Source file src/cmd/compile/internal/types2/infer.go
1 // Copyright 2018 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 parameter inference. 6 7 package types2 8 9 import ( 10 "cmd/compile/internal/syntax" 11 "fmt" 12 . "internal/types/errors" 13 "strings" 14 ) 15 16 // If enableReverseTypeInference is set, uninstantiated and 17 // partially instantiated generic functions may be assigned 18 // (incl. returned) to variables of function type and type 19 // inference will attempt to infer the missing type arguments. 20 // Available with go1.21. 21 const enableReverseTypeInference = true // disable for debugging 22 23 // infer attempts to infer the complete set of type arguments for generic function instantiation/call 24 // based on the given type parameters tparams, type arguments targs, function parameters params, and 25 // function arguments args, if any. There must be at least one type parameter, no more type arguments 26 // than type parameters, and params and args must match in number (incl. zero). 27 // If reverse is set, an error message's contents are reversed for a better error message for some 28 // errors related to reverse type inference (where the function call is synthetic). 29 // If successful, infer returns the complete list of given and inferred type arguments, one for each 30 // type parameter. Otherwise the result is nil and appropriate errors will be reported. 31 func (check *Checker) infer(pos syntax.Pos, tparams []*TypeParam, targs []Type, params *Tuple, args []*operand, reverse bool) (inferred []Type) { 32 // Don't verify result conditions if there's no error handler installed: 33 // in that case, an error leads to an exit panic and the result value may 34 // be incorrect. But in that case it doesn't matter because callers won't 35 // be able to use it either. 36 if check.conf.Error != nil { 37 defer func() { 38 assert(inferred == nil || len(inferred) == len(tparams) && !containsNil(inferred)) 39 }() 40 } 41 42 if traceInference { 43 check.dump("== infer : %s%s ➞ %s", tparams, params, targs) // aligned with rename print below 44 defer func() { 45 check.dump("=> %s ➞ %s\n", tparams, inferred) 46 }() 47 } 48 49 // There must be at least one type parameter, and no more type arguments than type parameters. 50 n := len(tparams) 51 assert(n > 0 && len(targs) <= n) 52 53 // Parameters and arguments must match in number. 54 assert(params.Len() == len(args)) 55 56 // If we already have all type arguments, we're done. 57 if len(targs) == n && !containsNil(targs) { 58 return targs 59 } 60 61 // If we have invalid (ordinary) arguments, an error was reported before. 62 // Avoid additional inference errors and exit early (go.dev/issue/60434). 63 for _, arg := range args { 64 if arg.mode == invalid { 65 return nil 66 } 67 } 68 69 // Make sure we have a "full" list of type arguments, some of which may 70 // be nil (unknown). Make a copy so as to not clobber the incoming slice. 71 if len(targs) < n { 72 targs2 := make([]Type, n) 73 copy(targs2, targs) 74 targs = targs2 75 } 76 // len(targs) == n 77 78 // Continue with the type arguments we have. Avoid matching generic 79 // parameters that already have type arguments against function arguments: 80 // It may fail because matching uses type identity while parameter passing 81 // uses assignment rules. Instantiate the parameter list with the type 82 // arguments we have, and continue with that parameter list. 83 84 // Substitute type arguments for their respective type parameters in params, 85 // if any. Note that nil targs entries are ignored by check.subst. 86 // We do this for better error messages; it's not needed for correctness. 87 // For instance, given: 88 // 89 // func f[P, Q any](P, Q) {} 90 // 91 // func _(s string) { 92 // f[int](s, s) // ERROR 93 // } 94 // 95 // With substitution, we get the error: 96 // "cannot use s (variable of type string) as int value in argument to f[int]" 97 // 98 // Without substitution we get the (worse) error: 99 // "type string of s does not match inferred type int for P" 100 // even though the type int was provided (not inferred) for P. 101 // 102 // TODO(gri) We might be able to finesse this in the error message reporting 103 // (which only happens in case of an error) and then avoid doing 104 // the substitution (which always happens). 