1 // Copyright 2020 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 type unification. 6 // 7 // Type unification attempts to make two types x and y structurally 8 // equivalent by determining the types for a given list of (bound) 9 // type parameters which may occur within x and y. If x and y are 10 // structurally different (say []T vs chan T), or conflicting 11 // types are determined for type parameters, unification fails. 12 // If unification succeeds, as a side-effect, the types of the 13 // bound type parameters may be determined. 14 // 15 // Unification typically requires multiple calls u.unify(x, y) to 16 // a given unifier u, with various combinations of types x and y. 17 // In each call, additional type parameter types may be determined 18 // as a side effect and recorded in u. 19 // If a call fails (returns false), unification fails. 20 // 21 // In the unification context, structural equivalence of two types 22 // ignores the difference between a defined type and its underlying 23 // type if one type is a defined type and the other one is not. 24 // It also ignores the difference between an (external, unbound) 25 // type parameter and its core type. 26 // If two types are not structurally equivalent, they cannot be Go 27 // identical types. On the other hand, if they are structurally 28 // equivalent, they may be Go identical or at least assignable, or 29 // they may be in the type set of a constraint. 30 // Whether they indeed are identical or assignable is determined 31 // upon instantiation and function argument passing. 32 33 package types2 34 35 import ( 36 "bytes" 37 "fmt" 38 "sort" 39 "strings" 40 ) 41 42 const ( 43 // Upper limit for recursion depth. Used to catch infinite recursions 44 // due to implementation issues (e.g., see issues go.dev/issue/48619, go.dev/issue/48656). 45 unificationDepthLimit = 50 46 47 // Whether to panic when unificationDepthLimit is reached. 48 // If disabled, a recursion depth overflow results in a (quiet) 49 // unification failure. 50 panicAtUnificationDepthLimit = true 51 52 // If enableCoreTypeUnification is set, unification will consider 53 // the core types, if any, of non-local (unbound) type parameters. 54 enableCoreTypeUnification = true 55 56 // If traceInference is set, unification will print a trace of its operation. 57 // Interpretation of trace: 58 // x ≡ y attempt to unify types x and y 59 // p ➞ y type parameter p is set to type y (p is inferred to be y) 60 // p ⇄ q type parameters p and q match (p is inferred to be q and vice versa) 61 // x ≢ y types x and y cannot be unified 62 // [p, q, ...] ➞ [x, y, ...] mapping from type parameters to types 63 traceInference = false 64 ) 65 66 // A unifier maintains a list of type parameters and 67 // corresponding types inferred for each type parameter. 68 // A unifier is created by calling newUnifier. 69 type unifier struct { 70 check *Checker 71 // handles maps each type parameter to its inferred type through 72 // an indirection *Type called (inferred type) "handle". 73 // Initially, each type parameter has its own, separate handle, 74 // with a nil (i.e., not yet inferred) type. 75 // After a type parameter P is unified with a type parameter Q, 76 // P and Q share the same handle (and thus type). This ensures 77 // that inferring the type for a given type parameter P will 78 // automatically infer the same type for all other parameters 79 // unified (joined) with P. 80 handles map[*TypeParam]*Type 81 depth int // recursion depth during unification 82 enableInterfaceInference bool // use shared methods for better inference 83 } 84 85 // newUnifier returns a new unifier initialized with the given type parameter 86 // and corresponding type argument lists. The type argument list may be shorter 87 // than the type parameter list, and it may contain nil types. Matching type 88 // parameters and arguments must have the same index. 89 func newUnifier(check *Checker, tparams []*TypeParam, targs []Type, enableInterfaceInference bool) *unifier { 90 assert(len(tparams) >= len(targs)) 91 handles := make(map[*TypeParam]*Type, len(tparams)) 92 // Allocate all handles up-front: in a correct program, all type parameters 93 // must be resolved and thus eventually will get a handle. 