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.
5 // This file implements type unification.
15 // The unifier maintains two separate sets of type parameters x and y
16 // which are used to resolve type parameters in the x and y arguments
17 // provided to the unify call. For unidirectional unification, only
18 // one of these sets (say x) is provided, and then type parameters are
19 // only resolved for the x argument passed to unify, not the y argument
20 // (even if that also contains possibly the same type parameters). This
21 // is crucial to infer the type parameters of self-recursive calls:
23 // func f[P any](a P) { f(a) }
25 // For the call f(a) we want to infer that the type argument for P is P.
26 // During unification, the parameter type P must be resolved to the type
27 // parameter P ("x" side), but the argument type P must be left alone so
28 // that unification resolves the type parameter P to P.
30 // For bidirection unification, both sets are provided. This enables
31 // unification to go from argument to parameter type and vice versa.
32 // For constraint type inference, we use bidirectional unification
33 // where both the x and y type parameters are identical. This is done
34 // by setting up one of them (using init) and then assigning its value
37 // A unifier maintains the current type parameters for x and y
38 // and the respective types inferred for each type parameter.
39 // A unifier is created by calling newUnifier.
42 x, y tparamsList // x and y must initialized via tparamsList.init
43 types []Type // inferred types, shared by x and y
46 // newUnifier returns a new unifier.
47 // If exact is set, unification requires unified types to match
48 // exactly. If exact is not set, a named type's underlying type
49 // is considered if unification would fail otherwise, and the
50 // direction of channels is ignored.
51 func newUnifier(exact bool) *unifier {
52 u := &unifier{exact: exact}
58 // unify attempts to unify x and y and reports whether it succeeded.
59 func (u *unifier) unify(x, y Type) bool {
60 return u.nify(x, y, nil)
63 // A tparamsList describes a list of type parameters and the types inferred for them.
64 type tparamsList struct {
67 // For each tparams element, there is a corresponding type slot index in indices.
68 // index < 0: unifier.types[-index-1] == nil
69 // index == 0: no type slot allocated yet
70 // index > 0: unifier.types[index-1] == typ
71 // Joined tparams elements share the same type slot and thus have the same index.
72 // By using a negative index for nil types we don't need to check unifier.types
73 // to see if we have a type or not.
74 indices []int // len(d.indices) == len(d.tparams)
77 // String returns a string representation for a tparamsList. For debugging.
78 func (d *tparamsList) String() string {
80 w := newTypeWriter(&buf, nil)
82 for i, tpar := range d.tparams {
94 // init initializes d with the given type parameters.
95 // The type parameters must be in the order in which they appear in their declaration
96 // (this ensures that the tparams indices match the respective type parameter index).
97 func (d *tparamsList) init(tparams []*TypeParam) {
98 if len(tparams) == 0 {
102 for i, tpar := range tparams {
103 assert(i == tpar.index)
107 d.indices = make([]int, len(tparams))
110 // join unifies the i'th type parameter of x with the j'th type parameter of y.
111 // If both type parameters already have a type associated with them and they are
112 // not joined, join fails and returns false.
113 func (u *unifier) join(i, j int) bool {
117 case ti == 0 && tj == 0:
118 // Neither type parameter has a type slot associated with them.
119 // Allocate a new joined nil type slot (negative index).
120 u.types = append(u.types, nil)
121 u.x.indices[i] = -len(u.types)
122 u.y.indices[j] = -len(u.types)
124 // The type parameter for x has no type slot yet. Use slot of y.
127 // The type parameter for y has no type slot yet. Use slot of x.
130 // Both type parameters have a slot: ti != 0 && tj != 0.
132 // Both type parameters already share the same slot. Nothing to do.
134 case ti > 0 && tj > 0:
135 // Both type parameters have (possibly different) inferred types. Cannot join.
136 // TODO(gri) Should we check if types are identical? Investigate.
139 // Only the type parameter for x has an inferred type. Use x slot for y.
141 // This case is handled like the default case.
143 // // Only the type parameter for y has an inferred type. Use y slot for x.
144 // u.x.setIndex(i, tj)
146 // Neither type parameter has an inferred type. Use y slot for x
147 // (or x slot for y, it doesn't matter).
153 // If typ is a type parameter of d, index returns the type parameter index.
154 // Otherwise, the result is < 0.