105 if params.Len() > 0 { 106 smap := makeSubstMap(tparams, targs) 107 params = check.subst(nopos, params, smap, nil, check.context()).(*Tuple) 108 } 109 110 // Unify parameter and argument types for generic parameters with typed arguments 111 // and collect the indices of generic parameters with untyped arguments. 112 // Terminology: generic parameter = function parameter with a type-parameterized type 113 u := newUnifier(tparams, targs, check.allowVersion(check.pkg, pos, go1_21)) 114 115 errorf := func(tpar, targ Type, arg *operand) { 116 // provide a better error message if we can 117 targs := u.inferred(tparams) 118 if targs[0] == nil { 119 // The first type parameter couldn't be inferred. 120 // If none of them could be inferred, don't try 121 // to provide the inferred type in the error msg. 122 allFailed := true 123 for _, targ := range targs { 124 if targ != nil { 125 allFailed = false 126 break 127 } 128 } 129 if allFailed { 130 check.errorf(arg, CannotInferTypeArgs, "type %s of %s does not match %s (cannot infer %s)", targ, arg.expr, tpar, typeParamsString(tparams)) 131 return 132 } 133 } 134 smap := makeSubstMap(tparams, targs) 135 // TODO(gri): pass a poser here, rather than arg.Pos(). 136 inferred := check.subst(arg.Pos(), tpar, smap, nil, check.context()) 137 // CannotInferTypeArgs indicates a failure of inference, though the actual 138 // error may be better attributed to a user-provided type argument (hence 139 // InvalidTypeArg). We can't differentiate these cases, so fall back on 140 // the more general CannotInferTypeArgs. 141 if inferred != tpar { 142 if reverse { 143 check.errorf(arg, CannotInferTypeArgs, "inferred type %s for %s does not match type %s of %s", inferred, tpar, targ, arg.expr) 144 } else { 145 check.errorf(arg, CannotInferTypeArgs, "type %s of %s does not match inferred type %s for %s", targ, arg.expr, inferred, tpar) 146 } 147 } else { 148 check.errorf(arg, CannotInferTypeArgs, "type %s of %s does not match %s", targ, arg.expr, tpar) 149 } 150 } 151 152 // indices of generic parameters with untyped arguments, for later use 153 var untyped []int 154 155 // --- 1 --- 156 // use information from function arguments 157 158 if traceInference { 159 u.tracef("== function parameters: %s", params) 160 u.tracef("-- function arguments : %s", args) 161 } 162 163 for i, arg := range args { 164 if arg.mode == invalid { 165 // An error was reported earlier. Ignore this arg 166 // and continue, we may still be able to infer all 167 // targs resulting in fewer follow-on errors. 168 // TODO(gri) determine if we still need this check 169 continue 170 } 171 par := params.At(i) 172 if isParameterized(tparams, par.typ) || isParameterized(tparams, arg.typ) { 173 // Function parameters are always typed. Arguments may be untyped. 174 // Collect the indices of untyped arguments and handle them later. 175 if isTyped(arg.typ) { 176 if !u.unify(par.typ, arg.typ, assign) { 177 errorf(par.typ, arg.typ, arg) 178 return nil 179 } 180 } else if _, ok := par.typ.(*TypeParam); ok && !arg.isNil() { 181 // Since default types are all basic (i.e., non-composite) types, an 182 // untyped argument will never match a composite parameter type; the 183 // only parameter type it can possibly match against is a *TypeParam. 184 // Thus, for untyped arguments we only need to look at parameter types 185 // that are single type parameters. 186 // Also, untyped nils don't have a default type and can be ignored. 