//
// The process stops as soon as all type arguments are known or an error occurs.
func (check *Checker) infer(pos syntax.Pos, tparams []*TypeParam, targs []Type, params *Tuple, args []*operand) (result []Type) {
+ if useNewTypeInference {
+ return check.infer2(pos, tparams, targs, params, args)
+ }
+
if debug {
defer func() {
assert(result == nil || len(result) == len(tparams))
--- /dev/null
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements type parameter inference.
+
+package types2
+
+import (
+ "cmd/compile/internal/syntax"
+ . "internal/types/errors"
+)
+
+const useNewTypeInference = false
+
+// infer2 attempts to infer the complete set of type arguments for generic function instantiation/call
+// based on the given type parameters tparams, type arguments targs, function parameters params, and
+// function arguments args, if any. There must be at least one type parameter, no more type arguments
+// than type parameters, and params and args must match in number (incl. zero).
+// If successful, infer returns the complete list of given and inferred type arguments, one for each
+// type parameter. Otherwise the result is nil and appropriate errors will be reported.
+func (check *Checker) infer2(pos syntax.Pos, tparams []*TypeParam, targs []Type, params *Tuple, args []*operand) (inferred []Type) {
+ if debug {
+ defer func() {
+ assert(inferred == nil || len(inferred) == len(tparams))
+ for _, targ := range inferred {
+ assert(targ != nil)
+ }
+ }()
+ }
+
+ if traceInference {
+ check.dump("-- infer2 %s%s ➞ %s", tparams, params, targs)
+ defer func() {
+ check.dump("=> %s ➞ %s\n", tparams, inferred)
+ }()
+ }
+
+ // There must be at least one type parameter, and no more type arguments than type parameters.
+ n := len(tparams)
+ assert(n > 0 && len(targs) <= n)
+
+ // Function parameters and arguments must match in number.
+ assert(params.Len() == len(args))
+
+ // If we already have all type arguments, we're done.
+ if len(targs) == n {
+ return targs
+ }
+ // len(targs) < n
+
+ // Rename type parameters to avoid conflicts in recursive instantiation scenarios.
+ tparams, params = check.renameTParams(pos, tparams, params)
+
+ // If we have more than 2 arguments, we may have arguments with named and unnamed types.
+ // If that is the case, permutate params and args such that the arguments with named
+ // types are first in the list. This doesn't affect type inference if all types are taken
+ // as is. But when we have inexact unification enabled (as is the case for function type
+ // inference), when a named type is unified with an unnamed type, unification proceeds
+ // with the underlying type of the named type because otherwise unification would fail
+ // right away. This leads to an asymmetry in type inference: in cases where arguments of
+ // named and unnamed types are passed to parameters with identical type, different types
+ // (named vs underlying) may be inferred depending on the order of the arguments.
+ // By ensuring that named types are seen first, order dependence is avoided and unification
+ // succeeds where it can (go.dev/issue/43056).
+ const enableArgSorting = true
+ if m := len(args); m >= 2 && enableArgSorting {
+ // Determine indices of arguments with named and unnamed types.
+ var named, unnamed []int
+ for i, arg := range args {
+ if hasName(arg.typ) {
+ named = append(named, i)
+ } else {
+ unnamed = append(unnamed, i)
+ }
+ }
+
+ // If we have named and unnamed types, move the arguments with
+ // named types first. Update the parameter list accordingly.
+ // Make copies so as not to clobber the incoming slices.
+ if len(named) != 0 && len(unnamed) != 0 {
+ params2 := make([]*Var, m)
+ args2 := make([]*operand, m)
+ i := 0
+ for _, j := range named {
+ params2[i] = params.At(j)
+ args2[i] = args[j]
+ i++
+ }
+ for _, j := range unnamed {
+ params2[i] = params.At(j)
+ args2[i] = args[j]
+ i++
+ }
+ params = NewTuple(params2...)
+ args = args2
+ }
+ }
+
+ // Make sure we have a "full" list of type arguments, some of which may
+ // be nil (unknown). Make a copy so as to not clobber the incoming slice.
+ if len(targs) < n {
+ targs2 := make([]Type, n)
+ copy(targs2, targs)
+ targs = targs2
+ }
+ // len(targs) == n
+
+ // Continue with the type arguments we have. Avoid matching generic
+ // parameters that already have type arguments against function arguments:
+ // It may fail because matching uses type identity while parameter passing
+ // uses assignment rules. Instantiate the parameter list with the type
+ // arguments we have, and continue with that parameter list.
