go/types, types2: add cause parameter to missingMethod, (new)assertableTo

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

// Copyright 2013 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.

// This file implements various field and method lookup functions.

package types2

import (
        "bytes"
        "strings"
)

// Internal use of LookupFieldOrMethod: If the obj result is a method
// associated with a concrete (non-interface) type, the method's signature
// may not be fully set up. Call Checker.objDecl(obj, nil) before accessing
// the method's type.

// LookupFieldOrMethod looks up a field or method with given package and name
// in T and returns the corresponding *Var or *Func, an index sequence, and a
// bool indicating if there were any pointer indirections on the path to the
// field or method. If addressable is set, T is the type of an addressable
// variable (only matters for method lookups). T must not be nil.
//
// The last index entry is the field or method index in the (possibly embedded)
// type where the entry was found, either:
//
//  1. the list of declared methods of a named type; or
//  2. the list of all methods (method set) of an interface type; or
//  3. the list of fields of a struct type.
//
// The earlier index entries are the indices of the embedded struct fields
// traversed to get to the found entry, starting at depth 0.
//
// If no entry is found, a nil object is returned. In this case, the returned
// index and indirect values have the following meaning:
//
//   - If index != nil, the index sequence points to an ambiguous entry
//     (the same name appeared more than once at the same embedding level).
//
//   - If indirect is set, a method with a pointer receiver type was found
//     but there was no pointer on the path from the actual receiver type to
//     the method's formal receiver base type, nor was the receiver addressable.
func LookupFieldOrMethod(T Type, addressable bool, pkg *Package, name string) (obj Object, index []int, indirect bool) {
        if T == nil {
                panic("LookupFieldOrMethod on nil type")
        }

        // Methods cannot be associated to a named pointer type.
        // (spec: "The type denoted by T is called the receiver base type;
        // it must not be a pointer or interface type and it must be declared
        // in the same package as the method.").
        // Thus, if we have a named pointer type, proceed with the underlying
        // pointer type but discard the result if it is a method since we would
        // not have found it for T (see also go.dev/issue/8590).
        if t, _ := T.(*Named); t != nil {
                if p, _ := t.Underlying().(*Pointer); p != nil {
                        obj, index, indirect = lookupFieldOrMethod(p, false, pkg, name, false)
                        if _, ok := obj.(*Func); ok {
                                return nil, nil, false
                        }
                        return
                }
        }

        obj, index, indirect = lookupFieldOrMethod(T, addressable, pkg, name, false)

        // If we didn't find anything and if we have a type parameter with a core type,
        // see if there is a matching field (but not a method, those need to be declared
        // explicitly in the constraint). If the constraint is a named pointer type (see
        // above), we are ok here because only fields are accepted as results.
        const enableTParamFieldLookup = false // see go.dev/issue/51576
        if enableTParamFieldLookup && obj == nil && isTypeParam(T) {
                if t := coreType(T); t != nil {
                        obj, index, indirect = lookupFieldOrMethod(t, addressable, pkg, name, false)
                        if _, ok := obj.(*Var); !ok {
                                obj, index, indirect = nil, nil, false // accept fields (variables) only
                        }
                }
        }
        return
}

// lookupFieldOrMethod should only be called by LookupFieldOrMethod and missingMethod.
// If foldCase is true, the lookup for methods will include looking for any method
// which case-folds to the same as 'name' (used for giving helpful error messages).
//
// The resulting object may not be fully type-checked.
func lookupFieldOrMethod(T Type, addressable bool, pkg *Package, name string, foldCase bool) (obj Object, index []int, indirect bool) {
        // WARNING: The code in this function is extremely subtle - do not modify casually!

        if name == "_" {
                return // blank fields/methods are never found
        }

        typ, isPtr := deref(T)

        // *typ where typ is an interface (incl. a type parameter) has no methods.
        if isPtr {
                if _, ok := under(typ).(*Interface); ok {
                        return
                }
        }

        // Start with typ as single entry at shallowest depth.
        current := []embeddedType{{typ, nil, isPtr, false}}

        // seen tracks named types that we have seen already, allocated lazily.
        // Used to avoid endless searches in case of recursive types.
        //
        // We must use a lookup on identity rather than a simple map[*Named]bool as
        // instantiated types may be identical but not equal.
        var seen instanceLookup

        // search current depth
        for len(current) > 0 {
                var next []embeddedType // embedded types found at current depth

                // look for (pkg, name) in all types at current depth
                for _, e := range current {
                        typ := e.typ

                        // If we have a named type, we may have associated methods.
                        // Look for those first.
                        if named, _ := typ.(*Named); named != nil {
                                if alt := seen.lookup(named); alt != nil {
                                        // We have seen this type before, at a more shallow depth
                                        // (note that multiples of this type at the current depth
                                        // were consolidated before). The type at that depth shadows
                                        // this same type at the current depth, so we can ignore
                                        // this one.
                                        continue
                                }
                                seen.add(named)