187 untyped = append(untyped, i) 188 } 189 } 190 } 191 192 if traceInference { 193 inferred := u.inferred(tparams) 194 u.tracef("=> %s ➞ %s\n", tparams, inferred) 195 } 196 197 // --- 2 --- 198 // use information from type parameter constraints 199 200 if traceInference { 201 u.tracef("== type parameters: %s", tparams) 202 } 203 204 // Unify type parameters with their constraints as long 205 // as progress is being made. 206 // 207 // This is an O(n^2) algorithm where n is the number of 208 // type parameters: if there is progress, at least one 209 // type argument is inferred per iteration, and we have 210 // a doubly nested loop. 211 // 212 // In practice this is not a problem because the number 213 // of type parameters tends to be very small (< 5 or so). 214 // (It should be possible for unification to efficiently 215 // signal newly inferred type arguments; then the loops 216 // here could handle the respective type parameters only, 217 // but that will come at a cost of extra complexity which 218 // may not be worth it.) 219 for i := 0; ; i++ { 220 nn := u.unknowns() 221 if traceInference { 222 if i > 0 { 223 fmt.Println() 224 } 225 u.tracef("-- iteration %d", i) 226 } 227 228 for _, tpar := range tparams { 229 tx := u.at(tpar) 230 core, single := coreTerm(tpar) 231 if traceInference { 232 u.tracef("-- type parameter %s = %s: core(%s) = %s, single = %v", tpar, tx, tpar, core, single) 233 } 234 235 // If there is a core term (i.e., a core type with tilde information) 236 // unify the type parameter with the core type. 237 if core != nil { 238 // A type parameter can be unified with its core type in two cases. 239 switch { 240 case tx != nil: 241 // The corresponding type argument tx is known. There are 2 cases: 242 // 1) If the core type has a tilde, per spec requirement for tilde 243 // elements, the core type is an underlying (literal) type. 244 // And because of the tilde, the underlying type of tx must match 245 // against the core type. 246 // But because unify automatically matches a defined type against 247 // an underlying literal type, we can simply unify tx with the 248 // core type. 249 // 2) If the core type doesn't have a tilde, we also must unify tx 250 // with the core type. 251 if !u.unify(tx, core.typ, 0) { 252 // TODO(gri) Type parameters that appear in the constraint and 253 // for which we have type arguments inferred should 254 // use those type arguments for a better error message. 255 check.errorf(pos, CannotInferTypeArgs, "%s (type %s) does not satisfy %s", tpar, tx, tpar.Constraint()) 256 return nil 257 } 258 case single && !core.tilde: 259 // The corresponding type argument tx is unknown and there's a single 260 // specific type and no tilde. 261 // In this case the type argument must be that single type; set it. 262 u.set(tpar, core.typ) 263 } 264 } else { 265 if tx != nil { 266 // We don't have a core type, but the type argument tx is known. 267 // It must have (at least) all the methods of the type constraint, 268 // and the method signatures must unify; otherwise tx cannot satisfy 269 // the constraint. 270 // TODO(gri) Now that unification handles interfaces, this code can 271 // be reduced to calling u.unify(tx, tpar.iface(), assign) 272 // (which will compare signatures exactly as we do below). 273 // We leave it as is for now because missingMethod provides 274 // a failure cause which allows for a better error message. 275 // Eventually, unify should return an error with cause. 276 var cause string 277 constraint := tpar.iface() 278 if m, _ := check.missingMethod(tx, constraint, true, func(x, y Type) bool { return u.unify(x, y, exact) }, &cause); m != nil { 279 // TODO(gri) better error message (see TODO above) 280 check.errorf(pos, CannotInferTypeArgs, "%s (type %s) does not satisfy %s %s", tpar, tx, tpar.Constraint(), cause) 281 return nil 282 } 283 } 284 } 285 } 286 287 if u.unknowns() == nn { 288 break // no progress 289 } 290 } 291 292 if traceInference { 293 inferred := u.inferred(tparams) 294 u.tracef("=> %s ➞ %s\n", tparams, inferred) 295 } 296 297 // --- 3 --- 298 // use information from untyped constants 299 300 if traceInference { 301 u.tracef("== untyped arguments: %v", untyped) 302 } 303 304 // Some generic parameters with untyped arguments may have been given a type by now. 305 // Collect all remaining parameters that don't have a type yet and determine the 306 // maximum untyped type for each of those parameters, if possible. 