Source file src/go/types/unify.go
1 // Code generated by "go test -run=Generate -write=all"; DO NOT EDIT. 2 // Source: ../../cmd/compile/internal/types2/unify.go 3 4 // Copyright 2020 The Go Authors. All rights reserved. 5 // Use of this source code is governed by a BSD-style 6 // license that can be found in the LICENSE file. 7 8 // This file implements type unification. 9 // 10 // Type unification attempts to make two types x and y structurally 11 // equivalent by determining the types for a given list of (bound) 12 // type parameters which may occur within x and y. If x and y are 13 // structurally different (say []T vs chan T), or conflicting 14 // types are determined for type parameters, unification fails. 15 // If unification succeeds, as a side-effect, the types of the 16 // bound type parameters may be determined. 17 // 18 // Unification typically requires multiple calls u.unify(x, y) to 19 // a given unifier u, with various combinations of types x and y. 20 // In each call, additional type parameter types may be determined 21 // as a side effect and recorded in u. 22 // If a call fails (returns false), unification fails. 23 // 24 // In the unification context, structural equivalence of two types 25 // ignores the difference between a defined type and its underlying 26 // type if one type is a defined type and the other one is not. 27 // It also ignores the difference between an (external, unbound) 28 // type parameter and its core type. 29 // If two types are not structurally equivalent, they cannot be Go 30 // identical types. On the other hand, if they are structurally 31 // equivalent, they may be Go identical or at least assignable, or 32 // they may be in the type set of a constraint. 33 // Whether they indeed are identical or assignable is determined 34 // upon instantiation and function argument passing. 35 36 package types 37 38 import ( 39 "bytes" 40 "fmt" 41 "sort" 42 "strings" 43 ) 44 45 const ( 46 // Upper limit for recursion depth. Used to catch infinite recursions 47 // due to implementation issues (e.g., see issues go.dev/issue/48619, go.dev/issue/48656). 48 unificationDepthLimit = 50 49 50 // Whether to panic when unificationDepthLimit is reached. 51 // If disabled, a recursion depth overflow results in a (quiet) 52 // unification failure. 53 panicAtUnificationDepthLimit = true 54 55 // If enableCoreTypeUnification is set, unification will consider 56 // the core types, if any, of non-local (unbound) type parameters. 57 enableCoreTypeUnification = true 58 59 // If traceInference is set, unification will print a trace of its operation. 60 // Interpretation of trace: 61 // x ≡ y attempt to unify types x and y 62 // p ➞ y type parameter p is set to type y (p is inferred to be y) 63 // p ⇄ q type parameters p and q match (p is inferred to be q and vice versa) 64 // x ≢ y types x and y cannot be unified 65 // [p, q, ...] ➞ [x, y, ...] mapping from type parameters to types 66 traceInference = false 67 ) 68 69 // A unifier maintains a list of type parameters and 70 // corresponding types inferred for each type parameter. 71 // A unifier is created by calling newUnifier. 72 type unifier struct { 73 // handles maps each type parameter to its inferred type through 74 // an indirection *Type called (inferred type) "handle". 75 // Initially, each type parameter has its own, separate handle, 76 // with a nil (i.e., not yet inferred) type. 77 // After a type parameter P is unified with a type parameter Q, 78 // P and Q share the same handle (and thus type). This ensures 79 // that inferring the type for a given type parameter P will 80 // automatically infer the same type for all other parameters 81 // unified (joined) with P. 82 handles map[*TypeParam]*Type 83 depth int // recursion depth during unification 84 enableInterfaceInference bool // use shared methods for better inference 85 } 86 87 // newUnifier returns a new unifier initialized with the given type parameter 88 // and corresponding type argument lists. The type argument list may be shorter 89 // than the type parameter list, and it may contain nil types. Matching type 90 // parameters and arguments must have the same index. 