+
+ // Substitute type arguments for their respective type parameters in params,
+ // if any. Note that nil targs entries are ignored by check.subst.
+ // TODO(gri) Can we avoid this (we're setting known type arguments below,
+ // but that doesn't impact the isParameterized check for now).
+ if params.Len() > 0 {
+ smap := makeSubstMap(tparams, targs)
+ params = check.subst(nopos, params, smap, nil, check.context()).(*Tuple)
+ }
+
+ // Unify parameter and argument types for generic parameters with typed arguments
+ // and collect the indices of generic parameters with untyped arguments.
+ // Terminology: generic parameter = function parameter with a type-parameterized type
+ u := newUnifier(tparams, targs)
+
+ errorf := func(kind string, tpar, targ Type, arg *operand) {
+ // provide a better error message if we can
+ targs, index := u.inferred()
+ if index == 0 {
+ // The first type parameter couldn't be inferred.
+ // If none of them could be inferred, don't try
+ // to provide the inferred type in the error msg.
+ allFailed := true
+ for _, targ := range targs {
+ if targ != nil {
+ allFailed = false
+ break
+ }
+ }
+ if allFailed {
+ check.errorf(arg, CannotInferTypeArgs, "%s %s of %s does not match %s (cannot infer %s)", kind, targ, arg.expr, tpar, typeParamsString(tparams))
+ return
+ }
+ }
+ smap := makeSubstMap(tparams, targs)
+ // TODO(gri): pass a poser here, rather than arg.Pos().
+ inferred := check.subst(arg.Pos(), tpar, smap, nil, check.context())
+ // CannotInferTypeArgs indicates a failure of inference, though the actual
+ // error may be better attributed to a user-provided type argument (hence
+ // InvalidTypeArg). We can't differentiate these cases, so fall back on
+ // the more general CannotInferTypeArgs.
+ if inferred != tpar {
+ check.errorf(arg, CannotInferTypeArgs, "%s %s of %s does not match inferred type %s for %s", kind, targ, arg.expr, inferred, tpar)
+ } else {
+ check.errorf(arg, CannotInferTypeArgs, "%s %s of %s does not match %s", kind, targ, arg.expr, tpar)
+ }
+ }
+
+ // indices of generic parameters with untyped arguments, for later use
+ var untyped []int
+
+ // --- 1 ---
+ // use information from function arguments
+
+ if traceInference {
+ u.tracef("parameters: %s", params)
+ u.tracef("arguments : %s", args)
+ }
+
+ for i, arg := range args {
+ par := params.At(i)
+ // If we permit bidirectional unification, this conditional code needs to be
+ // executed even if par.typ is not parameterized since the argument may be a
+ // generic function (for which we want to infer its type arguments).
+ if isParameterized(tparams, par.typ) {
+ if arg.mode == invalid {
+ // An error was reported earlier. Ignore this targ
+ // and continue, we may still be able to infer all
+ // targs resulting in fewer follow-on errors.
+ continue
+ }
+ if isTyped(arg.typ) {
+ if !u.unify(par.typ, arg.typ) {
+ errorf("type", par.typ, arg.typ, arg)
+ return nil
+ }
+ } else if _, ok := par.typ.(*TypeParam); ok {
+ // Since default types are all basic (i.e., non-composite) types, an
+ // untyped argument will never match a composite parameter type; the
+ // only parameter type it can possibly match against is a *TypeParam.
+ // Thus, for untyped arguments we only need to look at parameter types
+ // that are single type parameters.
+ untyped = append(untyped, i)
+ }
+ }
+ }
+
+ if traceInference {
+ inferred, _ := u.inferred()
+ u.tracef("=> %s ➞ %s\n", tparams, inferred)
+ }
+
+ // --- 2 ---
+ // use information from type parameter constraints
+
+ if traceInference {
+ u.tracef("type parameters: %s", tparams)
+ }
+
+ // Repeatedly apply constraint type inference as long as
+ // progress is being made.
+ //
+ // This is an O(n^2) algorithm where n is the number of
+ // type parameters: if there is progress, at least one
+ // type argument is inferred per iteration and we have
+ // a doubly nested loop.
+ //
+ // In practice this is not a problem because the number
+ // of type parameters tends to be very small (< 5 or so).