94 // Also, sharing of handles caused by unified type parameters is rare and 95 // so it's ok to not optimize for that case (and delay handle allocation). 96 for i, x := range tparams { 97 var t Type 98 if i < len(targs) { 99 t = targs[i] 100 } 101 handles[x] = &t 102 } 103 return &unifier{check, handles, 0, enableInterfaceInference} 104 } 105 106 // unifyMode controls the behavior of the unifier. 107 type unifyMode uint 108 109 const ( 110 // If assign is set, we are unifying types involved in an assignment: 111 // they may match inexactly at the top, but element types must match 112 // exactly. 113 assign unifyMode = 1 << iota 114 115 // If exact is set, types unify if they are identical (or can be 116 // made identical with suitable arguments for type parameters). 117 // Otherwise, a named type and a type literal unify if their 118 // underlying types unify, channel directions are ignored, and 119 // if there is an interface, the other type must implement the 120 // interface. 121 exact 122 ) 123 124 func (m unifyMode) String() string { 125 switch m { 126 case 0: 127 return "inexact" 128 case assign: 129 return "assign" 130 case exact: 131 return "exact" 132 case assign | exact: 133 return "assign, exact" 134 } 135 return fmt.Sprintf("mode %d", m) 136 } 137 138 // unify attempts to unify x and y and reports whether it succeeded. 139 // As a side-effect, types may be inferred for type parameters. 140 // The mode parameter controls how types are compared. 141 func (u *unifier) unify(x, y Type, mode unifyMode) bool { 142 return u.nify(x, y, mode, nil) 143 } 144 145 func (u *unifier) tracef(format string, args ...any) { 146 // TODO(gri) consider adjusting this to use Checker.trace 147 fmt.Println(strings.Repeat(". ", u.depth) + sprintf(nil, true, format, args...)) 148 } 149 150 // String returns a string representation of the current mapping 151 // from type parameters to types. 152 func (u *unifier) String() string { 153 // sort type parameters for reproducible strings 154 tparams := make(typeParamsById, len(u.handles)) 155 i := 0 156 for tpar := range u.handles { 157 tparams[i] = tpar 158 i++ 159 } 160 sort.Sort(tparams) 161 162 var buf bytes.Buffer 163 w := newTypeWriter(&buf, nil) 164 w.byte('[') 165 for i, x := range tparams { 166 if i > 0 { 167 w.string(", ") 168 } 169 w.typ(x) 170 w.string(": ") 171 w.typ(u.at(x)) 172 } 173 w.byte(']') 174 return buf.String() 175 } 176 177 type typeParamsById []*TypeParam 178 179 func (s typeParamsById) Len() int { return len(s) } 180 func (s typeParamsById) Less(i, j int) bool { return s[i].id < s[j].id } 181 func (s typeParamsById) Swap(i, j int) { s[i], s[j] = s[j], s[i] } 182 183 // join unifies the given type parameters x and y. 184 // If both type parameters already have a type associated with them 185 // and they are not joined, join fails and returns false. 186 func (u *unifier) join(x, y *TypeParam) bool { 187 if traceInference { 188 u.tracef("%s ⇄ %s", x, y) 189 } 190 switch hx, hy := u.handles[x], u.handles[y]; { 191 case hx == hy: 192 // Both type parameters already share the same handle. Nothing to do. 193 case *hx != nil && *hy != nil: 194 // Both type parameters have (possibly different) inferred types. Cannot join. 195 return false 196 case *hx != nil: 197 // Only type parameter x has an inferred type. Use handle of x. 198 u.setHandle(y, hx) 199 // This case is treated like the default case. 200 // case *hy != nil: 201 // // Only type parameter y has an inferred type. Use handle of y. 202 // u.setHandle(x, hy) 203 default: 204 // Neither type parameter has an inferred type. Use handle of y. 205 u.setHandle(x, hy) 206 } 207 return true 208 } 209 210 // asBoundTypeParam returns x.(*TypeParam) if x is a type parameter recorded with u. 211 // Otherwise, the result is nil. 212 func (u *unifier) asBoundTypeParam(x Type) *TypeParam { 213 if x, _ := Unalias(x).