91 func newUnifier(tparams []*TypeParam, targs []Type, enableInterfaceInference bool) *unifier { 92 assert(len(tparams) >= len(targs)) 93 handles := make(map[*TypeParam]*Type, len(tparams)) 94 // Allocate all handles up-front: in a correct program, all type parameters 95 // must be resolved and thus eventually will get a handle. 96 // Also, sharing of handles caused by unified type parameters is rare and 97 // so it's ok to not optimize for that case (and delay handle allocation). 98 for i, x := range tparams { 99 var t Type 100 if i < len(targs) { 101 t = targs[i] 102 } 103 handles[x] = &t 104 } 105 return &unifier{handles, 0, enableInterfaceInference} 106 } 107 108 // unifyMode controls the behavior of the unifier. 109 type unifyMode uint 110 111 const ( 112 // If assign is set, we are unifying types involved in an assignment: 113 // they may match inexactly at the top, but element types must match 114 // exactly. 115 assign unifyMode = 1 << iota 116 117 // If exact is set, types unify if they are identical (or can be 118 // made identical with suitable arguments for type parameters). 119 // Otherwise, a named type and a type literal unify if their 120 // underlying types unify, channel directions are ignored, and 121 // if there is an interface, the other type must implement the 122 // interface. 123 exact 124 ) 125 126 func (m unifyMode) String() string { 127 switch m { 128 case 0: 129 return "inexact" 130 case assign: 131 return "assign" 132 case exact: 133 return "exact" 134 case assign | exact: 135 return "assign, exact" 136 } 137 return fmt.Sprintf("mode %d", m) 138 } 139 140 // unify attempts to unify x and y and reports whether it succeeded. 141 // As a side-effect, types may be inferred for type parameters. 142 // The mode parameter controls how types are compared. 143 func (u *unifier) unify(x, y Type, mode unifyMode) bool { 144 return u.nify(x, y, mode, nil) 145 } 146 147 func (u *unifier) tracef(format string, args ...interface{}) { 148 fmt.Println(strings.Repeat(". ", u.depth) + sprintf(nil, nil, true, format, args...)) 149 } 150 151 // String returns a string representation of the current mapping 152 // from type parameters to types. 153 func (u *unifier) String() string { 154 // sort type parameters for reproducible strings 155 tparams := make(typeParamsById, len(u.handles)) 156 i := 0 157 for tpar := range u.handles { 158 tparams[i] = tpar 159 i++ 160 } 161 sort.Sort(tparams) 162 163 var buf bytes.Buffer 164 w := newTypeWriter(&buf, nil) 165 w.byte('[') 166 for i, x := range tparams { 167 if i > 0 { 168 w.string(", ") 169 } 170 w.typ(x) 171 w.string(": ") 172 w.typ(u.at(x)) 173 } 174 w.byte(']') 175 return buf.String() 176 } 177 178 type typeParamsById []*TypeParam 179 180 func (s typeParamsById) Len() int { return len(s) } 181 func (s typeParamsById) Less(i, j int) bool { return s[i].id < s[j].id } 182 func (s typeParamsById) Swap(i, j int) { s[i], s[j] = s[j], s[i] } 183 184 // join unifies the given type parameters x and y. 185 // If both type parameters already have a type associated with them 186 // and they are not joined, join fails and returns false. 187 func (u *unifier) join(x, y *TypeParam) bool { 188 if traceInference { 189 u.tracef("%s ⇄ %s", x, y) 190 } 191 switch hx, hy := u.handles[x], u.handles[y]; { 192 case hx == hy: 193 // Both type parameters already share the same handle. Nothing to do. 194 case *hx != nil && *hy != nil: 195 // Both type parameters have (possibly different) inferred types. Cannot join. 196 return false 197 case *hx != nil: 198 // Only type parameter x has an inferred type. Use handle of x. 199 u.setHandle(y, hx) 200 // This case is treated like the default case. 201 // case *hy != nil: 202 // // Only type parameter y has an inferred type. Use handle of y. 203 // u.setHandle(x, hy) 204 default: 205 // Neither type parameter has an inferred type. Use handle of y. 206 u.setHandle(x, hy) 207 } 208 return true 209 } 210 211 // asBoundTypeParam returns x.(*TypeParam) if x is a type parameter recorded with u. 212 // Otherwise, the result is nil. 213 func (u *unifier) asBoundTypeParam(x Type) *TypeParam { 214 if x, _ := Unalias(x).