+ // (It should be possible for unification to efficiently
+ // signal newly inferred type arguments; then the loops
+ // here could handle the respective type parameters only,
+ // but that will come at a cost of extra complexity which
+ // may not be worth it.)
+ for {
+ nn := u.unknowns()
+
+ for _, tpar := range tparams {
+ // If there is a core term (i.e., a core type with tilde information)
+ // unify the type parameter with the core type.
+ if core, single := coreTerm(tpar); core != nil {
+ if traceInference {
+ u.tracef("core(%s) = %s (single = %v)", tpar, core, single)
+ }
+ // A type parameter can be unified with its core type in two cases.
+ tx := u.at(tpar)
+ switch {
+ case tx != nil:
+ // The corresponding type argument tx is known.
+ // In this case, if the core type has a tilde, the type argument's underlying
+ // type must match the core type, otherwise the type argument and the core type
+ // must match.
+ // If tx is an external type parameter, don't consider its underlying type
+ // (which is an interface). Core type unification will attempt to unify against
+ // core.typ.
+ // Note also that even with inexact unification we cannot leave away the under
+ // call here because it's possible that both tx and core.typ are named types,
+ // with under(tx) being a (named) basic type matching core.typ. Such cases do
+ // not match with inexact unification.
+ if core.tilde && !isTypeParam(tx) {
+ tx = under(tx)
+ }
+ // Unification may fail because it operates with limited information (core type),
+ // even if a given type argument satisfies the corresponding type constraint.
+ // For instance, given [P T1|T2, ...] where the type argument for P is (named
+ // type) T1, and T1 and T2 have the same built-in (named) type T0 as underlying
+ // type, the core type will be the named type T0, which doesn't match T1.
+ // Yet the instantiation of P with T1 is clearly valid (see go.dev/issue/53650).
+ // Reporting an error if unification fails would be incorrect in this case.
+ // On the other hand, it is safe to ignore failing unification during constraint
+ // type inference because if the failure is true, an error will be reported when
+ // checking instantiation.
+ // TODO(gri) we should be able to report an error here and fix the issue in
+ // unification
+ u.unify(tx, core.typ)
+
+ case single && !core.tilde:
+ // The corresponding type argument tx is unknown and there's a single
+ // specific type and no tilde.
+ // In this case the type argument must be that single type; set it.
+ u.set(tpar, core.typ)
+
+ default:
+ // Unification is not possible and no progress was made.
+ continue
+ }
+ } else {
+ if traceInference {
+ u.tracef("core(%s) = nil", tpar)
+ }
+ }
+ }
+
+ if u.unknowns() == nn {
+ break // no progress
+ }
+ }
+
+ if traceInference {
+ inferred, _ := u.inferred()
+ u.tracef("=> %s ➞ %s\n", tparams, inferred)
+ }
+
+ // --- 3 ---
+ // use information from untyped contants
+
+ if traceInference {
+ u.tracef("untyped: %v", untyped)
+ }
+
+ // Some generic parameters with untyped arguments may have been given a type by now.
+ // Collect all remaining parameters that don't have a type yet and unify them with
+ // the default types of the untyped arguments.
+ // We need to collect them all before unifying them with their untyped arguments;
+ // otherwise a parameter type that appears multiple times will have a type after
+ // the first unification and will be skipped later on, leading to incorrect results.
+ j := 0
+ for _, i := range untyped {
+ tpar := params.At(i).typ.(*TypeParam) // is type parameter by construction of untyped
+ if u.at(tpar) == nil {
+ untyped[j] = i
+ j++
+ }
+ }
+ // untyped[:j] are the undices of parameters without a type yet
+ for _, i := range untyped[:j] {
+ tpar := params.At(i).typ.(*TypeParam)
+ arg := args[i]
+ typ := Default(arg.typ)
+ // The default type for an untyped nil is untyped nil which must
+ // not be inferred as type parameter type. Ignore them by making
+ // sure all default types are typed.
+ if isTyped(typ) && !u.unify(tpar, typ) {
+ errorf("default type", tpar, typ, arg)
+ return nil
+ }
+ }
+
+ // --- simplify ---
+
+ // u.inferred() now contains the incoming type arguments plus any additional type
+ // arguments which were inferred. The inferred non-nil entries may still contain
+ // references to other type parameters found in constraints.
