1 // Copyright 2013 The Go Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style
3 // license that can be found in the LICENSE file.
5 // This file implements various field and method lookup functions.
14 // Internal use of LookupFieldOrMethod: If the obj result is a method
15 // associated with a concrete (non-interface) type, the method's signature
16 // may not be fully set up. Call Checker.objDecl(obj, nil) before accessing
19 // LookupFieldOrMethod looks up a field or method with given package and name
20 // in T and returns the corresponding *Var or *Func, an index sequence, and a
21 // bool indicating if there were any pointer indirections on the path to the
22 // field or method. If addressable is set, T is the type of an addressable
23 // variable (only matters for method lookups). T must not be nil.
25 // The last index entry is the field or method index in the (possibly embedded)
26 // type where the entry was found, either:
28 // 1. the list of declared methods of a named type; or
29 // 2. the list of all methods (method set) of an interface type; or
30 // 3. the list of fields of a struct type.
32 // The earlier index entries are the indices of the embedded struct fields
33 // traversed to get to the found entry, starting at depth 0.
35 // If no entry is found, a nil object is returned. In this case, the returned
36 // index and indirect values have the following meaning:
38 // - If index != nil, the index sequence points to an ambiguous entry
39 // (the same name appeared more than once at the same embedding level).
41 // - If indirect is set, a method with a pointer receiver type was found
42 // but there was no pointer on the path from the actual receiver type to
43 // the method's formal receiver base type, nor was the receiver addressable.
44 func LookupFieldOrMethod(T Type, addressable bool, pkg *Package, name string) (obj Object, index []int, indirect bool) {
46 panic("LookupFieldOrMethod on nil type")
49 // Methods cannot be associated to a named pointer type.
50 // (spec: "The type denoted by T is called the receiver base type;
51 // it must not be a pointer or interface type and it must be declared
52 // in the same package as the method.").
53 // Thus, if we have a named pointer type, proceed with the underlying
54 // pointer type but discard the result if it is a method since we would
55 // not have found it for T (see also go.dev/issue/8590).
56 if t, _ := T.(*Named); t != nil {
57 if p, _ := t.Underlying().(*Pointer); p != nil {
58 obj, index, indirect = lookupFieldOrMethod(p, false, pkg, name, false)
59 if _, ok := obj.(*Func); ok {
60 return nil, nil, false
66 obj, index, indirect = lookupFieldOrMethod(T, addressable, pkg, name, false)
68 // If we didn't find anything and if we have a type parameter with a core type,
69 // see if there is a matching field (but not a method, those need to be declared
70 // explicitly in the constraint). If the constraint is a named pointer type (see
71 // above), we are ok here because only fields are accepted as results.
72 const enableTParamFieldLookup = false // see go.dev/issue/51576
73 if enableTParamFieldLookup && obj == nil && isTypeParam(T) {
74 if t := coreType(T); t != nil {
75 obj, index, indirect = lookupFieldOrMethod(t, addressable, pkg, name, false)
76 if _, ok := obj.(*Var); !ok {
77 obj, index, indirect = nil, nil, false // accept fields (variables) only
84 // lookupFieldOrMethod should only be called by LookupFieldOrMethod and missingMethod.
85 // If foldCase is true, the lookup for methods will include looking for any method
86 // which case-folds to the same as 'name' (used for giving helpful error messages).
88 // The resulting object may not be fully type-checked.
89 func lookupFieldOrMethod(T Type, addressable bool, pkg *Package, name string, foldCase bool) (obj Object, index []int, indirect bool) {
90 // WARNING: The code in this function is extremely subtle - do not modify casually!
93 return // blank fields/methods are never found
96 typ, isPtr := deref(T)
98 // *typ where typ is an interface (incl. a type parameter) has no methods.
100 if _, ok := under(typ).(*Interface); ok {
105 // Start with typ as single entry at shallowest depth.
106 current := []embeddedType{{typ, nil, isPtr, false}}
108 // seen tracks named types that we have seen already, allocated lazily.
109 // Used to avoid endless searches in case of recursive types.