(*TypeParam); x != nil { 215 if _, found := u.handles[x]; found { 216 return x 217 } 218 } 219 return nil 220 } 221 222 // setHandle sets the handle for type parameter x 223 // (and all its joined type parameters) to h. 224 func (u *unifier) setHandle(x *TypeParam, h *Type) { 225 hx := u.handles[x] 226 assert(hx != nil) 227 for y, hy := range u.handles { 228 if hy == hx { 229 u.handles[y] = h 230 } 231 } 232 } 233 234 // at returns the (possibly nil) type for type parameter x. 235 func (u *unifier) at(x *TypeParam) Type { 236 return *u.handles[x] 237 } 238 239 // set sets the type t for type parameter x; 240 // t must not be nil. 241 func (u *unifier) set(x *TypeParam, t Type) { 242 assert(t != nil) 243 if traceInference { 244 u.tracef("%s ➞ %s", x, t) 245 } 246 *u.handles[x] = t 247 } 248 249 // unknowns returns the number of type parameters for which no type has been set yet. 250 func (u *unifier) unknowns() int { 251 n := 0 252 for _, h := range u.handles { 253 if *h == nil { 254 n++ 255 } 256 } 257 return n 258 } 259 260 // inferred returns the list of inferred types for the given type parameter list. 261 // The result is never nil and has the same length as tparams; result types that 262 // could not be inferred are nil. Corresponding type parameters and result types 263 // have identical indices. 264 func (u *unifier) inferred(tparams []*TypeParam) []Type { 265 list := make([]Type, len(tparams)) 266 for i, x := range tparams { 267 list[i] = u.at(x) 268 } 269 return list 270 } 271 272 // asInterface returns the underlying type of x as an interface if 273 // it is a non-type parameter interface. Otherwise it returns nil. 274 func asInterface(x Type) (i *Interface) { 275 if _, ok := Unalias(x).(*TypeParam); !ok { 276 i, _ = under(x).(*Interface) 277 } 278 return i 279 } 280 281 // nify implements the core unification algorithm which is an 282 // adapted version of Checker.identical. For changes to that 283 // code the corresponding changes should be made here. 284 // Must not be called directly from outside the unifier. 285 func (u *unifier) nify(x, y Type, mode unifyMode, p *ifacePair) (result bool) { 286 u.depth++ 287 if traceInference { 288 u.tracef("%s ≡ %s\t// %s", x, y, mode) 289 } 290 defer func() { 291 if traceInference && !result { 292 u.tracef("%s ≢ %s", x, y) 293 } 294 u.depth-- 295 }() 296 297 // nothing to do if x == y 298 if x == y || Unalias(x) == Unalias(y) { 299 return true 300 } 301 302 // Stop gap for cases where unification fails. 303 if u.depth > unificationDepthLimit { 304 if traceInference { 305 u.tracef("depth %d >= %d", u.depth, unificationDepthLimit) 306 } 307 if panicAtUnificationDepthLimit { 308 panic("unification reached recursion depth limit") 309 } 310 return false 311 } 312 313 // Unification is symmetric, so we can swap the operands. 314 // Ensure that if we have at least one 315 // - defined type, make sure one is in y 316 // - type parameter recorded with u, make sure one is in x 317 if asNamed(x) != nil || u.asBoundTypeParam(y) != nil { 318 if traceInference { 319 u.tracef("%s ≡ %s\t// swap", y, x) 320 } 321 x, y = y, x 322 } 323 324 // Unification will fail if we match a defined type against a type literal. 325 // If we are matching types in an assignment, at the top-level, types with 326 // the same type structure are permitted as long as at least one of them 327 // is not a defined type. To accommodate for that possibility, we continue 328 // unification with the underlying type of a defined type if the other type 329 // is a type literal. This is controlled by the exact unification mode. 330 // We also continue if the other type is a basic type because basic types 331 // are valid underlying types and may appear as core types of type constraints. 332 // If we exclude them, inferred defined types for type parameters may not 333 // match against the core types of their constraints (even though they might 334 // correctly match against some of the types in the constraint's type set). 335 // Finally, if unification (incorrectly) succeeds by matching the underlying 336 // type of a defined type against a basic type (because we include basic types 337 // as type literals here), and if that leads to an incorrectly inferred type, 338 // we will fail at function instantiation or argument assignment time. 339 // 340 // If we have at least one defined type, there is one in y. 341 if ny := asNamed(y); mode&exact == 0 && ny != nil && isTypeLit(x) && !