cmd/compile: don't convert to interface{} for un-comparable types in generic switch

Fixes #53635

Change-Id: I41f383be8870432fc0d29fa83687911ddd8217f1
Reviewed-on: https://go-review.googlesource.com/c/go/+/415634
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Cuong Manh Le <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Matthew Dempsky <[email protected]>

Author: cuonglm

src/cmd/compile/internal/noder/stencil.go

@@ -1214,6 +1214,9 @@ 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 && !types.IsComparable(m.Tag.Type()) {
+				break // Nothing to do here for un-comparable types.
+			}
 			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{}.

test/fixedbugs/issue53635.go

@@ -0,0 +1,31 @@
+// 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.
+
+package main
+
+func main() {
+	f[int]()
+}
+
+func f[T any]() {
+	switch []T(nil) {
+	case nil:
+	default:
+		panic("FAIL")
+	}
+
+	switch (func() T)(nil) {
+	case nil:
+	default:
+		panic("FAIL")
+	}
+
+	switch (map[int]T)(nil) {
+	case nil:
+	default:
+		panic("FAIL")
+	}
+}