go/types, types2: use exact matching when unifying constraint method signatures

Fixes #60556.

Change-Id: I203a0bc79eff607654c3e8350d259e694cb035b2
Reviewed-on: https://go-review.googlesource.com/c/go/+/499995
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>

Author: griesemer

src/cmd/compile/internal/types2/infer.go

@@ -249,9 +249,15 @@ func (check *Checker) infer(pos syntax.Pos, tparams []*TypeParam, targs []Type,
 					// It must have (at least) all the methods of the type constraint,
 					// and the method signatures must unify; otherwise tx cannot satisfy
 					// the constraint.
+					// TODO(gri) Now that unification handles interfaces, this code can
+					//           be reduced to calling u.unify(tx, tpar.iface(), assign)
+					//           (which will compare signatures exactly as we do below).
+					//           We leave it as is for now because missingMethod provides
+					//           a failure cause which allows for a better error message.
+					//           Eventually, unify should return an error with cause.
 					var cause string
 					constraint := tpar.iface()
-					if m, _ := check.missingMethod(tx, constraint, true, func(x, y Type) bool { return u.unify(x, y, 0) }, &cause); m != nil {
+					if m, _ := check.missingMethod(tx, constraint, true, func(x, y Type) bool { return u.unify(x, y, exact) }, &cause); m != nil {
 						// TODO(gri) better error message (see TODO above)
 						check.errorf(pos, CannotInferTypeArgs, "%s (type %s) does not satisfy %s %s", tpar, tx, tpar.Constraint(), cause)
 						return nil

src/go/types/infer.go

@@ -0,0 +1,19 @@
+// 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.
+
+package p
+
+type I[T any] interface {
+	m(I[T])
+}
+
+type S[T any] struct{}
+
+func (S[T]) m(I[T]) {}
+
+func f[T I[E], E any](T) {}
+
+func _() {
+	f(S[int]{})
+}