(*TypeParam); x != nil { 214 if _, found := u.handles[x]; found { 215 return x 216 } 217 } 218 return nil 219 } 220 221 // setHandle sets the handle for type parameter x 222 // (and all its joined type parameters) to h. 223 func (u *unifier) setHandle(x *TypeParam, h *Type) { 224 hx := u.handles[x] 225 assert(hx != nil) 226 for y, hy := range u.handles { 227 if hy == hx { 228 u.handles[y] = h 229 } 230 } 231 } 232 233 // at returns the (possibly nil) type for type parameter x. 234 func (u *unifier) at(x *TypeParam) Type { 235 return *u.handles[x] 236 } 237 238 // set sets the type t for type parameter x; 239 // t must not be nil. 240 func (u *unifier) set(x *TypeParam, t Type) { 241 assert(t != nil) 242 if traceInference { 243 u.tracef("%s ➞ %s", x, t) 244 } 245 *u.handles[x] = t 246 } 247 248 // unknowns returns the number of type parameters for which no type has been set yet. 249 func (u *unifier) unknowns() int { 250 n := 0 251 for _, h := range u.handles { 252 if *h == nil { 253 n++ 254 } 255 } 256 return n 257 } 258 259 // inferred returns the list of inferred types for the given type parameter list. 260 // The result is never nil and has the same length as tparams; result types that 261 // could not be inferred are nil. Corresponding type parameters and result types 262 // have identical indices. 263 func (u *unifier) inferred(tparams []*TypeParam) []Type { 264 list := make([]Type, len(tparams)) 265 for i, x := range tparams { 266 list[i] = u.at(x) 267 } 268 return list 269 } 270 271 // asInterface returns the underlying type of x as an interface if 272 // it is a non-type parameter interface. Otherwise it returns nil. 273 func asInterface(x Type) (i *Interface) { 274 if _, ok := Unalias(x).(*TypeParam); !ok { 275 i, _ = x.Underlying().(*Interface) 276 } 277 return i 278 } 279 280 // nify implements the core unification algorithm which is an 281 // adapted version of Checker.identical. For changes to that 282 // code the corresponding changes should be made here. 283 // Must not be called directly from outside the unifier. 284 func (u *unifier) nify(x, y Type, mode unifyMode, p *ifacePair) (result bool) { 285 u.depth++ 286 if traceInference { 287 u.tracef("%s ≡ %s\t// %s", x, y, mode) 288 } 289 defer func() { 290 if traceInference && !result { 291 u.tracef("%s ≢ %s", x, y) 292 } 293 u.depth-- 294 }() 295 296 // nothing to do if x == y 297 if x == y || Unalias(x) == Unalias(y) { 298 return true 299 } 300 301 // Stop gap for cases where unification fails. 302 if u.depth > unificationDepthLimit { 303 if traceInference { 304 u.tracef("depth %d >= %d", u.depth, unificationDepthLimit) 305 } 306 if panicAtUnificationDepthLimit { 307 panic("unification reached recursion depth limit") 308 } 309 return false 310 } 311 312 // Unification is symmetric, so we can swap the operands. 313 // Ensure that if we have at least one 314 // - defined type, make sure one is in y 315 // - type parameter recorded with u, make sure one is in x 316 if asNamed(x) != nil || u.asBoundTypeParam(y) != nil { 317 if traceInference { 318 u.tracef("%s ≡ %s\t// swap", y, x) 319 } 320 x, y = y, x 321 } 322 323 // Unification will fail if we match a defined type against a type literal. 324 // If we are matching types in an assignment, at the top-level, types with 325 // the same type structure are permitted as long as at least one of them 326 // is not a defined type. To accommodate for that possibility, we continue 327 // unification with the underlying type of a defined type if the other type 328 // is a type literal. This is controlled by the exact unification mode. 329 // We also continue if the other type is a basic type because basic types 330 // are valid underlying types and may appear as core types of type constraints. 331 // If we exclude them, inferred defined types for type parameters may not 332 // match against the core types of their constraints (even though they might 333 // correctly match against some of the types in the constraint's type set). 334 // Finally, if unification (incorrectly) succeeds by matching the underlying 335 // type of a defined type against a basic type (because we include basic types 336 // as type literals here), and if that leads to an incorrectly inferred type, 337 // we will fail at function instantiation or argument assignment time. 338 // 339 // If we have at least one defined type, there is one in y. 340 if ny := asNamed(y); mode&exact == 0 && ny != nil && isTypeLit(x) && !(u.enableInterfaceInference && IsInterface(x)) { 341 if traceInference { 342 u.tracef("%s ≡ under %s", x, ny) 343 } 344 y = ny.Underlying() 345 // Per the spec, a defined type cannot have an underlying type 346 // that is a type parameter. 