                                // look for a matching attached method
                                if i, m := named.lookupMethod(pkg, name, foldCase); m != nil {
                                        // potential match
                                        // caution: method may not have a proper signature yet
                                        index = concat(e.index, i)
                                        if obj != nil || e.multiples {
                                                return nil, index, false // collision
                                        }
                                        obj = m
                                        indirect = e.indirect
                                        continue // we can't have a matching field or interface method
                                }
                        }

                        switch t := under(typ).(type) {
                        case *Struct:
                                // look for a matching field and collect embedded types
                                for i, f := range t.fields {
                                        if f.sameId(pkg, name) {
                                                assert(f.typ != nil)
                                                index = concat(e.index, i)
                                                if obj != nil || e.multiples {
                                                        return nil, index, false // collision
                                                }
                                                obj = f
                                                indirect = e.indirect
                                                continue // we can't have a matching interface method
                                        }
                                        // Collect embedded struct fields for searching the next
                                        // lower depth, but only if we have not seen a match yet
                                        // (if we have a match it is either the desired field or
                                        // we have a name collision on the same depth; in either
                                        // case we don't need to look further).
                                        // Embedded fields are always of the form T or *T where
                                        // T is a type name. If e.typ appeared multiple times at
                                        // this depth, f.typ appears multiple times at the next
                                        // depth.
                                        if obj == nil && f.embedded {
                                                typ, isPtr := deref(f.typ)
                                                // TODO(gri) optimization: ignore types that can't
                                                // have fields or methods (only Named, Struct, and
                                                // Interface types need to be considered).
                                                next = append(next, embeddedType{typ, concat(e.index, i), e.indirect || isPtr, e.multiples})
                                        }
                                }

                        case *Interface:
                                // look for a matching method (interface may be a type parameter)
                                if i, m := t.typeSet().LookupMethod(pkg, name, foldCase); m != nil {
                                        assert(m.typ != nil)
                                        index = concat(e.index, i)
                                        if obj != nil || e.multiples {
                                                return nil, index, false // collision
                                        }
                                        obj = m
                                        indirect = e.indirect
                                }
                        }
                }

                if obj != nil {
                        // found a potential match
                        // spec: "A method call x.m() is valid if the method set of (the type of) x
                        //        contains m and the argument list can be assigned to the parameter
                        //        list of m. If x is addressable and &x's method set contains m, x.m()
                        //        is shorthand for (&x).m()".
                        if f, _ := obj.(*Func); f != nil {
                                // determine if method has a pointer receiver
                                if f.hasPtrRecv() && !indirect && !addressable {
                                        return nil, nil, true // pointer/addressable receiver required
                                }
                        }
                        return
                }

                current = consolidateMultiples(next)
        }

        return nil, nil, false // not found
}

// embeddedType represents an embedded type
type embeddedType struct {
        typ       Type
        index     []int // embedded field indices, starting with index at depth 0
        indirect  bool  // if set, there was a pointer indirection on the path to this field
        multiples bool  // if set, typ appears multiple times at this depth
}

// consolidateMultiples collects multiple list entries with the same type
// into a single entry marked as containing multiples. The result is the
// consolidated list.
func consolidateMultiples(list []embeddedType) []embeddedType {
        if len(list) <= 1 {
                return list // at most one entry - nothing to do
        }

        n := 0                     // number of entries w/ unique type
        prev := make(map[Type]int) // index at which type was previously seen
        for _, e := range list {
                if i, found := lookupType(prev, e.typ); found {
                        list[i].multiples = true
                        // ignore this entry
                } else {
                        prev[e.typ] = n
                        list[n] = e
                        n++
                }
        }
        return list[:n]
}

func lookupType(m map[Type]int, typ Type) (int, bool) {
        // fast path: maybe the types are equal
        if i, found := m[typ]; found {
                return i, true
        }

        for t, i := range m {
                if Identical(t, typ) {
                        return i, true
                }
        }

        return 0, false
}

type instanceLookup struct {
        // buf is used to avoid allocating the map m in the common case of a small
        // number of instances.
        buf [3]*Named
        m   map[*Named][]*Named
}

func (l *instanceLookup) lookup(inst *Named) *Named {
        for _, t := range l.buf {
                if t != nil && Identical(inst, t) {
                        return t
                }
        }
        for _, t := range l.m[inst.Origin()] {
                if Identical(inst, t) {
                        return t
                }
        }
        return nil
}

func (l *instanceLookup) add(inst *Named) {
        for i, t := range l.buf {
                if t == nil {
                        l.buf[i] = inst
                        return
                }
        }
        if l.m == nil {
                l.m = make(map[*Named][]*Named)
        }
        insts := l.m[inst.Origin()]
        l.m[inst.Origin()] = append(insts, inst)
}