(u.enableInterfaceInference && IsInterface(x)) { 342 if traceInference { 343 u.tracef("%s ≡ under %s", x, ny) 344 } 345 y = ny.under() 346 // Per the spec, a defined type cannot have an underlying type 347 // that is a type parameter. 348 assert(!isTypeParam(y)) 349 // x and y may be identical now 350 if x == y || Unalias(x) == Unalias(y) { 351 return true 352 } 353 } 354 355 // Cases where at least one of x or y is a type parameter recorded with u. 356 // If we have at least one type parameter, there is one in x. 357 // If we have exactly one type parameter, because it is in x, 358 // isTypeLit(x) is false and y was not changed above. In other 359 // words, if y was a defined type, it is still a defined type 360 // (relevant for the logic below). 361 switch px, py := u.asBoundTypeParam(x), u.asBoundTypeParam(y); { 362 case px != nil && py != nil: 363 // both x and y are type parameters 364 if u.join(px, py) { 365 return true 366 } 367 // both x and y have an inferred type - they must match 368 return u.nify(u.at(px), u.at(py), mode, p) 369 370 case px != nil: 371 // x is a type parameter, y is not 372 if x := u.at(px); x != nil { 373 // x has an inferred type which must match y 374 if u.nify(x, y, mode, p) { 375 // We have a match, possibly through underlying types. 376 xi := asInterface(x) 377 yi := asInterface(y) 378 xn := asNamed(x) != nil 379 yn := asNamed(y) != nil 380 // If we have two interfaces, what to do depends on 381 // whether they are named and their method sets. 382 if xi != nil && yi != nil { 383 // Both types are interfaces. 384 // If both types are defined types, they must be identical 385 // because unification doesn't know which type has the "right" name. 386 if xn && yn { 387 return Identical(x, y) 388 } 389 // In all other cases, the method sets must match. 390 // The types unified so we know that corresponding methods 391 // match and we can simply compare the number of methods. 392 // TODO(gri) We may be able to relax this rule and select 393 // the more general interface. But if one of them is a defined 394 // type, it's not clear how to choose and whether we introduce 395 // an order dependency or not. Requiring the same method set 396 // is conservative. 397 if len(xi.typeSet().methods) != len(yi.typeSet().methods) { 398 return false 399 } 400 } else if xi != nil || yi != nil { 401 // One but not both of them are interfaces. 402 // In this case, either x or y could be viable matches for the corresponding 403 // type parameter, which means choosing either introduces an order dependence. 404 // Therefore, we must fail unification (go.dev/issue/60933). 405 return false 406 } 407 // If we have inexact unification and one of x or y is a defined type, select the 408 // defined type. This ensures that in a series of types, all matching against the 409 // same type parameter, we infer a defined type if there is one, independent of 410 // order. Type inference or assignment may fail, which is ok. 411 // Selecting a defined type, if any, ensures that we don't lose the type name; 412 // and since we have inexact unification, a value of equally named or matching 413 // undefined type remains assignable (go.dev/issue/43056). 414 // 415 // Similarly, if we have inexact unification and there are no defined types but 416 // channel types, select a directed channel, if any. This ensures that in a series 417 // of unnamed types, all matching against the same type parameter, we infer the 418 // directed channel if there is one, independent of order. 419 // Selecting a directional channel, if any, ensures that a value of another 420 // inexactly unifying channel type remains assignable (go.dev/issue/62157). 421 // 422 // If we have multiple defined channel types, they are either identical or we 423 // have assignment conflicts, so we can ignore directionality in this case. 424 // 425 // If we have defined and literal channel types, a defined type wins to avoid 426 // order dependencies. 427 if mode&exact == 0 { 428 switch { 429 case xn: 430 // x is a defined type: nothing to do. 431 case yn: 432 // x is not a defined type and y is a defined type: select y. 433 u.set(px, y) 434 default: 435 // Neither x nor y are defined types. 436 if yc, _ := under(y).