111 // We must use a lookup on identity rather than a simple map[*Named]bool as
112 // instantiated types may be identical but not equal.
113 var seen instanceLookup
115 // search current depth
116 for len(current) > 0 {
117 var next []embeddedType // embedded types found at current depth
119 // look for (pkg, name) in all types at current depth
120 for _, e := range current {
123 // If we have a named type, we may have associated methods.
124 // Look for those first.
125 if named, _ := typ.(*Named); named != nil {
126 if alt := seen.lookup(named); alt != nil {
127 // We have seen this type before, at a more shallow depth
128 // (note that multiples of this type at the current depth
129 // were consolidated before). The type at that depth shadows
130 // this same type at the current depth, so we can ignore
136 // look for a matching attached method
137 if i, m := named.lookupMethod(pkg, name, foldCase); m != nil {
139 // caution: method may not have a proper signature yet
140 index = concat(e.index, i)
141 if obj != nil || e.multiples {
142 return nil, index, false // collision
145 indirect = e.indirect
146 continue // we can't have a matching field or interface method
150 switch t := under(typ).(type) {
152 // look for a matching field and collect embedded types
153 for i, f := range t.fields {
154 if f.sameId(pkg, name) {
156 index = concat(e.index, i)
157 if obj != nil || e.multiples {
158 return nil, index, false // collision
161 indirect = e.indirect
162 continue // we can't have a matching interface method
164 // Collect embedded struct fields for searching the next
165 // lower depth, but only if we have not seen a match yet
166 // (if we have a match it is either the desired field or
167 // we have a name collision on the same depth; in either
168 // case we don't need to look further).
169 // Embedded fields are always of the form T or *T where
170 // T is a type name. If e.typ appeared multiple times at
171 // this depth, f.typ appears multiple times at the next
173 if obj == nil && f.embedded {
174 typ, isPtr := deref(f.typ)
175 // TODO(gri) optimization: ignore types that can't
176 // have fields or methods (only Named, Struct, and
177 // Interface types need to be considered).
178 next = append(next, embeddedType{typ, concat(e.index, i), e.indirect || isPtr, e.multiples})
183 // look for a matching method (interface may be a type parameter)
184 if i, m := t.typeSet().LookupMethod(pkg, name, foldCase); m != nil {
186 index = concat(e.index, i)
187 if obj != nil || e.multiples {
188 return nil, index, false // collision
191 indirect = e.indirect
197 // found a potential match
198 // spec: "A method call x.m() is valid if the method set of (the type of) x
199 // contains m and the argument list can be assigned to the parameter
200 // list of m. If x is addressable and &x's method set contains m, x.m()
201 // is shorthand for (&x).m()".
202 if f, _ := obj.(*Func); f != nil {
203 // determine if method has a pointer receiver
204 if f.hasPtrRecv() && !indirect && !addressable {
205 return nil, nil, true // pointer/addressable receiver required
211 current = consolidateMultiples(next)
214 return nil, nil, false // not found
217 // embeddedType represents an embedded type
218 type embeddedType struct {
220 index []int // embedded field indices, starting with index at depth 0
221 indirect bool // if set, there was a pointer indirection on the path to this field
222 multiples bool // if set, typ appears multiple times at this depth
225 // consolidateMultiples collects multiple list entries with the same type
226 // into a single entry marked as containing multiples. The result is the
227 // consolidated list.
228 func consolidateMultiples(list []embeddedType) []embeddedType {
230 return list // at most one entry - nothing to do
233 n := 0 // number of entries w/ unique type
234 prev := make(map[Type]int) // index at which type was previously seen
235 for _, e := range list {
236 if i, found := lookupType(prev, e.typ); found {
237 list[i].multiples = true
248 func lookupType(m map[Type]int, typ Type) (int, bool) {
249 // fast path: maybe the types are equal
250 if i, found := m[typ]; found {
254 for t, i := range m {
255 if Identical(t, typ) {
263 type instanceLookup struct {
264 // buf is used to avoid allocating the map m in the common case of a small
265 // number of instances.
