test: add regress test for #53477

This test already passes for GOEXPERIMENT=unified; add regress test to
ensure it stays that way.

Updates #53477.

Change-Id: Ib7aa7428260595077052207899edcc044a6ab1c8
Reviewed-on: https://go-review.googlesource.com/c/go/+/413394
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: David Chase <drchase@google.com>

src/go/internal/gcimporter/gcimporter_test.go

@@ -178,6 +178,7 @@ func TestImportTypeparamTests(t *testing.T) {
 		"nested.go":     "fails to compile",              // TODO(rfindley): investigate this.
 		"issue50417.go": "inconsistent interface member sorting",
 		"issue53419.go": "fails to compile",
+		"issue53477.go": "fails to compile",
 	}
 
 	for _, entry := range list {

test/run.go

@@ -1967,6 +1967,7 @@ var go118Failures = setOf(
 	"typeparam/nested.go",     // 1.18 compiler doesn't support function-local types with generics
 	"typeparam/issue51521.go", // 1.18 compiler produces bad panic message and link error
 	"typeparam/issue53419.go", // 1.18 compiler mishandles generic selector resolution
+	"typeparam/issue53477.go", // 1.18 compiler mishandles generic interface-interface comparisons from value switch statements
 )
 
 // In all of these cases, the 1.17 compiler reports reasonable errors, but either the

test/typeparam/issue53477.go

@@ -0,0 +1,34 @@
+// run
+
+// 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.
+
+// Test that generic interface-interface comparisons resulting from
+// value switch statements are handled correctly.
+
+package main
+
+func main() {
+	f[X](0)
+}
+
+type Mer[T any] interface{ M(T) }
+type MNer[T any] interface {
+	Mer[T]
+	N()
+}
+
+type X int
+
+func (X) M(X) {}
+func (X) N()  {}
+
+func f[T MNer[T]](t T) {
+	switch Mer[T](t) {
+	case MNer[T](t):
+		// ok
+	default:
+		panic("FAIL")
+	}
+}