1 // Code generated by "go test -run=Generate -write=all"; DO NOT EDIT.
3 // Copyright 2020 The Go Authors. All rights reserved.
4 // Use of this source code is governed by a BSD-style
5 // license that can be found in the LICENSE file.
7 // This file implements type unification.
9 // Type unification attempts to make two types x and y structurally
10 // equivalent by determining the types for a given list of (bound)
11 // type parameters which may occur within x and y. If x and y are
12 // structurally different (say []T vs chan T), or conflicting
13 // types are determined for type parameters, unification fails.
14 // If unification succeeds, as a side-effect, the types of the
15 // bound type parameters may be determined.
17 // Unification typically requires multiple calls u.unify(x, y) to
18 // a given unifier u, with various combinations of types x and y.
19 // In each call, additional type parameter types may be determined
20 // as a side effect and recorded in u.
21 // If a call fails (returns false), unification fails.
23 // In the unification context, structural equivalence of two types
24 // ignores the difference between a defined type and its underlying
25 // type if one type is a defined type and the other one is not.
26 // It also ignores the difference between an (external, unbound)
27 // type parameter and its core type.
28 // If two types are not structurally equivalent, they cannot be Go
29 // identical types. On the other hand, if they are structurally
30 // equivalent, they may be Go identical or at least assignable, or
31 // they may be in the type set of a constraint.
32 // Whether they indeed are identical or assignable is determined
33 // upon instantiation and function argument passing.
45 // Upper limit for recursion depth. Used to catch infinite recursions
46 // due to implementation issues (e.g., see issues go.dev/issue/48619, go.dev/issue/48656).
47 unificationDepthLimit = 50
49 // Whether to panic when unificationDepthLimit is reached.
50 // If disabled, a recursion depth overflow results in a (quiet)
51 // unification failure.
52 panicAtUnificationDepthLimit = true
54 // If enableCoreTypeUnification is set, unification will consider
55 // the core types, if any, of non-local (unbound) type parameters.
56 enableCoreTypeUnification = true
58 // If enableInterfaceInference is set, type inference uses
59 // shared methods for improved type inference involving
61 enableInterfaceInference = true
63 // If traceInference is set, unification will print a trace of its operation.
64 // Interpretation of trace:
65 // x ≡ y attempt to unify types x and y
66 // p ➞ y type parameter p is set to type y (p is inferred to be y)
67 // p ⇄ q type parameters p and q match (p is inferred to be q and vice versa)
68 // x ≢ y types x and y cannot be unified
69 // [p, q, ...] ➞ [x, y, ...] mapping from type parameters to types
70 traceInference = false
73 // A unifier maintains a list of type parameters and
74 // corresponding types inferred for each type parameter.
75 // A unifier is created by calling newUnifier.
77 // handles maps each type parameter to its inferred type through
78 // an indirection *Type called (inferred type) "handle".
79 // Initially, each type parameter has its own, separate handle,
80 // with a nil (i.e., not yet inferred) type.
81 // After a type parameter P is unified with a type parameter Q,
82 // P and Q share the same handle (and thus type). This ensures
83 // that inferring the type for a given type parameter P will
84 // automatically infer the same type for all other parameters
85 // unified (joined) with P.
86 handles map[*TypeParam]*Type
87 depth int // recursion depth during unification
90 // newUnifier returns a new unifier initialized with the given type parameter
91 // and corresponding type argument lists. The type argument list may be shorter
92 // than the type parameter list, and it may contain nil types. Matching type
93 // parameters and arguments must have the same index.
94 func newUnifier(tparams []*TypeParam, targs []Type) *unifier {
95 assert(len(tparams) >= len(targs))
96 handles := make(map[*TypeParam]*Type, len(tparams))
97 // Allocate all handles up-front: in a correct program, all type parameters
98 // must be resolved and thus eventually will get a handle.
99 // Also, sharing of handles caused by unified type parameters is rare and
100 // so it's ok to not optimize for that case (and delay handle allocation).
101 for i, x := range tparams {
108 return &unifier{handles, 0}
111 // unifyMode controls the behavior of the unifier.
115 // If assign is set, we are unifying types involved in an assignment:
116 // they may match inexactly at the top, but element types must match
118 assign unifyMode = 1 << iota
120 // If exact is set, types unify if they are identical (or can be
121 // made identical with suitable arguments for type parameters).
122 // Otherwise, a named type and a type literal unify if their
123 // underlying types unify, channel directions are ignored, and
124 // if there is an interface, the other type must implement the
129 func (m unifyMode) String() string {
138 return "assign, exact"
140 return fmt.Sprintf("mode %d", m)
143 // unify attempts to unify x and y and reports whether it succeeded.