(*Chan); yc != nil && yc.dir != SendRecv { 437 // y is a directed channel type: select y. 438 u.set(px, y) 439 } 440 } 441 } 442 return true 443 } 444 return false 445 } 446 // otherwise, infer type from y 447 u.set(px, y) 448 return true 449 } 450 451 // x != y if we get here 452 assert(x != y && Unalias(x) != Unalias(y)) 453 454 // If u.EnableInterfaceInference is set and we don't require exact unification, 455 // if both types are interfaces, one interface must have a subset of the 456 // methods of the other and corresponding method signatures must unify. 457 // If only one type is an interface, all its methods must be present in the 458 // other type and corresponding method signatures must unify. 459 if u.enableInterfaceInference && mode&exact == 0 { 460 // One or both interfaces may be defined types. 461 // Look under the name, but not under type parameters (go.dev/issue/60564). 462 xi := asInterface(x) 463 yi := asInterface(y) 464 // If we have two interfaces, check the type terms for equivalence, 465 // and unify common methods if possible. 466 if xi != nil && yi != nil { 467 xset := xi.typeSet() 468 yset := yi.typeSet() 469 if xset.comparable != yset.comparable { 470 return false 471 } 472 // For now we require terms to be equal. 473 // We should be able to relax this as well, eventually. 474 if !xset.terms.equal(yset.terms) { 475 return false 476 } 477 // Interface types are the only types where cycles can occur 478 // that are not "terminated" via named types; and such cycles 479 // can only be created via method parameter types that are 480 // anonymous interfaces (directly or indirectly) embedding 481 // the current interface. Example: 482 // 483 // type T interface { 484 // m() interface{T} 485 // } 486 // 487 // If two such (differently named) interfaces are compared, 488 // endless recursion occurs if the cycle is not detected. 489 // 490 // If x and y were compared before, they must be equal 491 // (if they were not, the recursion would have stopped); 492 // search the ifacePair stack for the same pair. 493 // 494 // This is a quadratic algorithm, but in practice these stacks 495 // are extremely short (bounded by the nesting depth of interface 496 // type declarations that recur via parameter types, an extremely 497 // rare occurrence). An alternative implementation might use a 498 // "visited" map, but that is probably less efficient overall. 499 q := &ifacePair{xi, yi, p} 500 for p != nil { 501 if p.identical(q) { 502 return true // same pair was compared before 503 } 504 p = p.prev 505 } 506 // The method set of x must be a subset of the method set 507 // of y or vice versa, and the common methods must unify. 508 xmethods := xset.methods 509 ymethods := yset.methods 510 // The smaller method set must be the subset, if it exists. 511 if len(xmethods) > len(ymethods) { 512 xmethods, ymethods = ymethods, xmethods 513 } 514 // len(xmethods) <= len(ymethods) 515 // Collect the ymethods in a map for quick lookup. 516 ymap := make(map[string]*Func, len(ymethods)) 517 for _, ym := range ymethods { 518 ymap[ym.Id()] = ym 519 } 520 // All xmethods must exist in ymethods and corresponding signatures must unify. 521 for _, xm := range xmethods { 522 if ym := ymap[xm.Id()]; ym == nil || !u.nify(xm.typ, ym.typ, exact, p) { 523 return false 524 } 525 } 526 return true 527 } 528 529 // We don't have two interfaces. If we have one, make sure it's in xi. 530 if yi != nil { 531 xi = yi 532 y = x 533 } 534 535 // If we have one interface, at a minimum each of the interface methods 536 // must be implemented and thus unify with a corresponding method from 537 // the non-interface type, otherwise unification fails. 538 if xi != nil { 539 // All xi methods must exist in y and corresponding signatures must unify. 540 xmethods := xi.typeSet().methods 541 for _, xm := range xmethods { 542 obj, _, _ := LookupFieldOrMethod(y, false, xm.pkg, xm.name) 543 if ym, _ := obj.(*Func); ym == nil || !u.nify(xm.typ, ym.typ, exact, p) { 544 return false 545 } 546 } 547 return true 548 } 549 } 550 551 // Unless we have exact unification, neither x nor y are interfaces now. 552 // Except for unbound type parameters (see below), x and y must be structurally 553 // equivalent to unify. 554 555 // If we get here and x or y is a type parameter, they are unbound 556 // (not recorded with the unifier). 