347 assert(!isTypeParam(y)) 348 // x and y may be identical now 349 if x == y || Unalias(x) == Unalias(y) { 350 return true 351 } 352 } 353 354 // Cases where at least one of x or y is a type parameter recorded with u. 355 // If we have at least one type parameter, there is one in x. 356 // If we have exactly one type parameter, because it is in x, 357 // isTypeLit(x) is false and y was not changed above. In other 358 // words, if y was a defined type, it is still a defined type 359 // (relevant for the logic below). 360 switch px, py := u.asBoundTypeParam(x), u.asBoundTypeParam(y); { 361 case px != nil && py != nil: 362 // both x and y are type parameters 363 if u.join(px, py) { 364 return true 365 } 366 // both x and y have an inferred type - they must match 367 return u.nify(u.at(px), u.at(py), mode, p) 368 369 case px != nil: 370 // x is a type parameter, y is not 371 if x := u.at(px); x != nil { 372 // x has an inferred type which must match y 373 if u.nify(x, y, mode, p) { 374 // We have a match, possibly through underlying types. 375 xi := asInterface(x) 376 yi := asInterface(y) 377 xn := asNamed(x) != nil 378 yn := asNamed(y) != nil 379 // If we have two interfaces, what to do depends on 380 // whether they are named and their method sets. 381 if xi != nil && yi != nil { 382 // Both types are interfaces. 383 // If both types are defined types, they must be identical 384 // because unification doesn't know which type has the "right" name. 385 if xn && yn { 386 return Identical(x, y) 387 } 388 // In all other cases, the method sets must match. 389 // The types unified so we know that corresponding methods 390 // match and we can simply compare the number of methods. 391 // TODO(gri) We may be able to relax this rule and select 392 // the more general interface. But if one of them is a defined 393 // type, it's not clear how to choose and whether we introduce 394 // an order dependency or not. Requiring the same method set 395 // is conservative. 396 if len(xi.typeSet().methods) != len(yi.typeSet().methods) { 397 return false 398 } 399 } else if xi != nil || yi != nil { 400 // One but not both of them are interfaces. 401 // In this case, either x or y could be viable matches for the corresponding 402 // type parameter, which means choosing either introduces an order dependence. 403 // Therefore, we must fail unification (go.dev/issue/60933). 404 return false 405 } 406 // If we have inexact unification and one of x or y is a defined type, select the 407 // defined type. This ensures that in a series of types, all matching against the 408 // same type parameter, we infer a defined type if there is one, independent of 409 // order. Type inference or assignment may fail, which is ok. 410 // Selecting a defined type, if any, ensures that we don't lose the type name; 411 // and since we have inexact unification, a value of equally named or matching 412 // undefined type remains assignable (go.dev/issue/43056). 413 // 414 // Similarly, if we have inexact unification and there are no defined types but 415 // channel types, select a directed channel, if any. This ensures that in a series 416 // of unnamed types, all matching against the same type parameter, we infer the 417 // directed channel if there is one, independent of order. 418 // Selecting a directional channel, if any, ensures that a value of another 419 // inexactly unifying channel type remains assignable (go.dev/issue/62157). 420 // 421 // If we have multiple defined channel types, they are either identical or we 422 // have assignment conflicts, so we can ignore directionality in this case. 423 // 424 // If we have defined and literal channel types, a defined type wins to avoid 425 // order dependencies. 426 if mode&exact == 0 { 427 switch { 428 case xn: 429 // x is a defined type: nothing to do. 430 case yn: 431 // x is not a defined type and y is a defined type: select y. 432 u.set(px, y) 433 default: 434 // Neither x nor y are defined types. 435 if yc, _ := y.Underlying().(*Chan); yc != nil && yc.dir != SendRecv { 436 // y is a directed channel type: select y. 437 u.set(px, y) 438 } 439 } 440 } 441 return true 442 } 443 return false 444 } 445 // otherwise, infer type from y 446 u.set(px, y) 447 return true 448 } 449 450 // x != y if we get here 451 assert(x != y && Unalias(x) != Unalias(y)) 452 453 // If u.EnableInterfaceInference is set and we don't require exact unification, 454 // if both types are interfaces, one interface must have a subset of the 455 // methods of the other and corresponding method signatures must unify. 