144 // As a side-effect, types may be inferred for type parameters.
145 // The mode parameter controls how types are compared.
146 func (u *unifier) unify(x, y Type, mode unifyMode) bool {
147 return u.nify(x, y, mode, nil)
150 func (u *unifier) tracef(format string, args ...interface{}) {
151 fmt.Println(strings.Repeat(". ", u.depth) + sprintf(nil, nil, true, format, args...))
154 // String returns a string representation of the current mapping
155 // from type parameters to types.
156 func (u *unifier) String() string {
157 // sort type parameters for reproducible strings
158 tparams := make(typeParamsById, len(u.handles))
160 for tpar := range u.handles {
167 w := newTypeWriter(&buf, nil)
169 for i, x := range tparams {
181 type typeParamsById []*TypeParam
183 func (s typeParamsById) Len() int { return len(s) }
184 func (s typeParamsById) Less(i, j int) bool { return s[i].id < s[j].id }
185 func (s typeParamsById) Swap(i, j int) { s[i], s[j] = s[j], s[i] }
187 // join unifies the given type parameters x and y.
188 // If both type parameters already have a type associated with them
189 // and they are not joined, join fails and returns false.
190 func (u *unifier) join(x, y *TypeParam) bool {
192 u.tracef("%s ⇄ %s", x, y)
194 switch hx, hy := u.handles[x], u.handles[y]; {
196 // Both type parameters already share the same handle. Nothing to do.
197 case *hx != nil && *hy != nil:
198 // Both type parameters have (possibly different) inferred types. Cannot join.
201 // Only type parameter x has an inferred type. Use handle of x.
203 // This case is treated like the default case.
205 // // Only type parameter y has an inferred type. Use handle of y.
206 // u.setHandle(x, hy)
208 // Neither type parameter has an inferred type. Use handle of y.
214 // asTypeParam returns x.(*TypeParam) if x is a type parameter recorded with u.
215 // Otherwise, the result is nil.
216 func (u *unifier) asTypeParam(x Type) *TypeParam {
217 if x, _ := x.(*TypeParam); x != nil {
218 if _, found := u.handles[x]; found {
225 // setHandle sets the handle for type parameter x
226 // (and all its joined type parameters) to h.
227 func (u *unifier) setHandle(x *TypeParam, h *Type) {
230 for y, hy := range u.handles {
237 // at returns the (possibly nil) type for type parameter x.
238 func (u *unifier) at(x *TypeParam) Type {
242 // set sets the type t for type parameter x;
243 // t must not be nil.
244 func (u *unifier) set(x *TypeParam, t Type) {
247 u.tracef("%s ➞ %s", x, t)
252 // unknowns returns the number of type parameters for which no type has been set yet.
253 func (u *unifier) unknowns() int {
255 for _, h := range u.handles {
263 // inferred returns the list of inferred types for the given type parameter list.
264 // The result is never nil and has the same length as tparams; result types that
265 // could not be inferred are nil. Corresponding type parameters and result types
266 // have identical indices.
267 func (u *unifier) inferred(tparams []*TypeParam) []Type {
268 list := make([]Type, len(tparams))
269 for i, x := range tparams {
275 // asInterface returns the underlying type of x as an interface if
276 // it is a non-type parameter interface. Otherwise it returns nil.
277 func asInterface(x Type) (i *Interface) {
278 if _, ok := x.(*TypeParam); !ok {
279 i, _ = under(x).(*Interface)
284 // nify implements the core unification algorithm which is an
285 // adapted version of Checker.identical. For changes to that
286 // code the corresponding changes should be made here.
287 // Must not be called directly from outside the unifier.
288 func (u *unifier) nify(x, y Type, mode unifyMode, p *ifacePair) (result bool) {
291 u.tracef("%s ≡ %s\t// %s", x, y, mode)
294 if traceInference && !result {
295 u.tracef("%s ≢ %s", x, y)
300 // nothing to do if x == y
305 // Stop gap for cases where unification fails.
306 if u.depth > unificationDepthLimit {
308 u.tracef("depth %d >= %d", u.depth, unificationDepthLimit)
310 if panicAtUnificationDepthLimit {
311 panic("unification reached recursion depth limit")
316 // Unification is symmetric, so we can swap the operands.
317 // Ensure that if we have at least one
318 // - defined type, make sure one is in y
319 // - type parameter recorded with u, make sure one is in x
320 if _, ok := x.(*Named); ok || u.asTypeParam(y) != nil {
322 u.tracef("%s ≡ %s\t// swap", y, x)
327 // Unification will fail if we match a defined type against a type literal.