267 m map[*Named][]*Named
270 func (l *instanceLookup) lookup(inst *Named) *Named {
271 for _, t := range l.buf {
272 if t != nil && Identical(inst, t) {
276 for _, t := range l.m[inst.Origin()] {
277 if Identical(inst, t) {
284 func (l *instanceLookup) add(inst *Named) {
285 for i, t := range l.buf {
292 l.m = make(map[*Named][]*Named)
294 insts := l.m[inst.Origin()]
295 l.m[inst.Origin()] = append(insts, inst)
298 // MissingMethod returns (nil, false) if V implements T, otherwise it
299 // returns a missing method required by T and whether it is missing or
300 // just has the wrong type.
302 // For non-interface types V, or if static is set, V implements T if all
303 // methods of T are present in V. Otherwise (V is an interface and static
304 // is not set), MissingMethod only checks that methods of T which are also
305 // present in V have matching types (e.g., for a type assertion x.(T) where
306 // x is of interface type V).
307 func MissingMethod(V Type, T *Interface, static bool) (method *Func, wrongType bool) {
308 m, alt := (*Checker)(nil).missingMethod(V, T, static)
309 // Only report a wrong type if the alternative method has the same name as m.
310 return m, alt != nil && alt.name == m.name // alt != nil implies m != nil
313 // missingMethod is like MissingMethod but accepts a *Checker as receiver.
314 // The receiver may be nil if missingMethod is invoked through an exported
315 // API call (such as MissingMethod), i.e., when all methods have been type-
318 // If a method is missing on T but is found on *T, or if a method is found
319 // on T when looked up with case-folding, this alternative method is returned
320 // as the second result.
321 func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method, alt *Func) {
322 if T.NumMethods() == 0 {
327 if u, _ := under(V).(*Interface); u != nil {
329 for _, m := range T.typeSet().methods {
330 _, f := tset.LookupMethod(m.pkg, m.name, false)
339 if !Identical(f.typ, m.typ) {
347 // V is not an interface
348 for _, m := range T.typeSet().methods {
349 // TODO(gri) should this be calling LookupFieldOrMethod instead (and why not)?
350 obj, _, _ := lookupFieldOrMethod(V, false, m.pkg, m.name, false)
352 // check if m is on *V, or on V with case-folding
355 // TODO(gri) Instead of NewPointer(V) below, can we just set the "addressable" argument?
356 obj, _, _ = lookupFieldOrMethod(NewPointer(V), false, m.pkg, m.name, false)
358 obj, _, _ = lookupFieldOrMethod(V, false, m.pkg, m.name, true /* fold case */)
362 // we must have a method (not a struct field)
368 // methods may not have a fully set up signature yet
370 check.objDecl(f, nil)
373 if !found || !Identical(f.typ, m.typ) {
381 // missingMethodCause returns a string giving the detailed cause for a missing method m,
382 // where m is missing from V, but required by T. It puts the cause in parentheses,
383 // and may include more have/want info after that. If non-nil, alt is a relevant
384 // method that matches in some way. It may have the correct name, but wrong type, or
385 // it may have a pointer receiver, or it may have the correct name except wrong case.
387 func (check *Checker) missingMethodCause(V, T Type, m, alt *Func) string {
388 mname := "method " + m.Name()
391 if m.Name() != alt.Name() {
392 return check.sprintf("(missing %s)\n\t\thave %s\n\t\twant %s",
393 mname, check.funcString(alt, false), check.funcString(m, false))
396 if Identical(m.typ, alt.typ) {
397 return check.sprintf("(%s has pointer receiver)", mname)
400 altS, mS := check.funcString(alt, false), check.funcString(m, false)
402 // Would tell the user that Foo isn't a Foo, add package information to disambiguate. See #54258.