307 var maxUntyped map[*TypeParam]Type // lazily allocated (we may not need it) 308 for _, index := range untyped { 309 tpar := params.At(index).typ.(*TypeParam) // is type parameter by construction of untyped 310 if u.at(tpar) == nil { 311 arg := args[index] // arg corresponding to tpar 312 if maxUntyped == nil { 313 maxUntyped = make(map[*TypeParam]Type) 314 } 315 max := maxUntyped[tpar] 316 if max == nil { 317 max = arg.typ 318 } else { 319 m := maxType(max, arg.typ) 320 if m == nil { 321 check.errorf(arg, CannotInferTypeArgs, "mismatched types %s and %s (cannot infer %s)", max, arg.typ, tpar) 322 return nil 323 } 324 max = m 325 } 326 maxUntyped[tpar] = max 327 } 328 } 329 // maxUntyped contains the maximum untyped type for each type parameter 330 // which doesn't have a type yet. Set the respective default types. 331 for tpar, typ := range maxUntyped { 332 d := Default(typ) 333 assert(isTyped(d)) 334 u.set(tpar, d) 335 } 336 337 // --- simplify --- 338 339 // u.inferred(tparams) now contains the incoming type arguments plus any additional type 340 // arguments which were inferred. The inferred non-nil entries may still contain 341 // references to other type parameters found in constraints. 342 // For instance, for [A any, B interface{ []C }, C interface{ *A }], if A == int 343 // was given, unification produced the type list [int, []C, *A]. We eliminate the 344 // remaining type parameters by substituting the type parameters in this type list 345 // until nothing changes anymore. 346 inferred = u.inferred(tparams) 347 if debug { 348 for i, targ := range targs { 349 assert(targ == nil || inferred[i] == targ) 350 } 351 } 352 353 // The data structure of each (provided or inferred) type represents a graph, where 354 // each node corresponds to a type and each (directed) vertex points to a component 355 // type. The substitution process described above repeatedly replaces type parameter 356 // nodes in these graphs with the graphs of the types the type parameters stand for, 357 // which creates a new (possibly bigger) graph for each type. 358 // The substitution process will not stop if the replacement graph for a type parameter 359 // also contains that type parameter. 360 // For instance, for [A interface{ *A }], without any type argument provided for A, 361 // unification produces the type list [*A]. Substituting A in *A with the value for 362 // A will lead to infinite expansion by producing [**A], [****A], [********A], etc., 363 // because the graph A -> *A has a cycle through A. 364 // Generally, cycles may occur across multiple type parameters and inferred types 365 // (for instance, consider [P interface{ *Q }, Q interface{ func(P) }]). 366 // We eliminate cycles by walking the graphs for all type parameters. If a cycle 367 // through a type parameter is detected, killCycles nils out the respective type 368 // (in the inferred list) which kills the cycle, and marks the corresponding type 369 // parameter as not inferred. 370 // 371 // TODO(gri) If useful, we could report the respective cycle as an error. We don't 372 // do this now because type inference will fail anyway, and furthermore, 373 // constraints with cycles of this kind cannot currently be satisfied by 374 // any user-supplied type. But should that change, reporting an error 375 // would be wrong. 376 killCycles(tparams, inferred) 377 378 // dirty tracks the indices of all types that may still contain type parameters. 379 // We know that nil type entries and entries corresponding to provided (non-nil) 380 // type arguments are clean, so exclude them from the start. 381 var dirty []int 382 for i, typ := range inferred { 383 if typ != nil && (i >= len(targs) || targs[i] == nil) { 384 dirty = append(dirty, i) 385 } 386 } 387 388 for len(dirty) > 0 { 389 if traceInference { 390 u.tracef("-- simplify %s ➞ %s", tparams, inferred) 391 } 392 // TODO(gri) Instead of creating a new substMap for each iteration, 393 // provide an update operation for substMaps and only change when 394 // needed. Optimization. 