// MissingMethod returns (nil, false) if V implements T, otherwise it
// returns a missing method required by T and whether it is missing or
// just has the wrong type.
//
// For non-interface types V, or if static is set, V implements T if all
// methods of T are present in V. Otherwise (V is an interface and static
// is not set), MissingMethod only checks that methods of T which are also
// present in V have matching types (e.g., for a type assertion x.(T) where
// x is of interface type V).
func MissingMethod(V Type, T *Interface, static bool) (method *Func, wrongType bool) {
        m, alt := (*Checker)(nil).missingMethod(V, T, static, Identical, nil)
        // Only report a wrong type if the alternative method has the same name as m.
        return m, alt != nil && alt.name == m.name // alt != nil implies m != nil
}

// missingMethod is like MissingMethod but accepts a *Checker as receiver,
// a comparator equivalent for type comparison, and a *string for error causes.
// The receiver may be nil if missingMethod is invoked through an exported
// API call (such as MissingMethod), i.e., when all methods have been type-
// checked.
// The underlying type of T must be an interface; T (rather than its under-
// lying type) is used for better error messages (reported through *cause).
// The comparator is used to compare signatures.
// If a method is missing and cause is not nil, *cause is set to the error cause.
//
// If a method is missing on T but is found on *T, or if a method is found
// on T when looked up with case-folding, this alternative method is returned
// as the second result.
func (check *Checker) missingMethod(V, T Type, static bool, equivalent func(x, y Type) bool, cause *string) (method, alt *Func) {
        methods := under(T).(*Interface).typeSet().methods // T must be an interface
        if len(methods) == 0 {
                return
        }

        if cause != nil {
                defer func() {
                        if method != nil {
                                *cause = check.missingMethodCause(V, T, method, alt)
                        }
                }()
        }

        // V is an interface
        if u, _ := under(V).(*Interface); u != nil {
                tset := u.typeSet()
                for _, m := range methods {
                        _, f := tset.LookupMethod(m.pkg, m.name, false)

                        if f == nil {
                                if !static {
                                        continue
                                }
                                return m, nil
                        }

                        if !equivalent(f.typ, m.typ) {
                                return m, f
                        }
                }

                return
        }

        // V is not an interface
        for _, m := range methods {
                // TODO(gri) should this be calling LookupFieldOrMethod instead (and why not)?
                obj, _, _ := lookupFieldOrMethod(V, false, m.pkg, m.name, false)

                // check if m is on *V, or on V with case-folding
                found := obj != nil
                if !found {
                        // TODO(gri) Instead of NewPointer(V) below, can we just set the "addressable" argument?
                        obj, _, _ = lookupFieldOrMethod(NewPointer(V), false, m.pkg, m.name, false)
                        if obj == nil {
                                obj, _, _ = lookupFieldOrMethod(V, false, m.pkg, m.name, true /* fold case */)
                        }
                }

                // we must have a method (not a struct field)
                f, _ := obj.(*Func)
                if f == nil {
                        return m, nil
                }

                // methods may not have a fully set up signature yet
                if check != nil {
                        check.objDecl(f, nil)
                }

                if !found || !equivalent(f.typ, m.typ) {
                        return m, f
                }
        }

        return
}

// missingMethodCause returns a string giving the detailed cause for a missing method m,
// where m is missing from V, but required by T. It puts the cause in parentheses,
// and may include more have/want info after that. If non-nil, alt is a relevant
// method that matches in some way. It may have the correct name, but wrong type, or
// it may have a pointer receiver, or it may have the correct name except wrong case.
// check may be nil.
// missingMethodCause should only be called by missingMethod.
// TODO(gri) integrate this logic into missingMethod and get rid of this function.
func (check *Checker) missingMethodCause(V, T Type, m, alt *Func) string {
        mname := "method " + m.Name()

        if alt != nil {
                if m.Name() != alt.Name() {
                        return check.sprintf("(missing %s)\n\t\thave %s\n\t\twant %s",
                                mname, check.funcString(alt, false), check.funcString(m, false))
                }

                if Identical(m.typ, alt.typ) {
                        return check.sprintf("(%s has pointer receiver)", mname)
                }

                altS, mS := check.funcString(alt, false), check.funcString(m, false)
                if altS == mS {
                        // Would tell the user that Foo isn't a Foo, add package information to disambiguate.
                        // See go.dev/issue/54258.
                        altS, mS = check.funcString(alt, true), check.funcString(m, true)
                }

                return check.sprintf("(wrong type for %s)\n\t\thave %s\n\t\twant %s",
                        mname, altS, mS)
        }

        if isInterfacePtr(V) {
                return "(" + check.interfacePtrError(V) + ")"
        }

        if isInterfacePtr(T) {
                return "(" + check.interfacePtrError(T) + ")"
        }

        obj, _, _ := lookupFieldOrMethod(V, true /* auto-deref */, m.pkg, m.name, false)
        if fld, _ := obj.(*Var); fld != nil {
                return check.sprintf("(%s.%s is a field, not a method)", V, fld.Name())
        }

        return check.sprintf("(missing %s)", mname)
}

func isInterfacePtr(T Type) bool {
        p, _ := under(T).(*Pointer)
        return p != nil && IsInterface(p.base)
}