328 // If we are matching types in an assignment, at the top-level, types with
329 // the same type structure are permitted as long as at least one of them
330 // is not a defined type. To accommodate for that possibility, we continue
331 // unification with the underlying type of a defined type if the other type
332 // is a type literal. This is controlled by the exact unification mode.
333 // We also continue if the other type is a basic type because basic types
334 // are valid underlying types and may appear as core types of type constraints.
335 // If we exclude them, inferred defined types for type parameters may not
336 // match against the core types of their constraints (even though they might
337 // correctly match against some of the types in the constraint's type set).
338 // Finally, if unification (incorrectly) succeeds by matching the underlying
339 // type of a defined type against a basic type (because we include basic types
340 // as type literals here), and if that leads to an incorrectly inferred type,
341 // we will fail at function instantiation or argument assignment time.
343 // If we have at least one defined type, there is one in y.
344 if ny, _ := y.(*Named); mode&exact == 0 && ny != nil && isTypeLit(x) && !(enableInterfaceInference && IsInterface(x)) {
346 u.tracef("%s ≡ under %s", x, ny)
349 // Per the spec, a defined type cannot have an underlying type
350 // that is a type parameter.
351 assert(!isTypeParam(y))
352 // x and y may be identical now
358 // Cases where at least one of x or y is a type parameter recorded with u.
359 // If we have at least one type parameter, there is one in x.
360 // If we have exactly one type parameter, because it is in x,
361 // isTypeLit(x) is false and y was not changed above. In other
362 // words, if y was a defined type, it is still a defined type
363 // (relevant for the logic below).
364 switch px, py := u.asTypeParam(x), u.asTypeParam(y); {
365 case px != nil && py != nil:
366 // both x and y are type parameters
370 // both x and y have an inferred type - they must match
371 return u.nify(u.at(px), u.at(py), mode, p)
374 // x is a type parameter, y is not
375 if x := u.at(px); x != nil {
376 // x has an inferred type which must match y
377 if u.nify(x, y, mode, p) {
378 // We have a match, possibly through underlying types.
383 // If we have two interfaces, what to do depends on
384 // whether they are named and their method sets.
385 if xi != nil && yi != nil {
386 // Both types are interfaces.
387 // If both types are defined types, they must be identical
388 // because unification doesn't know which type has the "right" name.
390 return Identical(x, y)
392 // In all other cases, the method sets must match.
393 // The types unified so we know that corresponding methods
394 // match and we can simply compare the number of methods.
395 // TODO(gri) We may be able to relax this rule and select
396 // the more general interface. But if one of them is a defined
397 // type, it's not clear how to choose and whether we introduce
398 // an order dependency or not. Requiring the same method set
400 if len(xi.typeSet().methods) != len(yi.typeSet().methods) {
403 } else if xi != nil || yi != nil {
404 // One but not both of them are interfaces.
405 // In this case, either x or y could be viable matches for the corresponding
406 // type parameter, which means choosing either introduces an order dependence.
407 // Therefore, we must fail unification (go.dev/issue/60933).
410 // If y is a defined type, make sure we record that type
411 // for type parameter x, which may have until now only
412 // recorded an underlying type (go.dev/issue/43056).
413 // Either both types are interfaces, or neither type is.
414 // If both are interfaces, they have the same methods.
416 // Note: Changing the recorded type for a type parameter to
417 // a defined type is only ok when unification is inexact.
418 // But in exact unification, if we have a match, x and y must
419 // be identical, so changing the recorded type for x is a no-op.
427 // otherwise, infer type from y
432 // x != y if we get here
435 // Type elements (array, slice, etc. elements) use emode for unification.
436 // Element types must match exactly if the types are used in an assignment.
438 if mode&assign != 0 {
442 // If EnableInterfaceInference is set and we don't require exact unification,
443 // if both types are interfaces, one interface must have a subset of the
444 // methods of the other and corresponding method signatures must unify.
445 // If only one type is an interface, all its methods must be present in the
446 // other type and corresponding method signatures must unify.
447 if enableInterfaceInference && mode&exact == 0 {
448 // One or both interfaces may be defined types.
449 // Look under the name, but not under type parameters (go.dev/issue/60564).
452 // If we have two interfaces, check the type terms for equivalence,
453 // and unify common methods if possible.
454 if xi != nil && yi != nil {
457 if xset.comparable != yset.comparable {
460 // For now we require terms to be equal.
461 // We should be able to relax this as well, eventually.
