Source file src/go/types/lookup.go
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. 4 5 // This file implements various field and method lookup functions. 6 7 package types 8 9 import ( 10 "bytes" 11 "strings" 12 ) 13 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 17 // the method's type. 18 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. 24 // 25 // The last index entry is the field or method index in the (possibly embedded) 26 // type where the entry was found, either: 27 // 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. 31 // 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. 34 // 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: 37 // 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). 40 // 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) { 45 if T == nil { 46 panic("LookupFieldOrMethod on nil type") 47 } 48 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 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 61 } 62 return 63 } 64 } 65 66 obj, index, indirect = lookupFieldOrMethod(T, addressable, pkg, name, false) 67 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 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 78 } 79 } 80 } 81 return 82 } 83 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). 87 // 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! 91 92 if name == "_" { 93 return // blank fields/methods are never found 94 } 95 96 typ, isPtr := deref(T) 97 98 // *typ where typ is an interface (incl. a type parameter) has no methods. 99 if isPtr { 100 if _, ok := under(typ).(*Interface); ok { 101 return 102 } 103 } 104 105 // Start with typ as single entry at shallowest depth. 106 current := []embeddedType{{typ, nil, isPtr, false}} 107 108 // seen tracks named types that we have seen already, allocated lazily. 109 // Used to avoid endless searches in case of recursive types. 110 // 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 114 115 // search current depth 116 for len(current) > 0 { 117 var next []embeddedType // embedded types found at current depth 118 119 // look for (pkg, name) in all types at current depth 120 for _, e := range current { 121 typ := e.typ 122 123 // 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 131 // this one. 132 continue 133 } 134 seen.add(named) 135 136 // look for a matching attached method 137 named.resolve(nil) 138 if i, m := named.lookupMethod(pkg, name, foldCase); m != nil { 139 // potential match 140 // caution: method may not have a proper signature yet 141 index = concat(e.index, i) 142 if obj != nil || e.multiples { 143 return nil, index, false // collision 144 } 145 obj = m 146 indirect = e.indirect 147 continue // we can't have a matching field or interface method 148 } 149 } 150 151 switch t := under(typ).(type) { 152 case *Struct: 153 // look for a matching field and collect embedded types 154 for i, f := range t.fields { 155 if f.sameId(pkg, name) { 156 assert(f.typ != nil) 157 index = concat(e.index, i) 158 if obj != nil || e.multiples { 159 return nil, index, false // collision 160 } 161 obj = f 162 indirect = e.indirect 163 continue // we can't have a matching interface method 164 } 165 // Collect embedded struct fields for searching the next 166 // lower depth, but only if we have not seen a match yet 167 // (if we have a match it is either the desired field or 168 // we have a name collision on the same depth; in either 169 // case we don't need to look further). 170 // Embedded fields are always of the form T or *T where 171 // T is a type name. If e.typ appeared multiple times at 172 // this depth, f.typ appears multiple times at the next 173 // depth. 174 if obj == nil && f.embedded { 175 typ, isPtr := deref(f.typ) 176 // TODO(gri) optimization: ignore types that can't 177 // have fields or methods (only Named, Struct, and 178 // Interface types need to be considered). 179 next = append(next, embeddedType{typ, concat(e.index, i), e.indirect || isPtr, e.multiples}) 180 } 181 } 182 183 case *Interface: 184 // look for a matching method (interface may be a type parameter) 185 if i, m := t.typeSet().LookupMethod(pkg, name, foldCase); m != nil { 186 assert(m.typ != nil) 187 index = concat(e.index, i) 188 if obj != nil || e.multiples { 189 return nil, index, false // collision 190 } 191 obj = m 192 indirect = e.indirect 193 } 194 } 195 } 196 197 if obj != nil { 198 // found a potential match 199 // spec: "A method call x.m() is valid if the method set of (the type of) x 200 // contains m and the argument list can be assigned to the parameter 201 // list of m. If x is addressable and &x's method set contains m, x.m() 202 // is shorthand for (&x).m()". 203 if f, _ := obj.