+ // For instance, for [A any, B interface{ []C }, C interface{ *A }], if A == int
+ // was given, unification produced the type list [int, []C, *A]. We eliminate the
+ // remaining type parameters by substituting the type parameters in this type list
+ // until nothing changes anymore.
+ inferred, _ = u.inferred()
+ if debug {
+ for i, targ := range targs {
+ assert(targ == nil || inferred[i] == targ)
+ }
+ }
+
+ // The data structure of each (provided or inferred) type represents a graph, where
+ // each node corresponds to a type and each (directed) vertex points to a component
+ // type. The substitution process described above repeatedly replaces type parameter
+ // nodes in these graphs with the graphs of the types the type parameters stand for,
+ // which creates a new (possibly bigger) graph for each type.
+ // The substitution process will not stop if the replacement graph for a type parameter
+ // also contains that type parameter.
+ // For instance, for [A interface{ *A }], without any type argument provided for A,
+ // unification produces the type list [*A]. Substituting A in *A with the value for
+ // A will lead to infinite expansion by producing [**A], [****A], [********A], etc.,
+ // because the graph A -> *A has a cycle through A.
+ // Generally, cycles may occur across multiple type parameters and inferred types
+ // (for instance, consider [P interface{ *Q }, Q interface{ func(P) }]).
+ // We eliminate cycles by walking the graphs for all type parameters. If a cycle
+ // through a type parameter is detected, cycleFinder nils out the respective type
+ // which kills the cycle; this also means that the respective type could not be
+ // inferred.
+ //
+ // TODO(gri) If useful, we could report the respective cycle as an error. We don't
+ // do this now because type inference will fail anyway, and furthermore,
+ // constraints with cycles of this kind cannot currently be satisfied by
+ // any user-supplied type. But should that change, reporting an error
+ // would be wrong.
+ w := cycleFinder{tparams, inferred, make(map[Type]bool)}
+ for _, t := range tparams {
+ w.typ(t) // t != nil
+ }
+
+ // dirty tracks the indices of all types that may still contain type parameters.
+ // We know that nil type entries and entries corresponding to provided (non-nil)
+ // type arguments are clean, so exclude them from the start.
+ var dirty []int
+ for i, typ := range inferred {
+ if typ != nil && (i >= len(targs) || targs[i] == nil) {
+ dirty = append(dirty, i)
+ }
+ }
+
+ for len(dirty) > 0 {
+ // TODO(gri) Instead of creating a new substMap for each iteration,
+ // provide an update operation for substMaps and only change when
+ // needed. Optimization.
+ smap := makeSubstMap(tparams, inferred)
+ n := 0
+ for _, index := range dirty {
+ t0 := inferred[index]
+ if t1 := check.subst(nopos, t0, smap, nil, check.context()); t1 != t0 {
+ inferred[index] = t1
+ dirty[n] = index
+ n++
+ }
+ }
+ dirty = dirty[:n]
+ }
+
+ // Once nothing changes anymore, we may still have type parameters left;
+ // e.g., a constraint with core type *P may match a type parameter Q but
+ // we don't have any type arguments to fill in for *P or Q (go.dev/issue/45548).
+ // Don't let such inferences escape; instead treat them as unresolved.
+ for i, typ := range inferred {
+ if typ == nil || isParameterized(tparams, typ) {
+ obj := tparams[i].obj
+ check.errorf(pos, CannotInferTypeArgs, "cannot infer %s (%s)", obj.name, obj.pos)
+ return nil
+ }
+ }
+
+ return
+}
+
+// dummy function using syntax.Pos to satisfy go/types generator for now
+// TODO(gri) remove and adjust generator
+func _(syntax.Pos) {}
"gccgosizes.go": nil,
"hilbert_test.go": nil,
"infer.go": func(f *ast.File) { fixTokenPos(f); fixInferSig(f) },
+ "infer2.go": func(f *ast.File) { fixTokenPos(f); fixInferSig(f) },
// "initorder.go": fixErrErrorfCall, // disabled for now due to unresolved error_ use implications for gopls
"instantiate.go": func(f *ast.File) { fixTokenPos(f); fixCheckErrorfCall(f) },
"instantiate_test.go": func(f *ast.File) { renameImportPath(f, `"cmd/compile/internal/types2"`, `"go/types"`) },
ast.Inspect(f, func(n ast.Node) bool {
switch n := n.(type) {
case *ast.FuncDecl:
- if n.Name.Name == "infer" {
+ if n.Name.Name == "infer" || n.Name.Name == "infer2" {
// rewrite (pos token.Pos, ...) to (posn positioner, ...)
par := n.Type.Params.List[0]
if len(par.Names) == 1 && par.Names[0].Name == "pos" {
n.Args[0] = arg
return false
}
- case "errorf":
+ case "errorf", "infer2":
// rewrite check.errorf(pos, ...) to check.errorf(posn, ...)
