[dev.typeparams] go/types: don't permit method calls on ptr to type parameter receivers

[gostls13.git] / src / go / types / 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 types

import "go/token"

// 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).
//
// 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) {
        // 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 issue 8590).
        if t := asNamed(T); t != nil {
                if p, _ := t.Underlying().(*Pointer); p != nil {
                        obj, index, indirect = lookupFieldOrMethod(p, false, pkg, name)
                        if _, ok := obj.(*Func); ok {
                                return nil, nil, false
                        }
                        return
                }
        }

        return lookupFieldOrMethod(T, addressable, pkg, name)
}

// TODO(gri) The named type consolidation and seen maps below must be
//           indexed by unique keys for a given type. Verify that named
//           types always have only one representation (even when imported
//           indirectly via different packages.)

// lookupFieldOrMethod should only be called by LookupFieldOrMethod and missingMethod.
func lookupFieldOrMethod(T Type, addressable bool, pkg *Package, name string) (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 or type parameter has no methods.
        switch under(typ).(type) {
        case *Interface, *TypeParam:
                if isPtr {
                        return
                }
        }

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

        // Named types that we have seen already, allocated lazily.
        // Used to avoid endless searches in case of recursive types.
        // Since only Named types can be used for recursive types, we
        // only need to track those.
        // (If we ever allow type aliases to construct recursive types,
        // we must use type identity rather than pointer equality for
        // the map key comparison, as we do in consolidateMultiples.)
        var seen map[*Named]bool

        // 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
                var tpar *TypeParam // set if obj receiver is a type parameter
                for _, e := range current {
                        typ := e.typ

                        // If we have a named type, we may have associated methods.
                        // Look for those first.
                        if named := asNamed(typ); named != nil {
                                if seen[named] {
                                        // 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
                                }
                                if seen == nil {
                                        seen = make(map[*Named]bool)
                                }
                                seen[named] = true

                                // look for a matching attached method
                                named.expand()
                                if i, m := lookupMethod(named.methods, pkg, name); 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
                                }

                                // continue with underlying type, but only if it's not a type parameter
                                // TODO(gri) is this what we want to do for type parameters? (spec question)
                                // TODO(#45639) the error message produced as a result of skipping an
                                //              underlying type parameter should be improved.
                                typ = named.under()
                                if asTypeParam(typ) != nil {
                                        continue
                                }
                        }

                        tpar = nil
                        switch t := 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
                                if i, m := t.typeSet().LookupMethod(pkg, name); 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
                                }

                        case *TypeParam:
                                if i, m := t.Bound().typeSet().LookupMethod(pkg, name); m != nil {
                                        assert(m.typ != nil)
                                        index = concat(e.index, i)
                                        if obj != nil || e.multiples {
                                                return nil, index, false // collision
                                        }
                                        tpar = t
                                        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
                                hasPtrRecv := tpar == nil && ptrRecv(f)
                                if 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
}

// 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, typ := (*Checker)(nil).missingMethod(V, T, static)
        return m, typ != nil
}

// missingMethod is like MissingMethod but accepts a *Checker as
// receiver and an addressable flag.
// 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.
// If the type has the correctly named method, but with the wrong
// signature, the existing method is returned as well.
// To improve error messages, also report the wrong signature
// when the method exists on *V instead of V.
func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method, wrongType *Func) {
        // fast path for common case
        if T.Empty() {
                return
        }

        if ityp := asInterface(V); ityp != nil {
                // TODO(gri) the methods are sorted - could do this more efficiently
                for _, m := range T.typeSet().methods {
                        _, f := ityp.typeSet().LookupMethod(m.pkg, m.name)

                        if f == nil {
                                // if m is the magic method == we're ok (interfaces are comparable)
                                if m.name == "==" || !static {
                                        continue
                                }
                                return m, f
                        }