456 // If only one type is an interface, all its methods must be present in the 457 // other type and corresponding method signatures must unify. 458 if u.enableInterfaceInference && mode&exact == 0 { 459 // One or both interfaces may be defined types. 460 // Look under the name, but not under type parameters (go.dev/issue/60564). 461 xi := asInterface(x) 462 yi := asInterface(y) 463 // If we have two interfaces, check the type terms for equivalence, 464 // and unify common methods if possible. 465 if xi != nil && yi != nil { 466 xset := xi.typeSet() 467 yset := yi.typeSet() 468 if xset.comparable != yset.comparable { 469 return false 470 } 471 // For now we require terms to be equal. 472 // We should be able to relax this as well, eventually. 473 if !xset.terms.equal(yset.terms) { 474 return false 475 } 476 // Interface types are the only types where cycles can occur 477 // that are not "terminated" via named types; and such cycles 478 // can only be created via method parameter types that are 479 // anonymous interfaces (directly or indirectly) embedding 480 // the current interface. Example: 481 // 482 // type T interface { 483 // m() interface{T} 484 // } 485 // 486 // If two such (differently named) interfaces are compared, 487 // endless recursion occurs if the cycle is not detected. 488 // 489 // If x and y were compared before, they must be equal 490 // (if they were not, the recursion would have stopped); 491 // search the ifacePair stack for the same pair. 492 // 493 // This is a quadratic algorithm, but in practice these stacks 494 // are extremely short (bounded by the nesting depth of interface 495 // type declarations that recur via parameter types, an extremely 496 // rare occurrence). An alternative implementation might use a 497 // "visited" map, but that is probably less efficient overall. 498 q := &ifacePair{xi, yi, p} 499 for p != nil { 500 if p.identical(q) { 501 return true // same pair was compared before 502 } 503 p = p.prev 504 } 505 // The method set of x must be a subset of the method set 506 // of y or vice versa, and the common methods must unify. 507 xmethods := xset.methods 508 ymethods := yset.methods 509 // The smaller method set must be the subset, if it exists. 510 if len(xmethods) > len(ymethods) { 511 xmethods, ymethods = ymethods, xmethods 512 } 513 // len(xmethods) <= len(ymethods) 514 // Collect the ymethods in a map for quick lookup. 515 ymap := make(map[string]*Func, len(ymethods)) 516 for _, ym := range ymethods { 517 ymap[ym.Id()] = ym 518 } 519 // All xmethods must exist in ymethods and corresponding signatures must unify. 520 for _, xm := range xmethods { 521 if ym := ymap[xm.Id()]; ym == nil || !u.nify(xm.typ, ym.typ, exact, p) { 522 return false 523 } 524 } 525 return true 526 } 527 528 // We don't have two interfaces. If we have one, make sure it's in xi. 529 if yi != nil { 530 xi = yi 531 y = x 532 } 533 534 // If we have one interface, at a minimum each of the interface methods 535 // must be implemented and thus unify with a corresponding method from 536 // the non-interface type, otherwise unification fails. 537 if xi != nil { 538 // All xi methods must exist in y and corresponding signatures must unify. 539 // A generic method never satisfies an interface method, so fail rather 540 // than unify ym's own type parameter into an inference variable. 541 xmethods := xi.typeSet().methods 542 for _, xm := range xmethods { 543 obj, _, _ := LookupFieldOrMethod(y, false, xm.pkg, xm.name) 544 ym, _ := obj.(*Func) 545 if ym == nil { 546 return false 547 } 548 u.check.objDecl(ym) // ensure fully set-up signature 549 if ym.Signature().TypeParams() != nil || !u.nify(xm.typ, ym.typ, exact, p) { 550 return false 551 } 552 } 553 return true 554 } 555 } 556 557 // Unless we have exact unification, neither x nor y are interfaces now. 558 // Except for unbound type parameters (see below), x and y must be structurally 559 // equivalent to unify. 560 561 // If we get here and x or y is a type parameter, they are unbound 562 // (not recorded with the unifier). 