+ // rewrite check.infer2(pos, ...) to check.infer2(posn, ...)
if ident, _ := n.Args[0].(*ast.Ident); ident != nil && ident.Name == "pos" {
pos := n.Args[0].Pos()
arg := newIdent(pos, "posn")
//
// The process stops as soon as all type arguments are known or an error occurs.
func (check *Checker) infer(posn positioner, tparams []*TypeParam, targs []Type, params *Tuple, args []*operand) (result []Type) {
+ if useNewTypeInference {
+ return check.infer2(posn, tparams, targs, params, args)
+ }
+
if debug {
defer func() {
assert(result == nil || len(result) == len(tparams))
--- /dev/null
+// Code generated by "go test -run=Generate -write=all"; DO NOT EDIT.
+
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements type parameter inference.
+
+package types
+
+import (
+ "go/token"
+ . "internal/types/errors"
+)
+
+const useNewTypeInference = false
+
+// infer2 attempts to infer the complete set of type arguments for generic function instantiation/call
+// based on the given type parameters tparams, type arguments targs, function parameters params, and
+// function arguments args, if any. There must be at least one type parameter, no more type arguments
+// than type parameters, and params and args must match in number (incl. zero).
+// If successful, infer returns the complete list of given and inferred type arguments, one for each
+// type parameter. Otherwise the result is nil and appropriate errors will be reported.
+func (check *Checker) infer2(posn positioner, tparams []*TypeParam, targs []Type, params *Tuple, args []*operand) (inferred []Type) {
+ if debug {
+ defer func() {
+ assert(inferred == nil || len(inferred) == len(tparams))
+ for _, targ := range inferred {
+ assert(targ != nil)
+ }
+ }()
+ }
+
+ if traceInference {
+ check.dump("-- infer2 %s%s ➞ %s", tparams, params, targs)
+ defer func() {
+ check.dump("=> %s ➞ %s\n", tparams, inferred)
+ }()
+ }
+
+ // There must be at least one type parameter, and no more type arguments than type parameters.
+ n := len(tparams)
+ assert(n > 0 && len(targs) <= n)
+
+ // Function parameters and arguments must match in number.
+ assert(params.Len() == len(args))
+
+ // If we already have all type arguments, we're done.
+ if len(targs) == n {
+ return targs
+ }
+ // len(targs) < n
+
+ // Rename type parameters to avoid conflicts in recursive instantiation scenarios.
+ tparams, params = check.renameTParams(posn.Pos(), tparams, params)
+
+ // If we have more than 2 arguments, we may have arguments with named and unnamed types.
+ // If that is the case, permutate params and args such that the arguments with named
+ // types are first in the list. This doesn't affect type inference if all types are taken
+ // as is. But when we have inexact unification enabled (as is the case for function type
+ // inference), when a named type is unified with an unnamed type, unification proceeds
+ // with the underlying type of the named type because otherwise unification would fail
+ // right away. This leads to an asymmetry in type inference: in cases where arguments of
+ // named and unnamed types are passed to parameters with identical type, different types
+ // (named vs underlying) may be inferred depending on the order of the arguments.
+ // By ensuring that named types are seen first, order dependence is avoided and unification
+ // succeeds where it can (go.dev/issue/43056).
+ const enableArgSorting = true
+ if m := len(args); m >= 2 && enableArgSorting {
+ // Determine indices of arguments with named and unnamed types.
+ var named, unnamed []int
+ for i, arg := range args {
+ if hasName(arg.typ) {
+ named = append(named, i)
+ } else {
+ unnamed = append(unnamed, i)
+ }
+ }
+
+ // If we have named and unnamed types, move the arguments with
+ // named types first. Update the parameter list accordingly.
+ // Make copies so as not to clobber the incoming slices.
