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// 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 method sets.
package types
import (
// A MethodSet is an ordered set of concrete or abstract (interface) methods;
// a method is a MethodVal selection, and they are ordered by ascending m.Obj().Id().
// The zero value for a MethodSet is a ready-to-use empty method set.
type MethodSet struct {
list []*Selection
func (s *MethodSet) String() string {
if s.Len() == 0 {
return "MethodSet {}"
var buf strings.Builder
fmt.Fprintln(&buf, "MethodSet {")
for _, f := range s.list {
fmt.Fprintf(&buf, "\t%s\n", f)
fmt.Fprintln(&buf, "}")
return buf.String()
// Len returns the number of methods in s.
func (s *MethodSet) Len() int { return len(s.list) }
// At returns the i'th method in s for 0 <= i < s.Len().
func (s *MethodSet) At(i int) *Selection { return s.list[i] }
// Lookup returns the method with matching package and name, or nil if not found.
func (s *MethodSet) Lookup(pkg *Package, name string) *Selection {
if s.Len() == 0 {
return nil
key := Id(pkg, name)
i := sort.Search(len(s.list), func(i int) bool {
m := s.list[i]
return m.obj.Id() >= key
if i < len(s.list) {
m := s.list[i]
if m.obj.Id() == key {
return m
return nil
// Shared empty method set.
var emptyMethodSet MethodSet
// Note: NewMethodSet is intended for external use only as it
// requires interfaces to be complete. If may be used
// internally if LookupFieldOrMethod completed the same
// interfaces beforehand.
// NewMethodSet returns the method set for the given type T.
// It always returns a non-nil method set, even if it is empty.
func NewMethodSet(T Type) *MethodSet {
// WARNING: The code in this function is extremely subtle - do not modify casually!
// This function and lookupFieldOrMethod should be kept in sync.
// method set up to the current depth, allocated lazily
var base methodSet
typ, isPtr := deref(T)
// *typ where typ is an interface has no methods.
if isPtr && IsInterface(typ) {
return &emptyMethodSet
// 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
// collect methods at current depth
for len(current) > 0 {
var next []embeddedType // embedded types found at current depth
// field and method sets at current depth, indexed by names (Id's), and allocated lazily
var fset map[string]bool // we only care about the field names
var mset methodSet
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
mset = mset.add(named.methods, e.index, e.indirect, e.multiples)
// continue with underlying type
typ = named.underlying
switch t := typ.(type) {
case *Struct:
for i, f := range t.fields {
if fset == nil {
fset = make(map[string]bool)
fset[f.Id()] = true
// Embedded fields are always of the form T or *T where
// T is a type name. If typ appeared multiple times at
// this depth, f.Type appears multiple times at the next
// depth.
if 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:
mset = mset.add(t.allMethods, e.index, true, e.multiples)
// Add methods and collisions at this depth to base if no entries with matching
// names exist already.
for k, m := range mset {
if _, found := base[k]; !found {
// Fields collide with methods of the same name at this depth.
if fset[k] {
m = nil // collision
if base == nil {
base = make(methodSet)
base[k] = m
// Add all (remaining) fields at this depth as collisions (since they will
// hide any method further down) if no entries with matching names exist already.
for k := range fset {
if _, found := base[k]; !found {
if base == nil {
base = make(methodSet)
base[k] = nil // collision
// It's ok to call consolidateMultiples with a nil *Checker because
// MethodSets are not used internally (outside debug mode). When used
// externally, interfaces are expected to be completed and then we do
// not need a *Checker to complete them when (indirectly) calling
// Checker.identical via consolidateMultiples.
current = (*Checker)(nil).consolidateMultiples(next)
if len(base) == 0 {
return &emptyMethodSet
// collect methods
var list []*Selection
for _, m := range base {
if m != nil {
m.recv = T
list = append(list, m)
// sort by unique name
sort.Slice(list, func(i, j int) bool {
return list[i].obj.Id() < list[j].obj.Id()
return &MethodSet{list}
// A methodSet is a set of methods and name collisions.
// A collision indicates that multiple methods with the
// same unique id, or a field with that id appeared.
type methodSet map[string]*Selection // a nil entry indicates a name collision
// Add adds all functions in list to the method set s.
// If multiples is set, every function in list appears multiple times
// and is treated as a collision.
func (s methodSet) add(list []*Func, index []int, indirect bool, multiples bool) methodSet {
if len(list) == 0 {
return s
if s == nil {
s = make(methodSet)
for i, f := range list {
key := f.Id()
// if f is not in the set, add it
if !multiples {
// TODO(gri) A found method may not be added because it's not in the method set
// (!indirect && ptrRecv(f)). A 2nd method on the same level may be in the method
// set and may not collide with the first one, thus leading to a false positive.
// Is that possible? Investigate.
if _, found := s[key]; !found && (indirect || !ptrRecv(f)) {
s[key] = &Selection{MethodVal, nil, f, concat(index, i), indirect}
s[key] = nil // collision
return s
// ptrRecv reports whether the receiver is of the form *T.
func ptrRecv(f *Func) bool {
// If a method's receiver type is set, use that as the source of truth for the receiver.
// Caution: Checker.funcDecl (decl.go) marks a function by setting its type to an empty
// signature. We may reach here before the signature is fully set up: we must explicitly
// check if the receiver is set (we cannot just look for non-nil f.typ).
if sig, _ := f.typ.(*Signature); sig != nil && sig.recv != nil {
_, isPtr := deref(sig.recv.typ)
return isPtr
// If a method's type is not set it may be a method/function that is:
// 1) client-supplied (via NewFunc with no signature), or
// 2) internally created but not yet type-checked.
// For case 1) we can't do anything; the client must know what they are doing.
// For case 2) we can use the information gathered by the resolver.
return f.hasPtrRecv