563 // Ensure that if we have at least one type parameter, it is in x 564 // (the earlier swap checks for _recorded_ type parameters only). 565 // This ensures that the switch switches on the type parameter. 566 // 567 // TODO(gri) Factor out type parameter handling from the switch. 568 if isTypeParam(y) { 569 if traceInference { 570 u.tracef("%s ≡ %s\t// swap", y, x) 571 } 572 x, y = y, x 573 } 574 575 // Type elements (array, slice, etc. elements) use emode for unification. 576 // Element types must match exactly if the types are used in an assignment. 577 emode := mode 578 if mode&assign != 0 { 579 emode |= exact 580 } 581 582 // Continue with unaliased types but don't lose original alias names, if any (go.dev/issue/67628). 583 xorig, x := x, Unalias(x) 584 yorig, y := y, Unalias(y) 585 586 switch x := x.(type) { 587 case *Basic: 588 // Basic types are singletons except for the rune and byte 589 // aliases, thus we cannot solely rely on the x == y check 590 // above. See also comment in TypeName.IsAlias. 591 if y, ok := y.(*Basic); ok { 592 return x.kind == y.kind 593 } 594 595 case *Array: 596 // Two array types unify if they have the same array length 597 // and their element types unify. 598 if y, ok := y.(*Array); ok { 599 // If one or both array lengths are unknown (< 0) due to some error, 600 // assume they are the same to avoid spurious follow-on errors. 601 return (x.len < 0 || y.len < 0 || x.len == y.len) && u.nify(x.elem, y.elem, emode, p) 602 } 603 604 case *Slice: 605 // Two slice types unify if their element types unify. 606 if y, ok := y.(*Slice); ok { 607 return u.nify(x.elem, y.elem, emode, p) 608 } 609 610 case *Struct: 611 // Two struct types unify if they have the same sequence of fields, 612 // and if corresponding fields have the same names, their (field) types unify, 613 // and they have identical tags. Two embedded fields are considered to have the same 614 // name. Lower-case field names from different packages are always different. 615 if y, ok := y.(*Struct); ok { 616 if x.NumFields() == y.NumFields() { 617 for i, f := range x.fields { 618 g := y.fields[i] 619 if f.embedded != g.embedded || 620 x.Tag(i) != y.Tag(i) || 621 !f.sameId(g.pkg, g.name, false) || 622 !u.nify(f.typ, g.typ, emode, p) { 623 return false 624 } 625 } 626 return true 627 } 628 } 629 630 case *Pointer: 631 // Two pointer types unify if their base types unify. 632 if y, ok := y.(*Pointer); ok { 633 return u.nify(x.base, y.base, emode, p) 634 } 635 636 case *Tuple: 637 // Two tuples types unify if they have the same number of elements 638 // and the types of corresponding elements unify. 639 if y, ok := y.(*Tuple); ok { 640 if x.Len() == y.Len() { 641 if x != nil { 642 for i, v := range x.vars { 643 w := y.vars[i] 644 if !u.nify(v.typ, w.typ, mode, p) { 645 return false 646 } 647 } 648 } 649 return true 650 } 651 } 652 653 case *Signature: 654 // Two function types unify if they have the same number of parameters 655 // and result values, corresponding parameter and result types unify, 656 // and either both functions are variadic or neither is. 657 // Parameter and result names are not required to match. 658 // TODO(gri) handle type parameters or document why we can ignore them. 659 if y, ok := y.(*Signature); ok { 660 return x.variadic == y.variadic && 661 u.nify(x.params, y.params, emode, p) && 662 u.nify(x.results, y.results, emode, p) 663 } 664 665 case *Interface: 666 assert(!u.enableInterfaceInference || mode&exact != 0) // handled before this switch 667 668 // Two interface types unify if they have the same set of methods with 669 // the same names, and corresponding function types unify. 670 // Lower-case method names from different packages are always different. 671 // The order of the methods is irrelevant. 672 if y, ok := y.(*Interface); ok { 673 xset := x.typeSet() 674 yset := y.typeSet() 675 if xset.comparable != yset.comparable { 676 return false 677 } 678 if !xset.terms.equal(yset.terms) { 679 return false 680 } 681 a := xset.methods 682 b := yset.methods 683 if len(a) == len(b) { 684 // Interface types are the only types where cycles can occur 685 // that are not "terminated" via named types; and such cycles 686 // can only be created via method parameter types that are 687 // anonymous interfaces (directly or indirectly) embedding 688 // the current interface. Example: 689 // 690 // type T interface { 691 // m() interface{T} 692 // } 693 // 694 // If two such (differently named) interfaces are compared, 695 // endless recursion occurs if the cycle is not detected. 