403 altS, mS = check.funcString(alt, true), check.funcString(m, true)
406 return check.sprintf("(wrong type for %s)\n\t\thave %s\n\t\twant %s",
410 if isInterfacePtr(V) {
411 return "(" + check.interfacePtrError(V) + ")"
414 if isInterfacePtr(T) {
415 return "(" + check.interfacePtrError(T) + ")"
418 obj, _, _ := lookupFieldOrMethod(V, true /* auto-deref */, m.pkg, m.name, false)
419 if fld, _ := obj.(*Var); fld != nil {
420 return check.sprintf("(%s.%s is a field, not a method)", V, fld.Name())
423 return check.sprintf("(missing %s)", mname)
426 func isInterfacePtr(T Type) bool {
427 p, _ := under(T).(*Pointer)
428 return p != nil && IsInterface(p.base)
432 func (check *Checker) interfacePtrError(T Type) string {
433 assert(isInterfacePtr(T))
434 if p, _ := under(T).(*Pointer); isTypeParam(p.base) {
435 return check.sprintf("type %s is pointer to type parameter, not type parameter", T)
437 return check.sprintf("type %s is pointer to interface, not interface", T)
440 // funcString returns a string of the form name + signature for f.
442 func (check *Checker) funcString(f *Func, pkgInfo bool) string {
443 buf := bytes.NewBufferString(f.name)
445 if check != nil && !pkgInfo {
448 w := newTypeWriter(buf, qf)
451 w.signature(f.typ.(*Signature))
455 // assertableTo reports whether a value of type V can be asserted to have type T.
456 // It returns (nil, false) as affirmative answer. Otherwise it returns a missing
457 // method required by V and whether it is missing or just has the wrong type.
458 // The receiver may be nil if assertableTo is invoked through an exported API call
459 // (such as AssertableTo), i.e., when all methods have been type-checked.
460 // TODO(gri) replace calls to this function with calls to newAssertableTo.
461 func (check *Checker) assertableTo(V *Interface, T Type) (method, wrongType *Func) {
462 // no static check is required if T is an interface
463 // spec: "If T is an interface type, x.(T) asserts that the
464 // dynamic type of x implements the interface T."
468 // TODO(gri) fix this for generalized interfaces
469 return check.missingMethod(T, V, false)
472 // newAssertableTo reports whether a value of type V can be asserted to have type T.
473 // It also implements behavior for interfaces that currently are only permitted
474 // in constraint position (we have not yet defined that behavior in the spec).
475 func (check *Checker) newAssertableTo(V *Interface, T Type) bool {
476 // no static check is required if T is an interface
477 // spec: "If T is an interface type, x.(T) asserts that the
478 // dynamic type of x implements the interface T."
482 return check.implements(T, V, false, nil)
485 // deref dereferences typ if it is a *Pointer and returns its base and true.
486 // Otherwise it returns (typ, false).
487 func deref(typ Type) (Type, bool) {
488 if p, _ := typ.(*Pointer); p != nil {
489 // p.base should never be nil, but be conservative
492 panic("pointer with nil base type (possibly due to an invalid cyclic declaration)")
494 return Typ[Invalid], true
501 // derefStructPtr dereferences typ if it is a (named or unnamed) pointer to a
502 // (named or unnamed) struct and returns its base. Otherwise it returns typ.
503 func derefStructPtr(typ Type) Type {
504 if p, _ := under(typ).(*Pointer); p != nil {
505 if _, ok := under(p.base).(*Struct); ok {
512 // concat returns the result of concatenating list and i.
513 // The result does not share its underlying array with list.
514 func concat(list []int, i int) []int {
516 t = append(t, list...)
520 // fieldIndex returns the index for the field with matching package and name, or a value < 0.
521 func fieldIndex(fields []*Var, pkg *Package, name string) int {
523 for i, f := range fields {
524 if f.sameId(pkg, name) {
532 // lookupMethod returns the index of and method with matching package and name, or (-1, nil).
533 // If foldCase is true, method names are considered equal if they are equal with case folding.
534 func lookupMethod(methods []*Func, pkg *Package, name string, foldCase bool) (int, *Func) {
536 for i, m := range methods {
537 if (m.name == name || foldCase && strings.EqualFold(m.name, name)) && m.sameId(pkg, m.name) {