395 smap := makeSubstMap(tparams, inferred) 396 n := 0 397 for _, index := range dirty { 398 t0 := inferred[index] 399 if t1 := check.subst(nopos, t0, smap, nil, check.context()); t1 != t0 { 400 // t0 was simplified to t1. 401 // If t0 was a generic function, but the simplified signature t1 does 402 // not contain any type parameters anymore, the function is not generic 403 // anymore. Remove it's type parameters. (go.dev/issue/59953) 404 // Note that if t0 was a signature, t1 must be a signature, and t1 405 // can only be a generic signature if it originated from a generic 406 // function argument. Those signatures are never defined types and 407 // thus there is no need to call under below. 408 // TODO(gri) Consider doing this in Checker.subst. 409 // Then this would fall out automatically here and also 410 // in instantiation (where we also explicitly nil out 411 // type parameters). See the *Signature TODO in subst. 412 if sig, _ := t1.(*Signature); sig != nil && sig.TypeParams().Len() > 0 && !isParameterized(tparams, sig) { 413 sig.tparams = nil 414 } 415 inferred[index] = t1 416 dirty[n] = index 417 n++ 418 } 419 } 420 dirty = dirty[:n] 421 } 422 423 // Once nothing changes anymore, we may still have type parameters left; 424 // e.g., a constraint with core type *P may match a type parameter Q but 425 // we don't have any type arguments to fill in for *P or Q (go.dev/issue/45548). 426 // Don't let such inferences escape; instead treat them as unresolved. 427 for i, typ := range inferred { 428 if typ == nil || isParameterized(tparams, typ) { 429 obj := tparams[i].obj 430 check.errorf(pos, CannotInferTypeArgs, "cannot infer %s (%s)", obj.name, obj.pos) 431 return nil 432 } 433 } 434 435 return 436 } 437 438 // containsNil reports whether list contains a nil entry. 439 func containsNil(list []Type) bool { 440 for _, t := range list { 441 if t == nil { 442 return true 443 } 444 } 445 return false 446 } 447 448 // renameTParams renames the type parameters in the given type such that each type 449 // parameter is given a new identity. renameTParams returns the new type parameters 450 // and updated type. If the result type is unchanged from the argument type, none 451 // of the type parameters in tparams occurred in the type. 452 // If typ is a generic function, type parameters held with typ are not changed and 453 // must be updated separately if desired. 454 // The positions is only used for debug traces. 455 func (check *Checker) renameTParams(pos syntax.Pos, tparams []*TypeParam, typ Type) ([]*TypeParam, Type) { 456 // For the purpose of type inference we must differentiate type parameters 457 // occurring in explicit type or value function arguments from the type 458 // parameters we are solving for via unification because they may be the 459 // same in self-recursive calls: 460 // 461 // func f[P constraint](x P) { 462 // f(x) 463 // } 464 // 465 // In this example, without type parameter renaming, the P used in the 466 // instantiation f[P] has the same pointer identity as the P we are trying 467 // to solve for through type inference. This causes problems for type 468 // unification. Because any such self-recursive call is equivalent to 469 // a mutually recursive call, type parameter renaming can be used to 470 // create separate, disentangled type parameters. The above example 471 // can be rewritten into the following equivalent code: 472 // 473 // func f[P constraint](x P) { 474 // f2(x) 475 // } 476 // 477 // func f2[P2 constraint](x P2) { 478 // f(x) 479 // } 480 // 481 // Type parameter renaming turns the first example into the second 482 // example by renaming the type parameter P into P2. 