696 // 697 // If x and y were compared before, they must be equal 698 // (if they were not, the recursion would have stopped); 699 // search the ifacePair stack for the same pair. 700 // 701 // This is a quadratic algorithm, but in practice these stacks 702 // are extremely short (bounded by the nesting depth of interface 703 // type declarations that recur via parameter types, an extremely 704 // rare occurrence). An alternative implementation might use a 705 // "visited" map, but that is probably less efficient overall. 706 q := &ifacePair{x, y, p} 707 for p != nil { 708 if p.identical(q) { 709 return true // same pair was compared before 710 } 711 p = p.prev 712 } 713 if debug { 714 assertSortedMethods(a) 715 assertSortedMethods(b) 716 } 717 for i, f := range a { 718 g := b[i] 719 if f.Id() != g.Id() || !u.nify(f.typ, g.typ, exact, q) { 720 return false 721 } 722 } 723 return true 724 } 725 } 726 727 case *Map: 728 // Two map types unify if their key and value types unify. 729 if y, ok := y.(*Map); ok { 730 return u.nify(x.key, y.key, emode, p) && u.nify(x.elem, y.elem, emode, p) 731 } 732 733 case *Chan: 734 // Two channel types unify if their value types unify 735 // and if they have the same direction. 736 // The channel direction is ignored for inexact unification. 737 if y, ok := y.(*Chan); ok { 738 return (mode&exact == 0 || x.dir == y.dir) && u.nify(x.elem, y.elem, emode, p) 739 } 740 741 case *Named: 742 // Two named types unify if their type names originate in the same type declaration. 743 // If they are instantiated, their type argument lists must unify. 744 if y := asNamed(y); y != nil { 745 // Check type arguments before origins so they unify 746 // even if the origins don't match; for better error 747 // messages (see go.dev/issue/53692). 748 xargs := x.TypeArgs().list() 749 yargs := y.TypeArgs().list() 750 if len(xargs) != len(yargs) { 751 return false 752 } 753 for i, xarg := range xargs { 754 if !u.nify(xarg, yargs[i], mode, p) { 755 return false 756 } 757 } 758 return identicalOrigin(x, y) 759 } 760 761 case *TypeParam: 762 // x must be an unbound type parameter (see comment above). 763 if debug { 764 assert(u.asBoundTypeParam(x) == nil) 765 } 766 // By definition, a valid type argument must be in the type set of 767 // the respective type constraint. Therefore, the type argument's 768 // underlying type must be in the set of underlying types of that 769 // constraint. If there is a single such underlying type, it's the 770 // constraint's core type. It must match the type argument's under- 771 // lying type, irrespective of whether the actual type argument, 772 // which may be a defined type, is actually in the type set (that 773 // will be determined at instantiation time). 774 // Thus, if we have the core type of an unbound type parameter, 775 // we know the structure of the possible types satisfying such 776 // parameters. Use that core type for further unification 777 // (see go.dev/issue/50755 for a test case). 778 if enableCoreTypeUnification { 779 // Because the core type is always an underlying type, 780 // unification will take care of matching against a 781 // defined or literal type automatically. 782 // If y is also an unbound type parameter, we will end 783 // up here again with x and y swapped, so we don't 784 // need to take care of that case separately. 785 if cx, _ := commonUnder(x, nil); cx != nil { 786 if traceInference { 787 u.tracef("core %s ≡ %s", xorig, yorig) 788 } 789 // If y is a defined type, it may not match against cx which 790 // is an underlying type (incl. int, string, etc.). Use assign 791 // mode here so that the unifier automatically uses y.Underlying() 792 // if necessary. 793 return u.nify(cx, yorig, assign, p) 794 } 795 } 796 // x != y and there's nothing to do 797 798 case nil: 799 // avoid a crash in case of nil type 800 801 default: 802 panic(sprintf(nil, true, "u.nify(%s, %s, %d)", xorig, yorig, mode)) 803 } 804 805 return false 806 } 807