+ if len(named) != 0 && len(unnamed) != 0 {
+ params2 := make([]*Var, m)
+ args2 := make([]*operand, m)
+ i := 0
+ for _, j := range named {
+ params2[i] = params.At(j)
+ args2[i] = args[j]
+ i++
+ }
+ for _, j := range unnamed {
+ params2[i] = params.At(j)
+ args2[i] = args[j]
+ i++
+ }
+ params = NewTuple(params2...)
+ args = args2
+ }
+ }
+
+ // Make sure we have a "full" list of type arguments, some of which may
+ // be nil (unknown). Make a copy so as to not clobber the incoming slice.
+ if len(targs) < n {
+ targs2 := make([]Type, n)
+ copy(targs2, targs)
+ targs = targs2
+ }
+ // len(targs) == n
+
+ // Continue with the type arguments we have. Avoid matching generic
+ // parameters that already have type arguments against function arguments:
+ // It may fail because matching uses type identity while parameter passing
+ // uses assignment rules. Instantiate the parameter list with the type
+ // arguments we have, and continue with that parameter list.
+
+ // Substitute type arguments for their respective type parameters in params,
+ // if any. Note that nil targs entries are ignored by check.subst.
+ // TODO(gri) Can we avoid this (we're setting known type arguments below,
+ // but that doesn't impact the isParameterized check for now).
+ if params.Len() > 0 {
+ smap := makeSubstMap(tparams, targs)
+ params = check.subst(nopos, params, smap, nil, check.context()).(*Tuple)
+ }
+
+ // Unify parameter and argument types for generic parameters with typed arguments
+ // and collect the indices of generic parameters with untyped arguments.
+ // Terminology: generic parameter = function parameter with a type-parameterized type
+ u := newUnifier(tparams, targs)
+
+ errorf := func(kind string, tpar, targ Type, arg *operand) {
+ // provide a better error message if we can
+ targs, index := u.inferred()
+ if index == 0 {
+ // The first type parameter couldn't be inferred.
+ // If none of them could be inferred, don't try
+ // to provide the inferred type in the error msg.
+ allFailed := true
+ for _, targ := range targs {
+ if targ != nil {
+ allFailed = false
+ break
+ }
+ }
+ if allFailed {
+ check.errorf(arg, CannotInferTypeArgs, "%s %s of %s does not match %s (cannot infer %s)", kind, targ, arg.expr, tpar, typeParamsString(tparams))
+ return
+ }
+ }
+ smap := makeSubstMap(tparams, targs)
+ // TODO(gri): pass a poser here, rather than arg.Pos().
+ inferred := check.subst(arg.Pos(), tpar, smap, nil, check.context())
+ // CannotInferTypeArgs indicates a failure of inference, though the actual
+ // error may be better attributed to a user-provided type argument (hence
+ // InvalidTypeArg). We can't differentiate these cases, so fall back on
+ // the more general CannotInferTypeArgs.
+ if inferred != tpar {
+ check.errorf(arg, CannotInferTypeArgs, "%s %s of %s does not match inferred type %s for %s", kind, targ, arg.expr, inferred, tpar)
+ } else {
+ check.errorf(arg, CannotInferTypeArgs, "%s %s of %s does not match %s", kind, targ, arg.expr, tpar)
+ }
+ }
+
+ // indices of generic parameters with untyped arguments, for later use
+ var untyped []int
+
+ // --- 1 ---
+ // use information from function arguments
+
+ if traceInference {
+ u.tracef("parameters: %s", params)
+ u.tracef("arguments : %s", args)
+ }
+
+ for i, arg := range args {
+ par := params.At(i)
+ // If we permit bidirectional unification, this conditional code needs to be
+ // executed even if par.typ is not parameterized since the argument may be a
+ // generic function (for which we want to infer its type arguments).
+ if isParameterized(tparams, par.typ) {
+ if arg.mode == invalid {
+ // An error was reported earlier. Ignore this targ
+ // and continue, we may still be able to infer all
+ // targs resulting in fewer follow-on errors.
+ continue
+ }
+ if isTyped(arg.typ) {
+ if !u.unify(par.typ, arg.typ) {
+ errorf("type", par.typ, arg.typ, arg)
+ return nil
+ }
+ } else if _, ok := par.typ.(*TypeParam); ok {
+ // Since default types are all basic (i.e., non-composite) types, an
+ // untyped argument will never match a composite parameter type; the
+ // only parameter type it can possibly match against is a *TypeParam.
+ // Thus, for untyped arguments we only need to look at parameter types
+ // that are single type parameters.