483 if len(tparams) == 0 { 484 return nil, typ // nothing to do 485 } 486 487 tparams2 := make([]*TypeParam, len(tparams)) 488 for i, tparam := range tparams { 489 tname := NewTypeName(tparam.Obj().Pos(), tparam.Obj().Pkg(), tparam.Obj().Name(), nil) 490 tparams2[i] = NewTypeParam(tname, nil) 491 tparams2[i].index = tparam.index // == i 492 } 493 494 renameMap := makeRenameMap(tparams, tparams2) 495 for i, tparam := range tparams { 496 tparams2[i].bound = check.subst(pos, tparam.bound, renameMap, nil, check.context()) 497 } 498 499 return tparams2, check.subst(pos, typ, renameMap, nil, check.context()) 500 } 501 502 // typeParamsString produces a string containing all the type parameter names 503 // in list suitable for human consumption. 504 func typeParamsString(list []*TypeParam) string { 505 // common cases 506 n := len(list) 507 switch n { 508 case 0: 509 return "" 510 case 1: 511 return list[0].obj.name 512 case 2: 513 return list[0].obj.name + " and " + list[1].obj.name 514 } 515 516 // general case (n > 2) 517 var buf strings.Builder 518 for i, tname := range list[:n-1] { 519 if i > 0 { 520 buf.WriteString(", ") 521 } 522 buf.WriteString(tname.obj.name) 523 } 524 buf.WriteString(", and ") 525 buf.WriteString(list[n-1].obj.name) 526 return buf.String() 527 } 528 529 // isParameterized reports whether typ contains any of the type parameters of tparams. 530 // If typ is a generic function, isParameterized ignores the type parameter declarations; 531 // it only considers the signature proper (incoming and result parameters). 532 func isParameterized(tparams []*TypeParam, typ Type) bool { 533 w := tpWalker{ 534 tparams: tparams, 535 seen: make(map[Type]bool), 536 } 537 return w.isParameterized(typ) 538 } 539 540 type tpWalker struct { 541 tparams []*TypeParam 542 seen map[Type]bool 543 } 544 545 func (w *tpWalker) isParameterized(typ Type) (res bool) { 546 // detect cycles 547 if x, ok := w.seen[typ]; ok { 548 return x 549 } 550 w.seen[typ] = false 551 defer func() { 552 w.seen[typ] = res 553 }() 554 555 switch t := typ.(type) { 556 case *Basic: 557 // nothing to do 558 559 case *Alias: 560 return w.isParameterized(Unalias(t)) 561 562 case *Array: 563 return w.isParameterized(t.elem) 564 565 case *Slice: 566 return w.isParameterized(t.elem) 567 568 case *Struct: 569 return w.varList(t.fields) 570 571 case *Pointer: 572 return w.isParameterized(t.base) 573 574 case *Tuple: 575 // This case does not occur from within isParameterized 576 // because tuples only appear in signatures where they 577 // are handled explicitly. But isParameterized is also 578 // called by Checker.callExpr with a function result tuple 579 // if instantiation failed (go.dev/issue/59890). 580 return t != nil && w.varList(t.vars) 581 582 case *Signature: 583 // t.tparams may not be nil if we are looking at a signature 584 // of a generic function type (or an interface method) that is 585 // part of the type we're testing. We don't care about these type 586 // parameters. 587 // Similarly, the receiver of a method may declare (rather than 588 // use) type parameters, we don't care about those either. 589 // Thus, we only need to look at the input and result parameters. 590 return t.params != nil && w.varList(t.params.vars) || t.results != nil && w.varList(t.results.vars) 591 592 case *Interface: 593 tset := t.typeSet() 594 for _, m := range tset.methods { 595 if w.isParameterized(m.typ) { 596 return true 597 } 598 } 599 return tset.is(func(t *term) bool { 600 return t != nil && w.isParameterized(t.typ) 601 }) 602 603 case *Map: 604 return w.isParameterized(t.key) || w.isParameterized(t.elem) 605 606 case *Chan: 607 return w.isParameterized(t.elem) 608 609 case *Named: 610 for _, t := range t.TypeArgs().list() { 611 if w.isParameterized(t) { 612 return true 613 } 614 } 615 616 case *TypeParam: 617 return tparamIndex(w.tparams, t) >= 0 618 619 default: 620 panic(fmt.Sprintf("unexpected %T", typ)) 621 } 622 623 return false 624 } 625 626 func (w *tpWalker) varList(list []*Var) bool { 627 for _, v := range list { 628 if w.isParameterized(v.typ) { 629 return true 630 } 631 } 632 return false 633 } 634 635 // If the type parameter has a single specific type S, coreTerm returns (S, true). 