557 // Ensure that if we have at least one type parameter, it is in x 558 // (the earlier swap checks for _recorded_ type parameters only). 559 // This ensures that the switch switches on the type parameter. 560 // 561 // TODO(gri) Factor out type parameter handling from the switch. 562 if isTypeParam(y) { 563 if traceInference { 564 u.tracef("%s ≡ %s\t// swap", y, x) 565 } 566 x, y = y, x 567 } 568 569 // Type elements (array, slice, etc. elements) use emode for unification. 570 // Element types must match exactly if the types are used in an assignment. 571 emode := mode 572 if mode&assign != 0 { 573 emode |= exact 574 } 575 576 // Continue with unaliased types but don't lose original alias names, if any (go.dev/issue/67628). 577 xorig, x := x, Unalias(x) 578 yorig, y := y, Unalias(y) 579 580 switch x := x.(type) { 581 case *Basic: 582 // Basic types are singletons except for the rune and byte 583 // aliases, thus we cannot solely rely on the x == y check 584 // above. See also comment in TypeName.IsAlias. 585 if y, ok := y.(*Basic); ok { 586 return x.kind == y.kind 587 } 588 589 case *Array: 590 // Two array types unify if they have the same array length 591 // and their element types unify. 592 if y, ok := y.(*Array); ok { 593 // If one or both array lengths are unknown (< 0) due to some error, 594 // assume they are the same to avoid spurious follow-on errors. 595 return (x.len < 0 || y.len < 0 || x.len == y.len) && u.nify(x.elem, y.elem, emode, p) 596 } 597 598 case *Slice: 599 // Two slice types unify if their element types unify. 600 if y, ok := y.(*Slice); ok { 601 return u.nify(x.elem, y.elem, emode, p) 602 } 603 604 case *Struct: 605 // Two struct types unify if they have the same sequence of fields, 606 // and if corresponding fields have the same names, their (field) types unify, 607 // and they have identical tags. Two embedded fields are considered to have the same 608 // name. Lower-case field names from different packages are always different. 609 if y, ok := y.(*Struct); ok { 610 if x.NumFields() == y.NumFields() { 611 for i, f := range x.fields { 612 g := y.fields[i] 613 if f.embedded != g.embedded || 614 x.Tag(i) != y.Tag(i) || 615 !f.sameId(g.pkg, g.name, false) || 616 !u.nify(f.typ, g.typ, emode, p) { 617 return false 618 } 619 } 620 return true 621 } 622 } 623 624 case *Pointer: 625 // Two pointer types unify if their base types unify. 626 if y, ok := y.(*Pointer); ok { 627 return u.nify(x.base, y.base, emode, p) 628 } 629 630 case *Tuple: 631 // Two tuples types unify if they have the same number of elements 632 // and the types of corresponding elements unify. 633 if y, ok := y.(*Tuple); ok { 634 if x.Len() == y.Len() { 635 if x != nil { 636 for i, v := range x.vars { 637 w := y.vars[i] 638 if !u.nify(v.typ, w.typ, mode, p) { 639 return false 640 } 641 } 642 } 643 return true 644 } 645 } 646 647 case *Signature: 648 // Two function types unify if they have the same number of parameters 649 // and result values, corresponding parameter and result types unify, 650 // and either both functions are variadic or neither is. 651 // Parameter and result names are not required to match. 652 // TODO(gri) handle type parameters or document why we can ignore them. 653 if y, ok := y.(*Signature); ok { 654 return x.variadic == y.variadic && 655 u.nify(x.params, y.params, emode, p) && 656 u.nify(x.results, y.results, emode, p) 657 } 658 659 case *Interface: 660 assert(!u.enableInterfaceInference || mode&exact != 0) // handled before this switch 661 662 // Two interface types unify if they have the same set of methods with 663 // the same names, and corresponding function types unify. 664 // Lower-case method names from different packages are always different. 665 // The order of the methods is irrelevant. 666 if y, ok := y.(*Interface); ok { 667 xset := x.typeSet() 668 yset := y.typeSet() 669 if xset.comparable != yset.comparable { 670 return false 671 } 672 if !xset.terms.equal(yset.terms) { 673 return false 674 } 675 a := xset.methods 676 b := yset.methods 677 if len(a) == len(b) { 678 // Interface types are the only types where cycles can occur 679 // that are not "terminated" via named types; and such cycles 680 // can only be created via method parameter types that are 681 // anonymous interfaces (directly or indirectly) embedding 682 // the current interface. Example: 683 // 684 // type T interface { 685 // m() interface{T} 686 // } 687 // 688 // If two such (differently named) interfaces are compared, 689 // endless recursion occurs if the cycle is not detected. 