155 func (d *tparamsList) index(typ Type) int {
156 if tpar, ok := typ.(*TypeParam); ok {
157 return tparamIndex(d.tparams, tpar)
162 // If tpar is a type parameter in list, tparamIndex returns the type parameter index.
163 // Otherwise, the result is < 0. tpar must not be nil.
164 func tparamIndex(list []*TypeParam, tpar *TypeParam) int {
165 // Temporary work-around for getting around a crash
166 // with unified build.
167 // TODO(gri) investigate and implement proper fix
168 if buildcfg.Experiment.Unified && tpar.index < 0 {
171 if i := tpar.index; i < len(list) && list[i] == tpar {
177 // setIndex sets the type slot index for the i'th type parameter
178 // (and all its joined parameters) to tj. The type parameter
179 // must have a (possibly nil) type slot associated with it.
180 func (d *tparamsList) setIndex(i, tj int) {
182 assert(ti != 0 && tj != 0)
183 for k, tk := range d.indices {
190 // at returns the type set for the i'th type parameter; or nil.
191 func (d *tparamsList) at(i int) Type {
192 if ti := d.indices[i]; ti > 0 {
193 return d.unifier.types[ti-1]
198 // set sets the type typ for the i'th type parameter;
199 // typ must not be nil and it must not have been set before.
200 func (d *tparamsList) set(i int, typ Type) {
203 switch ti := d.indices[i]; {
208 u.types = append(u.types, typ)
209 d.indices[i] = len(u.types)
211 panic("type already set")
215 // types returns the list of inferred types (via unification) for the type parameters
216 // described by d, and an index. If all types were inferred, the returned index is < 0.
217 // Otherwise, it is the index of the first type parameter which couldn't be inferred;
218 // i.e., for which list[index] is nil.
219 func (d *tparamsList) types() (list []Type, index int) {
220 list = make([]Type, len(d.tparams))
222 for i := range d.tparams {
225 if index < 0 && t == nil {
232 func (u *unifier) nifyEq(x, y Type, p *ifacePair) bool {
233 return x == y || u.nify(x, y, p)
236 // nify implements the core unification algorithm which is an
237 // adapted version of Checker.identical. For changes to that
238 // code the corresponding changes should be made here.
239 // Must not be called directly from outside the unifier.
240 func (u *unifier) nify(x, y Type, p *ifacePair) bool {
242 // If exact unification is known to fail because we attempt to
243 // match a type name against an unnamed type literal, consider
244 // the underlying type of the named type.
245 // (We use !hasName to exclude any type with a name, including
246 // basic types and type parameters; the rest are unamed types.)
247 if nx, _ := x.(*Named); nx != nil && !hasName(y) {
248 return u.nify(nx.under(), y, p)
249 } else if ny, _ := y.(*Named); ny != nil && !hasName(x) {
250 return u.nify(x, ny.under(), p)
254 // Cases where at least one of x or y is a type parameter.
255 switch i, j := u.x.index(x), u.y.index(y); {
256 case i >= 0 && j >= 0:
257 // both x and y are type parameters
261 // both x and y have an inferred type - they must match
262 return u.nifyEq(u.x.at(i), u.y.at(j), p)
265 // x is a type parameter, y is not
266 if tx := u.x.at(i); tx != nil {
267 return u.nifyEq(tx, y, p)
269 // otherwise, infer type from y
274 // y is a type parameter, x is not
275 if ty := u.y.at(j); ty != nil {
276 return u.nifyEq(x, ty, p)
278 // otherwise, infer type from x
283 // For type unification, do not shortcut (x == y) for identical
284 // types. Instead keep comparing them element-wise to unify the
285 // matching (and equal type parameter types). A simple test case
286 // where this matters is: func f[P any](a P) { f(a) } .
288 switch x := x.(type) {
290 // Basic types are singletons except for the rune and byte
291 // aliases, thus we cannot solely rely on the x == y check
292 // above. See also comment in TypeName.IsAlias.
293 if y, ok := y.(*Basic); ok {
294 return x.kind == y.kind
298 // Two array types are identical if they have identical element types
299 // and the same array length.
300 if y, ok := y.(*Array); ok {
301 // If one or both array lengths are unknown (< 0) due to some error,
302 // assume they are the same to avoid spurious follow-on errors.
