[release-branch.go1.18] go/types, types2: clarify a comment and add an extra test

Confirm that the current implementation of core type unification
looks correct and update the respective comment. Add an extra test.

Fixes #51376.

Change-Id: I6a603a4baeee2ede5bb4a1d60766204a808936d7
Reviewed-on: https://go-review.googlesource.com/c/go/+/388294
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
(cherry picked from commit 6da16b6ad5)
Reviewed-on: https://go-review.googlesource.com/c/go/+/390016
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/types2/testdata/fixedbugs/issue51376.go2

@@ -0,0 +1,24 @@
+// Copyright 2022 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 Map map[string]int
+
+func f[M ~map[K]V, K comparable, V any](M) {}
+func g[M map[K]V, K comparable, V any](M) {}
+
+func _[M1 ~map[K]V, M2 map[K]V, K comparable, V any]() {
+        var m1 M1
+        f(m1)
+        g( /* ERROR M1 does not implement map\[K\]V */ m1) // M1 has tilde
+
+        var m2 M2
+        f(m2)
+        g(m2) // M1 does not have tilde
+
+        var m3 Map
+        f(m3)
+        g( /* ERROR Map does not implement map\[string\]int */ m3) // M in g does not have tilde
+}

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

@@ -359,17 +359,17 @@ func (u *unifier) nify(x, y Type, p *ifacePair) (result bool) {
 	// (see issue #50755 for a test case).
 	if enableCoreTypeUnification && !u.exact {
 		if isTypeParam(x) && !hasName(y) {
-			// Caution: This may not be correct in light of ~ constraints.
-			//          See issue #51376.
-			// TODO(gri) investigate!
-			//
 			// When considering the type parameter for unification
-			// we look at the adjusted core type (coreTerm).
+			// we look at the adjusted core term (adjusted core type
+			// with tilde information).
 			// If the adjusted core type is a named type N; the
 			// corresponding core type is under(N). Since !u.exact
 			// and y doesn't have a name, unification will end up
 			// comparing under(N) to y, so we can just use the core
-			// type instead. Optimization.
+			// type instead. And we can ignore the tilde because we
+			// already look at the underlying types on both sides
+			// and we have known types on both sides.
+			// Optimization.
 			if cx := coreType(x); cx != nil {
 				if traceInference {
 					u.tracef("core %s ≡ %s", x, y)

src/go/types/testdata/fixedbugs/issue51376.go2

@@ -0,0 +1,24 @@
+// Copyright 2022 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 Map map[string]int
+
+func f[M ~map[K]V, K comparable, V any](M) {}
+func g[M map[K]V, K comparable, V any](M) {}
+
+func _[M1 ~map[K]V, M2 map[K]V, K comparable, V any]() {
+        var m1 M1
+        f(m1)
+        g /* ERROR M1 does not implement map\[K\]V */ (m1) // M1 has tilde
+
+        var m2 M2
+        f(m2)
+        g(m2) // M1 does not have tilde
+
+        var m3 Map
+        f(m3)
+        g /* ERROR Map does not implement map\[string\]int */ (m3) // M in g does not have tilde
+}

src/go/types/unify.go

@@ -359,17 +359,17 @@ func (u *unifier) nify(x, y Type, p *ifacePair) (result bool) {
 	// (see issue #50755 for a test case).
 	if enableCoreTypeUnification && !u.exact {
 		if isTypeParam(x) && !hasName(y) {
-			// Caution: This may not be correct in light of ~ constraints.
-			//          See issue #51376.
-			// TODO(gri) investigate!
-			//
 			// When considering the type parameter for unification
-			// we look at the adjusted core type (coreTerm).
+			// we look at the adjusted core term (adjusted core type
+			// with tilde information).
 			// If the adjusted core type is a named type N; the
 			// corresponding core type is under(N). Since !u.exact
 			// and y doesn't have a name, unification will end up
 			// comparing under(N) to y, so we can just use the core
-			// type instead. Optimization.
+			// type instead. And we can ignore the tilde because we
+			// already look at the underlying types on both sides
+			// and we have known types on both sides.
+			// Optimization.
 			if cx := coreType(x); cx != nil {
 				if traceInference {
 					u.tracef("core %s ≡ %s", x, y)