636 // Otherwise, if tpar has a core type T, it returns a term corresponding to that 637 // core type and false. In that case, if any term of tpar has a tilde, the core 638 // term has a tilde. In all other cases coreTerm returns (nil, false). 639 func coreTerm(tpar *TypeParam) (*term, bool) { 640 n := 0 641 var single *term // valid if n == 1 642 var tilde bool 643 tpar.is(func(t *term) bool { 644 if t == nil { 645 assert(n == 0) 646 return false // no terms 647 } 648 n++ 649 single = t 650 if t.tilde { 651 tilde = true 652 } 653 return true 654 }) 655 if n == 1 { 656 if debug { 657 assert(debug && under(single.typ) == coreType(tpar)) 658 } 659 return single, true 660 } 661 if typ := coreType(tpar); typ != nil { 662 // A core type is always an underlying type. 663 // If any term of tpar has a tilde, we don't 664 // have a precise core type and we must return 665 // a tilde as well. 666 return &term{tilde, typ}, false 667 } 668 return nil, false 669 } 670 671 // killCycles walks through the given type parameters and looks for cycles 672 // created by type parameters whose inferred types refer back to that type 673 // parameter, either directly or indirectly. If such a cycle is detected, 674 // it is killed by setting the corresponding inferred type to nil. 675 // 676 // TODO(gri) Determine if we can simply abort inference as soon as we have 677 // found a single cycle. 678 func killCycles(tparams []*TypeParam, inferred []Type) { 679 w := cycleFinder{tparams, inferred, make(map[Type]bool)} 680 for _, t := range tparams { 681 w.typ(t) // t != nil 682 } 683 } 684 685 type cycleFinder struct { 686 tparams []*TypeParam 687 inferred []Type 688 seen map[Type]bool 689 } 690 691 func (w *cycleFinder) typ(typ Type) { 692 if w.seen[typ] { 693 // We have seen typ before. If it is one of the type parameters 694 // in w.tparams, iterative substitution will lead to infinite expansion. 695 // Nil out the corresponding type which effectively kills the cycle. 696 if tpar, _ := typ.(*TypeParam); tpar != nil { 697 if i := tparamIndex(w.tparams, tpar); i >= 0 { 698 // cycle through tpar 699 w.inferred[i] = nil 700 } 701 } 702 // If we don't have one of our type parameters, the cycle is due 703 // to an ordinary recursive type and we can just stop walking it. 704 return 705 } 706 w.seen[typ] = true 707 defer delete(w.seen, typ) 708 709 switch t := typ.(type) { 710 case *Basic: 711 // nothing to do 712 713 case *Alias: 714 w.typ(Unalias(t)) 715 716 case *Array: 717 w.typ(t.elem) 718 719 case *Slice: 720 w.typ(t.elem) 721 722 case *Struct: 723 w.varList(t.fields) 724 725 case *Pointer: 726 w.typ(t.base) 727 728 // case *Tuple: 729 // This case should not occur because tuples only appear 730 // in signatures where they are handled explicitly. 731 732 case *Signature: 733 if t.params != nil { 734 w.varList(t.params.vars) 735 } 736 if t.results != nil { 737 w.varList(t.results.vars) 738 } 739 740 case *Union: 741 for _, t := range t.terms { 742 w.typ(t.typ) 743 } 744 745 case *Interface: 746 for _, m := range t.methods { 747 w.typ(m.typ) 748 } 749 for _, t := range t.embeddeds { 750 w.typ(t) 751 } 752 753 case *Map: 754 w.typ(t.key) 755 w.typ(t.elem) 756 757 case *Chan: 758 w.typ(t.elem) 759 760 case *Named: 761 for _, tpar := range t.TypeArgs().list() { 762 w.typ(tpar) 763 } 764 765 case *TypeParam: 766 if i := tparamIndex(w.tparams, t); i >= 0 && w.inferred[i] != nil { 767 w.typ(w.inferred[i]) 768 } 769 770 default: 771 panic(fmt.Sprintf("unexpected %T", typ)) 772 } 773 } 774 775 func (w *cycleFinder) varList(list []*Var) { 776 for _, v := range list { 777 w.typ(v.typ) 778 } 779 } 780 781 // If tpar is a type parameter in list, tparamIndex returns the index 782 // of the type parameter in list. Otherwise the result is < 0. 783 func tparamIndex(list []*TypeParam, tpar *TypeParam) int { 784 for i, p := range list { 785 if p == tpar { 786 return i 787 } 788 } 789 return -1 790 } 791