+ untyped = append(untyped, i)
+ }
+ }
+ }
+
+ if traceInference {
+ inferred, _ := u.inferred()
+ u.tracef("=> %s ➞ %s\n", tparams, inferred)
+ }
+
+ // --- 2 ---
+ // use information from type parameter constraints
+
+ if traceInference {
+ u.tracef("type parameters: %s", tparams)
+ }
+
+ // Repeatedly apply constraint type inference as long as
+ // progress is being made.
+ //
+ // This is an O(n^2) algorithm where n is the number of
+ // type parameters: if there is progress, at least one
+ // type argument is inferred per iteration and we have
+ // a doubly nested loop.
+ //
+ // In practice this is not a problem because the number
+ // of type parameters tends to be very small (< 5 or so).
+ // (It should be possible for unification to efficiently
+ // signal newly inferred type arguments; then the loops
+ // here could handle the respective type parameters only,
+ // but that will come at a cost of extra complexity which
+ // may not be worth it.)
+ for {
+ nn := u.unknowns()
+
+ for _, tpar := range tparams {
+ // If there is a core term (i.e., a core type with tilde information)
+ // unify the type parameter with the core type.
+ if core, single := coreTerm(tpar); core != nil {
+ if traceInference {
+ u.tracef("core(%s) = %s (single = %v)", tpar, core, single)
+ }
+ // A type parameter can be unified with its core type in two cases.
+ tx := u.at(tpar)
+ switch {
+ case tx != nil:
+ // The corresponding type argument tx is known.
+ // In this case, if the core type has a tilde, the type argument's underlying
+ // type must match the core type, otherwise the type argument and the core type
+ // must match.
+ // If tx is an external type parameter, don't consider its underlying type
+ // (which is an interface). Core type unification will attempt to unify against
+ // core.typ.
+ // Note also that even with inexact unification we cannot leave away the under
+ // call here because it's possible that both tx and core.typ are named types,
+ // with under(tx) being a (named) basic type matching core.typ. Such cases do
+ // not match with inexact unification.
+ if core.tilde && !isTypeParam(tx) {
+ tx = under(tx)
+ }
+ // Unification may fail because it operates with limited information (core type),
+ // even if a given type argument satisfies the corresponding type constraint.
+ // For instance, given [P T1|T2, ...] where the type argument for P is (named
+ // type) T1, and T1 and T2 have the same built-in (named) type T0 as underlying
+ // type, the core type will be the named type T0, which doesn't match T1.
+ // Yet the instantiation of P with T1 is clearly valid (see go.dev/issue/53650).
+ // Reporting an error if unification fails would be incorrect in this case.
+ // On the other hand, it is safe to ignore failing unification during constraint
+ // type inference because if the failure is true, an error will be reported when
+ // checking instantiation.
+ // TODO(gri) we should be able to report an error here and fix the issue in
+ // unification
+ u.unify(tx, core.typ)
+
+ case single && !core.tilde:
+ // The corresponding type argument tx is unknown and there's a single
+ // specific type and no tilde.
+ // In this case the type argument must be that single type; set it.
+ u.set(tpar, core.typ)
+
+ default:
+ // Unification is not possible and no progress was made.
+ continue
+ }
+ } else {
+ if traceInference {
+ u.tracef("core(%s) = nil", tpar)
+ }
+ }
+ }
+
+ if u.unknowns() == nn {
+ break // no progress
+ }
+ }
+
+ if traceInference {
+ inferred, _ := u.inferred()
+ u.tracef("=> %s ➞ %s\n", tparams, inferred)
+ }
+
+ // --- 3 ---
+ // use information from untyped contants
+
+ if traceInference {
+ u.tracef("untyped: %v", untyped)
+ }
+
+ // Some generic parameters with untyped arguments may have been given a type by now.
+ // Collect all remaining parameters that don't have a type yet and unify them with
+ // the default types of the untyped arguments.
+ // We need to collect them all before unifying them with their untyped arguments;
+ // otherwise a parameter type that appears multiple times will have a type after
+ // the first unification and will be skipped later on, leading to incorrect results.
