
//              B *C
//              C A
//      )
//
// The type of C is the (named) type of A which is incomplete,
// and which has as its underlying type the named type B.
func (n0 *Named) under() Type {
        u := n0.Underlying()

        // If the underlying type of a defined type is not a defined
        // (incl. instance) type, then that is the desired underlying
        // type.
        var n1 *Named
        switch u1 := u.(type) {
        case nil:
                // After expansion via Underlying(), we should never encounter a nil
                // underlying.
                panic("nil underlying")
        default:
                // common case
                return u
        case *Named:
                // handled below
                n1 = u1
        }

        if n0.check == nil {
                panic("Named.check == nil but type is incomplete")
        }

        // Invariant: after this point n0 as well as any named types in its
        // underlying chain should be set up when this function exits.
        check := n0.check
        n := n0

        seen := make(map[*Named]int) // types that need their underlying type resolved
        var path []Object            // objects encountered, for cycle reporting

loop:
        for {
                seen[n] = len(seen)
                path = append(path, n.obj)
                n = n1
                if i, ok := seen[n]; ok {
                        // cycle
                        check.cycleError(path[i:])
                        u = Typ[Invalid]
                        break
                }
                u = n.Underlying()
                switch u1 := u.(type) {
                case nil:
                        u = Typ[Invalid]
                        break loop
                default:
                        break loop
                case *Named:
                        // Continue collecting *Named types in the chain.
                        n1 = u1
                }
        }

        for n := range seen {
                // We should never have to update the underlying type of an imported type;
                // those underlying types should have been resolved during the import.
                // Also, doing so would lead to a race condition (was go.dev/issue/31749).
                // Do this check always, not just in debug mode (it's cheap).
                if n.obj.pkg != check.pkg {
                        panic("imported type with unresolved underlying type")
                }
                n.underlying = u
        }

        return u
}

func (n *Named) lookupMethod(pkg *Package, name string, foldCase bool) (int, *Func) {
        n.resolve()
        // If n is an instance, we may not have yet instantiated all of its methods.
        // Look up the method index in orig, and only instantiate method at the
        // matching index (if any).
        i, _ := lookupMethod(n.Origin().methods, pkg, name, foldCase)
        if i < 0 {
                return -1, nil
        }
        // For instances, m.Method(i) will be different from the orig method.
        return i, n.Method(i)
}

// context returns the type-checker context.
func (check *Checker) context() *Context {
        if check.ctxt == nil {
                check.ctxt = NewContext()
        }
        return check.ctxt
}

// expandUnderlying substitutes type arguments in the underlying type n.orig,
// returning the result. Returns Typ[Invalid] if there was an error.
func (n *Named) expandUnderlying() Type {
        check := n.check
        if check != nil && check.conf.Trace {
                check.trace(n.obj.pos, "-- Named.expandUnderlying %s", n)
                check.indent++
                defer func() {
                        check.indent--
                        check.trace(n.obj.pos, "=> %s (tparams = %s, under = %s)", n, n.tparams.list(), n.underlying)
                }()
        }

        assert(n.inst.orig.underlying != nil)
        if n.inst.ctxt == nil {
                n.inst.ctxt = NewContext()
        }

        orig := n.inst.orig
        targs := n.inst.targs

        if asNamed(orig.underlying) != nil {
                // We should only get a Named underlying type here during type checking
                // (for example, in recursive type declarations).
                assert(check != nil)
        }

        if orig.tparams.Len() != targs.Len() {
                // Mismatching arg and tparam length may be checked elsewhere.
                return Typ[Invalid]
        }

        // Ensure that an instance is recorded before substituting, so that we
        // resolve n for any recursive references.
        h := n.inst.ctxt.instanceHash(orig, targs.list())
        n2 := n.inst.ctxt.update(h, orig, n.TypeArgs().list(), n)
        assert(n == n2)

        smap := makeSubstMap(orig.tparams.list(), targs.list())
        var ctxt *Context
        if check != nil {
                ctxt = check.context()
        }
        underlying := n.check.subst(n.obj.pos, orig.underlying, smap, n, ctxt)
        // If the underlying type of n is an interface, we need to set the receiver of
        // its methods accurately -- we set the receiver of interface methods on
        // the RHS of a type declaration to the defined type.
        if iface, _ := underlying.(*Interface); iface != nil {
                if methods, copied := replaceRecvType(iface.methods, orig, n); copied {
                        // If the underlying type doesn't actually use type parameters, it's
                        // possible that it wasn't substituted. In this case we need to create
                        // a new *Interface before modifying receivers.
                        if iface == orig.underlying {
                                old := iface
                                iface = check.newInterface()
                                iface.embeddeds = old.embeddeds
                                assert(old.complete) // otherwise we are copying incomplete data
                                iface.complete = old.complete
                                iface.implicit = old.implicit // should be false but be conservative
                                underlying = iface
                        }
                        iface.methods = methods
                        iface.tset = nil // recompute type set with new methods

                        // If check != nil, check.newInterface will have saved the interface for later completion.
                        if check == nil { // golang/go#61561: all newly created interfaces must be fully evaluated
                                iface.typeSet()
                        }
                }
        }

        return underlying
}

// safeUnderlying returns the underlying type of typ without expanding
// instances, to avoid infinite recursion.
//
// TODO(rfindley): eliminate this function or give it a better name.
func safeUnderlying(typ Type) Type {
        if t := asNamed(typ); t != nil {
                return t.underlying
        }
        return typ.Underlying()
}