462 if !xset.terms.equal(yset.terms) {
465 // Interface types are the only types where cycles can occur
466 // that are not "terminated" via named types; and such cycles
467 // can only be created via method parameter types that are
468 // anonymous interfaces (directly or indirectly) embedding
469 // the current interface. Example:
471 // type T interface {
475 // If two such (differently named) interfaces are compared,
476 // endless recursion occurs if the cycle is not detected.
478 // If x and y were compared before, they must be equal
479 // (if they were not, the recursion would have stopped);
480 // search the ifacePair stack for the same pair.
482 // This is a quadratic algorithm, but in practice these stacks
483 // are extremely short (bounded by the nesting depth of interface
484 // type declarations that recur via parameter types, an extremely
485 // rare occurrence). An alternative implementation might use a
486 // "visited" map, but that is probably less efficient overall.
487 q := &ifacePair{xi, yi, p}
490 return true // same pair was compared before
494 // The method set of x must be a subset of the method set
495 // of y or vice versa, and the common methods must unify.
496 xmethods := xset.methods
497 ymethods := yset.methods
498 // The smaller method set must be the subset, if it exists.
499 if len(xmethods) > len(ymethods) {
500 xmethods, ymethods = ymethods, xmethods
502 // len(xmethods) <= len(ymethods)
503 // Collect the ymethods in a map for quick lookup.
504 ymap := make(map[string]*Func, len(ymethods))
505 for _, ym := range ymethods {
508 // All xmethods must exist in ymethods and corresponding signatures must unify.
509 for _, xm := range xmethods {
510 if ym := ymap[xm.Id()]; ym == nil || !u.nify(xm.typ, ym.typ, emode, p) {
517 // We don't have two interfaces. If we have one, make sure it's in xi.
523 // If we have one interface, at a minimum each of the interface methods
524 // must be implemented and thus unify with a corresponding method from
525 // the non-interface type, otherwise unification fails.
527 // All xi methods must exist in y and corresponding signatures must unify.
528 xmethods := xi.typeSet().methods
529 for _, xm := range xmethods {
530 obj, _, _ := LookupFieldOrMethod(y, false, xm.pkg, xm.name)
531 if ym, _ := obj.(*Func); ym == nil || !u.nify(xm.typ, ym.typ, emode, p) {
539 // Unless we have exact unification, neither x nor y are interfaces now.
540 // Except for unbound type parameters (see below), x and y must be structurally
541 // equivalent to unify.
543 // If we get here and x or y is a type parameter, they are unbound
544 // (not recorded with the unifier).
545 // Ensure that if we have at least one type parameter, it is in x
546 // (the earlier swap checks for _recorded_ type parameters only).
547 // This ensures that the switch switches on the type parameter.
549 // TODO(gri) Factor out type parameter handling from the switch.
552 u.tracef("%s ≡ %s\t// swap", y, x)
557 switch x := x.(type) {
559 // Basic types are singletons except for the rune and byte
560 // aliases, thus we cannot solely rely on the x == y check
561 // above. See also comment in TypeName.IsAlias.
562 if y, ok := y.(*Basic); ok {
563 return x.kind == y.kind
567 // Two array types unify if they have the same array length
568 // and their element types unify.
569 if y, ok := y.(*Array); ok {
570 // If one or both array lengths are unknown (< 0) due to some error,
571 // assume they are the same to avoid spurious follow-on errors.
572 return (x.len < 0 || y.len < 0 || x.len == y.len) && u.nify(x.elem, y.elem, emode, p)
576 // Two slice types unify if their element types unify.
577 if y, ok := y.(*Slice); ok {
578 return u.nify(x.elem, y.elem, emode, p)
582 // Two struct types unify if they have the same sequence of fields,
583 // and if corresponding fields have the same names, their (field) types unify,
584 // and they have identical tags. Two embedded fields are considered to have the same
585 // name. Lower-case field names from different packages are always different.
586 if y, ok := y.(*Struct); ok {
587 if x.NumFields() == y.NumFields() {
588 for i, f := range x.fields {
590 if f.embedded != g.embedded ||
591 x.Tag(i) != y.Tag(i) ||
592 !f.sameId(g.pkg, g.name) ||
593 !u.nify(f.typ, g.typ, emode, p) {
602 // Two pointer types unify if their base types unify.
603 if y, ok := y.(*Pointer); ok {
604 return u.nify(x.base, y.base, emode, p)
608 // Two tuples types unify if they have the same number of elements
609 // and the types of corresponding elements unify.