(*Func); f != nil { 204 // determine if method has a pointer receiver 205 if f.hasPtrRecv() && !indirect && !addressable { 206 return nil, nil, true // pointer/addressable receiver required 207 } 208 } 209 return 210 } 211 212 current = consolidateMultiples(next) 213 } 214 215 return nil, nil, false // not found 216 } 217 218 // embeddedType represents an embedded type 219 type embeddedType struct { 220 typ Type 221 index []int // embedded field indices, starting with index at depth 0 222 indirect bool // if set, there was a pointer indirection on the path to this field 223 multiples bool // if set, typ appears multiple times at this depth 224 } 225 226 // consolidateMultiples collects multiple list entries with the same type 227 // into a single entry marked as containing multiples. The result is the 228 // consolidated list. 229 func consolidateMultiples(list []embeddedType) []embeddedType { 230 if len(list) <= 1 { 231 return list // at most one entry - nothing to do 232 } 233 234 n := 0 // number of entries w/ unique type 235 prev := make(map[Type]int) // index at which type was previously seen 236 for _, e := range list { 237 if i, found := lookupType(prev, e.typ); found { 238 list[i].multiples = true 239 // ignore this entry 240 } else { 241 prev[e.typ] = n 242 list[n] = e 243 n++ 244 } 245 } 246 return list[:n] 247 } 248 249 func lookupType(m map[Type]int, typ Type) (int, bool) { 250 // fast path: maybe the types are equal 251 if i, found := m[typ]; found { 252 return i, true 253 } 254 255 for t, i := range m { 256 if Identical(t, typ) { 257 return i, true 258 } 259 } 260 261 return 0, false 262 } 263 264 type instanceLookup struct { 265 m map[*Named][]*Named 266 } 267 268 func (l *instanceLookup) lookup(inst *Named) *Named { 269 for _, t := range l.m[inst.Origin()] { 270 if Identical(inst, t) { 271 return t 272 } 273 } 274 return nil 275 } 276 277 func (l *instanceLookup) add(inst *Named) { 278 if l.m == nil { 279 l.m = make(map[*Named][]*Named) 280 } 281 insts := l.m[inst.Origin()] 282 l.m[inst.Origin()] = append(insts, inst) 283 } 284 285 // MissingMethod returns (nil, false) if V implements T, otherwise it 286 // returns a missing method required by T and whether it is missing or 287 // just has the wrong type. 288 // 289 // For non-interface types V, or if static is set, V implements T if all 290 // methods of T are present in V. Otherwise (V is an interface and static 291 // is not set), MissingMethod only checks that methods of T which are also 292 // present in V have matching types (e.g., for a type assertion x.(T) where 293 // x is of interface type V). 294 func MissingMethod(V Type, T *Interface, static bool) (method *Func, wrongType bool) { 295 m, alt := (*Checker)(nil).missingMethod(V, T, static) 296 // Only report a wrong type if the alternative method has the same name as m. 297 return m, alt != nil && alt.name == m.name // alt != nil implies m != nil 298 } 299 300 // missingMethod is like MissingMethod but accepts a *Checker as receiver. 301 // The receiver may be nil if missingMethod is invoked through an exported 302 // API call (such as MissingMethod), i.e., when all methods have been type- 303 // checked. 304 // 305 // If a method is missing on T but is found on *T, or if a method is found 306 // on T when looked up with case-folding, this alternative method is returned 307 // as the second result. 308 func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method, alt *Func) { 309 if T.NumMethods() == 0 { 310 return 311 } 312 313 // V is an interface 314 if u, _ := under(V).(*Interface); u != nil { 315 tset := u.typeSet() 316 for _, m := range T.typeSet().methods { 317 _, f := tset.LookupMethod(m.pkg, m.name, false) 318 319 if f == nil { 320 if !static { 321 continue 322 } 323 return m, nil 324 } 325 326 if !Identical(f.typ, m.typ) { 327 return m, f 328 } 329 } 330 331 return 332 } 333 334 // V is not an interface 335 for _, m := range T.typeSet().methods { 336 // TODO(gri) should this be calling LookupFieldOrMethod instead (and why not)? 337 obj, _, _ := lookupFieldOrMethod(V, false, m.pkg, m.name, false) 338 339 // check if m is on *V, or on V with case-folding 340 found := obj != nil 341 if !found { 342 // TODO(gri) Instead of NewPointer(V) below, can we just set the "addressable" argument? 343 obj, _, _ = lookupFieldOrMethod(NewPointer(V), false, m.pkg, m.name, false) 344 if obj == nil { 345 obj, _, _ = lookupFieldOrMethod(V, false, m.pkg, m.name, true /* fold case */) 346 } 347 } 348 349 // we must have a method (not a struct field) 350 f, _ := obj.(*Func) 351 if f == nil { 352 return m, nil 353 } 354 355 // methods may not have a fully set up signature yet 356 if check != nil { 357 check.objDecl(f, nil) 358 } 359 360 if !found || !Identical(f.typ, m.typ) { 361 return m, f 362 } 363 } 364 365 return 366 } 367 368 // missingMethodReason returns a string giving the detailed reason for a missing method m, 369 // where m is missing from V, but required by T. It puts the reason in parentheses, 370 // and may include more have/want info after that. If non-nil, alt is a relevant 371 // method that matches in some way. It may have the correct name, but wrong type, or 372 // it may have a pointer receiver, or it may have the correct name except wrong case. 373 // check may be nil. 374 func (check *Checker) missingMethodReason(V, T