                        // both methods must have the same number of type parameters
                        ftyp := f.typ.(*Signature)
                        mtyp := m.typ.(*Signature)
                        if len(ftyp.tparams) != len(mtyp.tparams) {
                                return m, f
                        }
                        if len(ftyp.tparams) > 0 {
                                panic("internal error: method with type parameters")
                        }

                        // If the methods have type parameters we don't care whether they
                        // are the same or not, as long as they match up. Use unification
                        // to see if they can be made to match.
                        // TODO(gri) is this always correct? what about type bounds?
                        // (Alternative is to rename/subst type parameters and compare.)
                        u := newUnifier(true)
                        u.x.init(ftyp.tparams)
                        if !u.unify(ftyp, mtyp) {
                                return m, f
                        }
                }

                return
        }

        // A concrete type implements T if it implements all methods of T.
        Vd, _ := deref(V)
        Vn := asNamed(Vd)
        for _, m := range T.typeSet().methods {
                // TODO(gri) should this be calling lookupFieldOrMethod instead (and why not)?
                obj, _, _ := lookupFieldOrMethod(V, false, m.pkg, m.name)

                // Check if *V implements this method of T.
                if obj == nil {
                        ptr := NewPointer(V)
                        obj, _, _ = lookupFieldOrMethod(ptr, false, m.pkg, m.name)
                        if obj != nil {
                                return m, obj.(*Func)
                        }
                }

                // we must have a method (not a field of matching function type)
                f, _ := obj.(*Func)
                if f == nil {
                        // if m is the magic method == and V is comparable, we're ok
                        if m.name == "==" && Comparable(V) {
                                continue
                        }
                        return m, nil
                }

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

                // both methods must have the same number of type parameters
                ftyp := f.typ.(*Signature)
                mtyp := m.typ.(*Signature)
                if len(ftyp.tparams) != len(mtyp.tparams) {
                        return m, f
                }
                if len(ftyp.tparams) > 0 {
                        panic("internal error: method with type parameters")
                }

                // If V is a (instantiated) generic type, its methods are still
                // parameterized using the original (declaration) receiver type
                // parameters (subst simply copies the existing method list, it
                // does not instantiate the methods).
                // In order to compare the signatures, substitute the receiver
                // type parameters of ftyp with V's instantiation type arguments.
                // This lazily instantiates the signature of method f.
                if Vn != nil && len(Vn.TParams()) > 0 {
                        // Be careful: The number of type arguments may not match
                        // the number of receiver parameters. If so, an error was
                        // reported earlier but the length discrepancy is still
                        // here. Exit early in this case to prevent an assertion
                        // failure in makeSubstMap.
                        // TODO(gri) Can we avoid this check by fixing the lengths?
                        if len(ftyp.rparams) != len(Vn.targs) {
                                return
                        }
                        ftyp = check.subst(token.NoPos, ftyp, makeSubstMap(ftyp.rparams, Vn.targs)).(*Signature)
                }

                // If the methods have type parameters we don't care whether they
                // are the same or not, as long as they match up. Use unification
                // to see if they can be made to match.
                // TODO(gri) is this always correct? what about type bounds?
                // (Alternative is to rename/subst type parameters and compare.)
                u := newUnifier(true)
                u.x.init(ftyp.rparams)
                if !u.unify(ftyp, mtyp) {
                        return m, f
                }
        }

        return
}

// 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.
// If the global constant forceStrict is set, assertions that are known to fail
// are not permitted.
func (check *Checker) assertableTo(V *Interface, T Type) (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 asInterface(T) != nil && !forceStrict {
                return
        }
        return check.missingMethod(T, V, false)
}

// 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 {
                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 := asPointer(typ); p != nil {
                if asStruct(p.base) != nil {
                        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).
func lookupMethod(methods []*Func, pkg *Package, name string) (int, *Func) {
        if name != "_" {
                for i, m := range methods {
                        if m.sameId(pkg, name) {
                                return i, m
                        }
                }
        }
        return -1, nil
}