610 if y, ok := y.(*Tuple); ok {
611 if x.Len() == y.Len() {
613 for i, v := range x.vars {
615 if !u.nify(v.typ, w.typ, mode, p) {
625 // Two function types unify if they have the same number of parameters
626 // and result values, corresponding parameter and result types unify,
627 // and either both functions are variadic or neither is.
628 // Parameter and result names are not required to match.
629 // TODO(gri) handle type parameters or document why we can ignore them.
630 if y, ok := y.(*Signature); ok {
631 return x.variadic == y.variadic &&
632 u.nify(x.params, y.params, emode, p) &&
633 u.nify(x.results, y.results, emode, p)
637 assert(!enableInterfaceInference || mode&exact != 0) // handled before this switch
639 // Two interface types unify if they have the same set of methods with
640 // the same names, and corresponding function types unify.
641 // Lower-case method names from different packages are always different.
642 // The order of the methods is irrelevant.
643 if y, ok := y.(*Interface); ok {
646 if xset.comparable != yset.comparable {
649 if !xset.terms.equal(yset.terms) {
654 if len(a) == len(b) {
655 // Interface types are the only types where cycles can occur
656 // that are not "terminated" via named types; and such cycles
657 // can only be created via method parameter types that are
658 // anonymous interfaces (directly or indirectly) embedding
659 // the current interface. Example:
661 // type T interface {
665 // If two such (differently named) interfaces are compared,
666 // endless recursion occurs if the cycle is not detected.
668 // If x and y were compared before, they must be equal
669 // (if they were not, the recursion would have stopped);
670 // search the ifacePair stack for the same pair.
672 // This is a quadratic algorithm, but in practice these stacks
673 // are extremely short (bounded by the nesting depth of interface
674 // type declarations that recur via parameter types, an extremely
675 // rare occurrence). An alternative implementation might use a
676 // "visited" map, but that is probably less efficient overall.
677 q := &ifacePair{x, y, p}
680 return true // same pair was compared before
685 assertSortedMethods(a)
686 assertSortedMethods(b)
688 for i, f := range a {
690 if f.Id() != g.Id() || !u.nify(f.typ, g.typ, emode, q) {
699 // Two map types unify if their key and value types unify.
700 if y, ok := y.(*Map); ok {
701 return u.nify(x.key, y.key, emode, p) && u.nify(x.elem, y.elem, emode, p)
705 // Two channel types unify if their value types unify
706 // and if they have the same direction.
707 // The channel direction is ignored for inexact unification.
708 if y, ok := y.(*Chan); ok {
709 return (mode&exact == 0 || x.dir == y.dir) && u.nify(x.elem, y.elem, emode, p)
713 // Two named types unify if their type names originate in the same type declaration.
714 // If they are instantiated, their type argument lists must unify.
715 if y, ok := y.(*Named); ok {
716 // Check type arguments before origins so they unify
717 // even if the origins don't match; for better error
718 // messages (see go.dev/issue/53692).
719 xargs := x.TypeArgs().list()
720 yargs := y.TypeArgs().list()
721 if len(xargs) != len(yargs) {
724 for i, xarg := range xargs {
725 if !u.nify(xarg, yargs[i], mode, p) {
729 return indenticalOrigin(x, y)
733 // x must be an unbound type parameter (see comment above).
735 assert(u.asTypeParam(x) == nil)
737 // By definition, a valid type argument must be in the type set of
738 // the respective type constraint. Therefore, the type argument's
739 // underlying type must be in the set of underlying types of that
740 // constraint. If there is a single such underlying type, it's the
741 // constraint's core type. It must match the type argument's under-
742 // lying type, irrespective of whether the actual type argument,
743 // which may be a defined type, is actually in the type set (that
744 // will be determined at instantiation time).
745 // Thus, if we have the core type of an unbound type parameter,
746 // we know the structure of the possible types satisfying such
747 // parameters. Use that core type for further unification
748 // (see go.dev/issue/50755 for a test case).
749 if enableCoreTypeUnification {
750 // Because the core type is always an underlying type,
751 // unification will take care of matching against a
752 // defined or literal type automatically.
753 // If y is also an unbound type parameter, we will end
754 // up here again with x and y swapped, so we don't
755 // need to take care of that case separately.
756 if cx := coreType(x); cx != nil {
758 u.tracef("core %s ≡ %s", x, y)
760 // If y is a defined type, it may not match against cx which
761 // is an underlying type (incl. int, string, etc.). Use assign
762 // mode here so that the unifier automatically takes under(y)
764 return u.nify(cx, y, assign, p)
767 // x != y and there's nothing to do
770 // avoid a crash in case of nil type
773 panic(sprintf(nil, nil, true, "u.nify(%s, %s, %d)", x, y, mode))