// check may be nil.
func (check *Checker) interfacePtrError(T Type) string {
        assert(isInterfacePtr(T))
        if p, _ := under(T).(*Pointer); isTypeParam(p.base) {
                return check.sprintf("type %s is pointer to type parameter, not type parameter", T)
        }
        return check.sprintf("type %s is pointer to interface, not interface", T)
}

// funcString returns a string of the form name + signature for f.
// check may be nil.
func (check *Checker) funcString(f *Func, pkgInfo bool) string {
        buf := bytes.NewBufferString(f.name)
        var qf Qualifier
        if check != nil && !pkgInfo {
                qf = check.qualifier
        }
        w := newTypeWriter(buf, qf)
        w.pkgInfo = pkgInfo
        w.paramNames = false
        w.signature(f.typ.(*Signature))
        return buf.String()
}

// assertableTo reports whether a value of type V can be asserted to have type T.
// It returns (nil, false) as affirmative answer. Otherwise it returns a missing
// method required by V and whether it is missing or just has the wrong type.
// The receiver may be nil if assertableTo is invoked through an exported API call
// (such as AssertableTo), i.e., when all methods have been type-checked.
// The underlying type of V must be an interface.
// If the result is negative and cause is not nil, *cause is set to the error cause.
// TODO(gri) replace calls to this function with calls to newAssertableTo.
func (check *Checker) assertableTo(V, T Type, cause *string) (method, wrongType *Func) {
        // no static check is required if T is an interface
        // spec: "If T is an interface type, x.(T) asserts that the
        //        dynamic type of x implements the interface T."
        if IsInterface(T) {
                return
        }
        // TODO(gri) fix this for generalized interfaces
        return check.missingMethod(T, V, false, Identical, cause)
}

// newAssertableTo reports whether a value of type V can be asserted to have type T.
// It also implements behavior for interfaces that currently are only permitted
// in constraint position (we have not yet defined that behavior in the spec).
// The underlying type of V must be an interface.
// If the result is false and cause is not nil, *cause is set to the error cause.
func (check *Checker) newAssertableTo(V, T Type, cause *string) bool {
        // no static check is required if T is an interface
        // spec: "If T is an interface type, x.(T) asserts that the
        //        dynamic type of x implements the interface T."
        if IsInterface(T) {
                return true
        }
        return check.implements(T, V, false, cause)
}

// deref dereferences typ if it is a *Pointer and returns its base and true.
// Otherwise it returns (typ, false).
func deref(typ Type) (Type, bool) {
        if p, _ := typ.(*Pointer); p != nil {
                // p.base should never be nil, but be conservative
                if p.base == nil {
                        if debug {
                                panic("pointer with nil base type (possibly due to an invalid cyclic declaration)")
                        }
                        return Typ[Invalid], true
                }
                return p.base, true
        }
        return typ, false
}

// derefStructPtr dereferences typ if it is a (named or unnamed) pointer to a
// (named or unnamed) struct and returns its base. Otherwise it returns typ.
func derefStructPtr(typ Type) Type {
        if p, _ := under(typ).(*Pointer); p != nil {
                if _, ok := under(p.base).(*Struct); ok {
                        return p.base
                }
        }
        return typ
}

// concat returns the result of concatenating list and i.
// The result does not share its underlying array with list.
func concat(list []int, i int) []int {
        var t []int
        t = append(t, list...)
        return append(t, i)
}

// fieldIndex returns the index for the field with matching package and name, or a value < 0.
func fieldIndex(fields []*Var, pkg *Package, name string) int {
        if name != "_" {
                for i, f := range fields {
                        if f.sameId(pkg, name) {
                                return i
                        }
                }
        }
        return -1
}

// lookupMethod returns the index of and method with matching package and name, or (-1, nil).
// If foldCase is true, method names are considered equal if they are equal with case folding.
func lookupMethod(methods []*Func, pkg *Package, name string, foldCase bool) (int, *Func) {
        if name != "_" {
                for i, m := range methods {
                        if (m.name == name || foldCase && strings.EqualFold(m.name, name)) && m.sameId(pkg, m.name) {
                                return i, m
                        }
                }
        }
        return -1, nil
}