690 // 691 // If x and y were compared before, they must be equal 692 // (if they were not, the recursion would have stopped); 693 // search the ifacePair stack for the same pair. 694 // 695 // This is a quadratic algorithm, but in practice these stacks 696 // are extremely short (bounded by the nesting depth of interface 697 // type declarations that recur via parameter types, an extremely 698 // rare occurrence). An alternative implementation might use a 699 // "visited" map, but that is probably less efficient overall. 700 q := &ifacePair{x, y, p} 701 for p != nil { 702 if p.identical(q) { 703 return true // same pair was compared before 704 } 705 p = p.prev 706 } 707 if debug { 708 assertSortedMethods(a) 709 assertSortedMethods(b) 710 } 711 for i, f := range a { 712 g := b[i] 713 if f.Id() != g.Id() || !u.nify(f.typ, g.typ, exact, q) { 714 return false 715 } 716 } 717 return true 718 } 719 } 720 721 case *Map: 722 // Two map types unify if their key and value types unify. 723 if y, ok := y.(*Map); ok { 724 return u.nify(x.key, y.key, emode, p) && u.nify(x.elem, y.elem, emode, p) 725 } 726 727 case *Chan: 728 // Two channel types unify if their value types unify 729 // and if they have the same direction. 730 // The channel direction is ignored for inexact unification. 731 if y, ok := y.(*Chan); ok { 732 return (mode&exact == 0 || x.dir == y.dir) && u.nify(x.elem, y.elem, emode, p) 733 } 734 735 case *Named: 736 // Two named types unify if their type names originate in the same type declaration. 737 // If they are instantiated, their type argument lists must unify. 738 if y := asNamed(y); y != nil { 739 // Check type arguments before origins so they unify 740 // even if the origins don't match; for better error 741 // messages (see go.dev/issue/53692). 742 xargs := x.TypeArgs().list() 743 yargs := y.TypeArgs().list() 744 if len(xargs) != len(yargs) { 745 return false 746 } 747 for i, xarg := range xargs { 748 if !u.nify(xarg, yargs[i], mode, p) { 749 return false 750 } 751 } 752 return identicalOrigin(x, y) 753 } 754 755 case *TypeParam: 756 // x must be an unbound type parameter (see comment above). 757 if debug { 758 assert(u.asBoundTypeParam(x) == nil) 759 } 760 // By definition, a valid type argument must be in the type set of 761 // the respective type constraint. Therefore, the type argument's 762 // underlying type must be in the set of underlying types of that 763 // constraint. If there is a single such underlying type, it's the 764 // constraint's core type. It must match the type argument's under- 765 // lying type, irrespective of whether the actual type argument, 766 // which may be a defined type, is actually in the type set (that 767 // will be determined at instantiation time). 768 // Thus, if we have the core type of an unbound type parameter, 769 // we know the structure of the possible types satisfying such 770 // parameters. Use that core type for further unification 771 // (see go.dev/issue/50755 for a test case). 772 if enableCoreTypeUnification { 773 // Because the core type is always an underlying type, 774 // unification will take care of matching against a 775 // defined or literal type automatically. 776 // If y is also an unbound type parameter, we will end 777 // up here again with x and y swapped, so we don't 778 // need to take care of that case separately. 779 if cx := coreType(x); cx != nil { 780 if traceInference { 781 u.tracef("core %s ≡ %s", xorig, yorig) 782 } 783 // If y is a defined type, it may not match against cx which 784 // is an underlying type (incl. int, string, etc.). Use assign 785 // mode here so that the unifier automatically takes under(y) 786 // if necessary. 787 return u.nify(cx, yorig, assign, p) 788 } 789 } 790 // x != y and there's nothing to do 791 792 case nil: 793 // avoid a crash in case of nil type 794 795 default: 796 panic(sprintf(nil, nil, true, "u.nify(%s, %s, %d)", xorig, yorig, mode)) 797 } 798 799 return false 800 } 801