blob: 3c9ff182ec7c2446628ad857fa9264dbe2126115 [file] [log] [blame]
// 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
// 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) {
return (*Checker)(nil).lookupFieldOrMethod(T, addressable, pkg, name)
// Internal use of Checker.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.
// TODO(gri) Now that we provide the *Checker, we can probably remove this
// caveat by calling Checker.objDecl from lookupFieldOrMethod. Investigate.
// lookupFieldOrMethod is like the external version but completes interfaces
// as necessary.
func (check *Checker) 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, _ := T.(*Named); t != nil {
if p, _ := t.underlying.(*Pointer); p != nil {
obj, index, indirect = check.rawLookupFieldOrMethod(p, false, pkg, name)
if _, ok := obj.(*Func); ok {
return nil, nil, false
return check.rawLookupFieldOrMethod(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.)
// rawLookupFieldOrMethod should only be called by lookupFieldOrMethod and missingMethod.
func (check *Checker) rawLookupFieldOrMethod(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!
// This function and NewMethodSet should be kept in sync.
if name == "_" {
return // blank fields/methods are never found
typ, isPtr := deref(T)
// *typ where typ is an interface has no methods.
if isPtr && IsInterface(typ) {
// 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
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 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.
if seen == nil {
seen = make(map[*Named]bool)
seen[named] = true
// look for a matching attached method
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
typ = named.underlying
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
// TODO(gri) t.allMethods is sorted - use binary search
if i, m := lookupMethod(t.allMethods, 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
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 && ptrRecv(f) && !indirect && !addressable {
return nil, nil, true // pointer/addressable receiver required
current = check.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 (check *Checker) 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 := check.lookupType(prev, e.typ); found {
list[i].multiples = true
// ignore this entry
} else {
prev[e.typ] = n
list[n] = e
return list[:n]
func (check *Checker) 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 check.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 receiver.
// 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() {
if ityp, _ := V.Underlying().(*Interface); ityp != nil {
// TODO(gri) allMethods is sorted - can do this more efficiently
for _, m := range T.allMethods {
_, obj := lookupMethod(ityp.allMethods, m.pkg,
switch {
case obj == nil:
if static {
return m, nil
case !check.identical(obj.Type(), m.typ):
return m, obj
// A concrete type implements T if it implements all methods of T.
for _, m := range T.allMethods {
obj, _, _ := check.rawLookupFieldOrMethod(V, false, m.pkg,
// Check if *V implements this method of T.
if obj == nil {
ptr := NewPointer(V)
obj, _, _ = check.rawLookupFieldOrMethod(ptr, false, m.pkg,
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 {
return m, nil
// methods may not have a fully set up signature yet
if check != nil {
check.objDecl(f, nil)
if !check.identical(f.typ, m.typ) {
return m, f
// 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.
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 _, ok := T.Underlying().(*Interface); ok && !strict {
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, _ := typ.Underlying().(*Pointer); p != nil {
if _, ok := p.base.Underlying().(*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).
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