+ j := 0
+ for _, i := range untyped {
+ tpar := params.At(i).typ.(*TypeParam) // is type parameter by construction of untyped
+ if u.at(tpar) == nil {
+ untyped[j] = i
+ j++
+ }
+ }
+ // untyped[:j] are the undices of parameters without a type yet
+ for _, i := range untyped[:j] {
+ tpar := params.At(i).typ.(*TypeParam)
+ arg := args[i]
+ typ := Default(arg.typ)
+ // The default type for an untyped nil is untyped nil which must
+ // not be inferred as type parameter type. Ignore them by making
+ // sure all default types are typed.
+ if isTyped(typ) && !u.unify(tpar, typ) {
+ errorf("default type", tpar, typ, arg)
+ return nil
+ }
+ }
+
+ // --- simplify ---
+
+ // u.inferred() now contains the incoming type arguments plus any additional type
+ // arguments which were inferred. The inferred non-nil entries may still contain
+ // references to other type parameters found in constraints.
+ // For instance, for [A any, B interface{ []C }, C interface{ *A }], if A == int
+ // was given, unification produced the type list [int, []C, *A]. We eliminate the
+ // remaining type parameters by substituting the type parameters in this type list
+ // until nothing changes anymore.
+ inferred, _ = u.inferred()
+ if debug {
+ for i, targ := range targs {
+ assert(targ == nil || inferred[i] == targ)
+ }
+ }
+
+ // The data structure of each (provided or inferred) type represents a graph, where
+ // each node corresponds to a type and each (directed) vertex points to a component
+ // type. The substitution process described above repeatedly replaces type parameter
+ // nodes in these graphs with the graphs of the types the type parameters stand for,
+ // which creates a new (possibly bigger) graph for each type.
+ // The substitution process will not stop if the replacement graph for a type parameter
+ // also contains that type parameter.
+ // For instance, for [A interface{ *A }], without any type argument provided for A,
+ // unification produces the type list [*A]. Substituting A in *A with the value for
+ // A will lead to infinite expansion by producing [**A], [****A], [********A], etc.,
+ // because the graph A -> *A has a cycle through A.
+ // Generally, cycles may occur across multiple type parameters and inferred types
+ // (for instance, consider [P interface{ *Q }, Q interface{ func(P) }]).
+ // We eliminate cycles by walking the graphs for all type parameters. If a cycle
+ // through a type parameter is detected, cycleFinder nils out the respective type
+ // which kills the cycle; this also means that the respective type could not be
+ // inferred.
+ //
+ // TODO(gri) If useful, we could report the respective cycle as an error. We don't
+ // do this now because type inference will fail anyway, and furthermore,
+ // constraints with cycles of this kind cannot currently be satisfied by
+ // any user-supplied type. But should that change, reporting an error
+ // would be wrong.
+ w := cycleFinder{tparams, inferred, make(map[Type]bool)}
+ for _, t := range tparams {
+ w.typ(t) // t != nil
+ }
+
+ // dirty tracks the indices of all types that may still contain type parameters.
+ // We know that nil type entries and entries corresponding to provided (non-nil)
+ // type arguments are clean, so exclude them from the start.
+ var dirty []int
+ for i, typ := range inferred {
+ if typ != nil && (i >= len(targs) || targs[i] == nil) {
+ dirty = append(dirty, i)
+ }
+ }
+
+ for len(dirty) > 0 {
+ // TODO(gri) Instead of creating a new substMap for each iteration,
+ // provide an update operation for substMaps and only change when
+ // needed. Optimization.
+ smap := makeSubstMap(tparams, inferred)
+ n := 0
+ for _, index := range dirty {
+ t0 := inferred[index]
+ if t1 := check.subst(nopos, t0, smap, nil, check.context()); t1 != t0 {
+ inferred[index] = t1
+ dirty[n] = index
+ n++
+ }
+ }
+ dirty = dirty[:n]
+ }
+
+ // Once nothing changes anymore, we may still have type parameters left;
+ // e.g., a constraint with core type *P may match a type parameter Q but
+ // we don't have any type arguments to fill in for *P or Q (go.dev/issue/45548).
+ // Don't let such inferences escape; instead treat them as unresolved.
+ for i, typ := range inferred {
+ if typ == nil || isParameterized(tparams, typ) {
+ obj := tparams[i].obj
+ check.errorf(posn, CannotInferTypeArgs, "cannot infer %s (%s)", obj.name, obj.pos)
+ return nil
+ }
+ }
+
+ return
+}
+
+// dummy function using syntax.Pos to satisfy go/types generator for now
+// TODO(gri) remove and adjust generator
+func _(token.Pos) {}