[release-branch.go1.18] cmd/compile: fix generic inter-inter comparisons from value switch statements

If value is a non-empty interface and has shape, we still need to
convert it to an interface{} first.

Fixes #53587

Change-Id: I516063ba4429a6cc24c483758387ec13815fc63e
Reviewed-on: https://go-review.googlesource.com/c/go/+/414834
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/414835

2 files changed, 36 insertions, 2 deletions

diff --git a/src/cmd/compile/internal/noder/stencil.go b/src/cmd/compile/internal/noder/stencil.go
index 7cfd70ec546..4c3dd37d94f 100644
--- a/src/cmd/compile/internal/noder/stencil.go
+++ b/src/cmd/compile/internal/noder/stencil.go
@@ -1186,7 +1186,7 @@ func (subst *subster) node(n ir.Node) ir.Node {
 			if m.Tag != nil && m.Tag.Op() == ir.OTYPESW {
 				break // Nothing to do here for type switches.
 			}
-			if m.Tag != nil && !m.Tag.Type().IsInterface() && m.Tag.Type().HasShape() {
+			if m.Tag != nil && !m.Tag.Type().IsEmptyInterface() && m.Tag.Type().HasShape() {
 				// To implement a switch on a value that is or has a type parameter, we first convert
 				// that thing we're switching on to an interface{}.
 				m.Tag = assignconvfn(m.Tag, types.Types[types.TINTER])
@@ -1195,7 +1195,7 @@ func (subst *subster) node(n ir.Node) ir.Node {
 				for i, x := range c.List {
 					// If we have a case that is or has a type parameter, convert that case
 					// to an interface{}.
-					if !x.Type().IsInterface() && x.Type().HasShape() {
+					if !x.Type().IsEmptyInterface() && x.Type().HasShape() {
 						c.List[i] = assignconvfn(x, types.Types[types.TINTER])
 					}
 				}
diff --git a/test/typeparam/issue53477.go b/test/typeparam/issue53477.go
new file mode 100644
index 00000000000..314171a7586
--- /dev/null
+++ b/test/typeparam/issue53477.go
@@ -0,0 +1,34 @@
+// run -gcflags=-G=3
+
+// 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")
+	}
+}