303 return (x.len < 0 || y.len < 0 || x.len == y.len) && u.nify(x.elem, y.elem, p)
307 // Two slice types are identical if they have identical element types.
308 if y, ok := y.(*Slice); ok {
309 return u.nify(x.elem, y.elem, p)
313 // Two struct types are identical if they have the same sequence of fields,
314 // and if corresponding fields have the same names, and identical types,
315 // and identical tags. Two embedded fields are considered to have the same
316 // name. Lower-case field names from different packages are always different.
317 if y, ok := y.(*Struct); ok {
318 if x.NumFields() == y.NumFields() {
319 for i, f := range x.fields {
321 if f.embedded != g.embedded ||
322 x.Tag(i) != y.Tag(i) ||
323 !f.sameId(g.pkg, g.name) ||
324 !u.nify(f.typ, g.typ, p) {
333 // Two pointer types are identical if they have identical base types.
334 if y, ok := y.(*Pointer); ok {
335 return u.nify(x.base, y.base, p)
339 // Two tuples types are identical if they have the same number of elements
340 // and corresponding elements have identical types.
341 if y, ok := y.(*Tuple); ok {
342 if x.Len() == y.Len() {
344 for i, v := range x.vars {
346 if !u.nify(v.typ, w.typ, p) {
356 // Two function types are identical if they have the same number of parameters
357 // and result values, corresponding parameter and result types are identical,
358 // and either both functions are variadic or neither is. Parameter and result
359 // names are not required to match.
360 // TODO(gri) handle type parameters or document why we can ignore them.
361 if y, ok := y.(*Signature); ok {
362 return x.variadic == y.variadic &&
363 u.nify(x.params, y.params, p) &&
364 u.nify(x.results, y.results, p)
368 // Two interface types are identical if they have the same set of methods with
369 // the same names and identical function types. Lower-case method names from
370 // different packages are always different. The order of the methods is irrelevant.
371 if y, ok := y.(*Interface); ok {
374 if !xset.terms.equal(yset.terms) {
379 if len(a) == len(b) {
380 // Interface types are the only types where cycles can occur
381 // that are not "terminated" via named types; and such cycles
382 // can only be created via method parameter types that are
383 // anonymous interfaces (directly or indirectly) embedding
384 // the current interface. Example:
386 // type T interface {
390 // If two such (differently named) interfaces are compared,
391 // endless recursion occurs if the cycle is not detected.
393 // If x and y were compared before, they must be equal
394 // (if they were not, the recursion would have stopped);
395 // search the ifacePair stack for the same pair.
397 // This is a quadratic algorithm, but in practice these stacks
398 // are extremely short (bounded by the nesting depth of interface
399 // type declarations that recur via parameter types, an extremely
400 // rare occurrence). An alternative implementation might use a
401 // "visited" map, but that is probably less efficient overall.
402 q := &ifacePair{x, y, p}
405 return true // same pair was compared before
410 assertSortedMethods(a)
411 assertSortedMethods(b)
413 for i, f := range a {
415 if f.Id() != g.Id() || !u.nify(f.typ, g.typ, q) {
424 // Two map types are identical if they have identical key and value types.
425 if y, ok := y.(*Map); ok {
426 return u.nify(x.key, y.key, p) && u.nify(x.elem, y.elem, p)
430 // Two channel types are identical if they have identical value types.
431 if y, ok := y.(*Chan); ok {
432 return (!u.exact || x.dir == y.dir) && u.nify(x.elem, y.elem, p)
436 // TODO(gri) This code differs now from the parallel code in Checker.identical. Investigate.
437 if y, ok := y.(*Named); ok {
438 xargs := x.targs.list()
439 yargs := y.targs.list()
441 // TODO(gri) This is not always correct: two types may have the same names
442 // in the same package if one of them is nested in a function.
443 // Extremely unlikely but we need an always correct solution.
444 if x.obj.pkg == y.obj.pkg && x.obj.name == y.obj.name {
445 assert(len(xargs) == len(yargs))
446 for i, x := range xargs {
447 if !u.nify(x, yargs[i], p) {
456 // Two type parameters (which are not part of the type parameters of the
457 // enclosing type as those are handled in the beginning of this function)
458 // are identical if they originate in the same declaration.
462 // avoid a crash in case of nil type
465 panic(fmt.Sprintf("### u.nify(%s, %s), u.x.tparams = %s", x, y, u.x.tparams))