Type, m, alt *Func) string { 375 var mname string 376 if check != nil && compilerErrorMessages { 377 mname = m.Name() + " method" 378 } else { 379 mname = "method " + m.Name() 380 } 381 382 if alt != nil { 383 if m.Name() != alt.Name() { 384 return check.sprintf("(missing %s)\n\t\thave %s\n\t\twant %s", 385 mname, check.funcString(alt), check.funcString(m)) 386 } 387 388 if Identical(m.typ, alt.typ) { 389 return check.sprintf("(%s has pointer receiver)", mname) 390 } 391 392 return check.sprintf("(wrong type for %s)\n\t\thave %s\n\t\twant %s", 393 mname, check.funcString(alt), check.funcString(m)) 394 } 395 396 if isInterfacePtr(V) { 397 return "(" + check.interfacePtrError(V) + ")" 398 } 399 400 if isInterfacePtr(T) { 401 return "(" + check.interfacePtrError(T) + ")" 402 } 403 404 return check.sprintf("(missing %s)", mname) 405 } 406 407 func isInterfacePtr(T Type) bool { 408 p, _ := under(T).(*Pointer) 409 return p != nil && IsInterface(p.base) 410 } 411 412 // check may be nil. 413 func (check *Checker) interfacePtrError(T Type) string { 414 assert(isInterfacePtr(T)) 415 if p, _ := under(T).(*Pointer); isTypeParam(p.base) { 416 return check.sprintf("type %s is pointer to type parameter, not type parameter", T) 417 } 418 return check.sprintf("type %s is pointer to interface, not interface", T) 419 } 420 421 // check may be nil. 422 func (check *Checker) funcString(f *Func) string { 423 buf := bytes.NewBufferString(f.name) 424 var qf Qualifier 425 if check != nil { 426 qf = check.qualifier 427 } 428 WriteSignature(buf, f.typ.(*Signature), qf) 429 return buf.String() 430 } 431 432 // assertableTo reports whether a value of type V can be asserted to have type T. 433 // It returns (nil, false) as affirmative answer. Otherwise it returns a missing 434 // method required by V and whether it is missing or just has the wrong type. 435 // The receiver may be nil if assertableTo is invoked through an exported API call 436 // (such as AssertableTo), i.e., when all methods have been type-checked. 437 // TODO(gri) replace calls to this function with calls to newAssertableTo. 438 func (check *Checker) assertableTo(V *Interface, T Type) (method, wrongType *Func) { 439 // no static check is required if T is an interface 440 // spec: "If T is an interface type, x.(T) asserts that the 441 // dynamic type of x implements the interface T." 442 if IsInterface(T) { 443 return 444 } 445 // TODO(gri) fix this for generalized interfaces 446 return check.missingMethod(T, V, false) 447 } 448 449 // newAssertableTo reports whether a value of type V can be asserted to have type T. 450 // It also implements behavior for interfaces that currently are only permitted 451 // in constraint position (we have not yet defined that behavior in the spec). 452 func (check *Checker) newAssertableTo(V *Interface, T Type) error { 453 // no static check is required if T is an interface 454 // spec: "If T is an interface type, x.(T) asserts that the 455 // dynamic type of x implements the interface T." 456 if IsInterface(T) { 457 return nil 458 } 459 return check.implements(T, V) 460 } 461 462 // deref dereferences typ if it is a *Pointer and returns its base and true. 463 // Otherwise it returns (typ, false). 464 func deref(typ Type) (Type, bool) { 465 if p, _ := typ.(*Pointer); p != nil { 466 // p.base should never be nil, but be conservative 467 if p.base == nil { 468 if debug { 469 panic("pointer with nil base type (possibly due to an invalid cyclic declaration)") 470 } 471 return Typ[Invalid], true 472 } 473 return p.base, true 474 } 475 return typ, false 476 } 477 478 // derefStructPtr dereferences typ if it is a (named or unnamed) pointer to a 479 // (named or unnamed) struct and returns its base. Otherwise it returns typ. 480 func derefStructPtr(typ Type) Type { 481 if p, _ := under(typ).(*Pointer); p != nil { 482 if _, ok := under(p.base).(*Struct); ok { 483 return p.base 484 } 485 } 486 return typ 487 } 488 489 // concat returns the result of concatenating list and i. 490 // The result does not share its underlying array with list. 491 func concat(list []int, i int) []int { 492 var t []int 493 t = append(t, list...) 494 return append(t, i) 495 } 496 497 // fieldIndex returns the index for the field with matching package and name, or a value < 0. 498 func fieldIndex(fields []*Var, pkg *Package, name string) int { 499 if name != "_" { 500 for i, f := range fields { 501 if f.sameId(pkg, name) { 502 return i 503 } 504 } 505 } 506 return -1 507 } 508 509 // lookupMethod returns the index of and method with matching package and name, or (-1, nil). 510 // If foldCase is true, method names are considered equal if they are equal with case folding. 511 func lookupMethod(methods []*Func, pkg *Package, name string, foldCase bool) (int, *Func) { 512 if name != "_" { 513 for i, m := range methods { 514 if (m.name == name || foldCase && strings.EqualFold(m.name, name)) && m.sameId(pkg, m.name) { 515 return i, m 516 } 517 } 518 } 519 return -1, nil 520 } 521