test: test for issue 53087

This issue has been fixed with unified IR, so just add a test.

Update #53087

Change-Id: I965d9f27529fa6b7c89e2921c65e5a100daeb9fe
Reviewed-on: https://go-review.googlesource.com/c/go/+/410197
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Keith Randall <khr@google.com>

test/typeparam/issue53087.go

@@ -0,0 +1,56 @@
+// 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
+
+import "fmt"
+
+type I interface {
+	M()
+}
+
+type S struct {
+	str string
+}
+
+func (s *S) M() {}
+
+var _ I = &S{}
+
+type CloningMap[K comparable, V any] struct {
+	inner map[K]V
+}
+
+func (cm CloningMap[K, V]) With(key K, value V) CloningMap[K, V] {
+	result := CloneBad(cm.inner)
+	result[key] = value
+	return CloningMap[K, V]{result}
+}
+
+func CloneBad[M ~map[K]V, K comparable, V any](m M) M {
+	r := make(M, len(m))
+	for k, v := range m {
+		r[k] = v
+	}
+	return r
+}
+
+func main() {
+	s1 := &S{"one"}
+	s2 := &S{"two"}
+
+	m := CloningMap[string, I]{inner: make(map[string]I)}
+	m = m.With("a", s1)
+	m = m.With("b", s2)
+
+	it, found := m.inner["a"]
+	if !found {
+		panic("a not found")
+	}
+	if _, ok := it.(*S); !ok {
+		panic(fmt.Sprintf("got %T want *main.S", it))
+	}
+}