test/typeparam: add a test case for issue46591

Fixes #46591

Change-Id: I4875092ecd7760b0cd487e793576ef7a9a569a0e
Reviewed-on: https://go-review.googlesource.com/c/go/+/343970
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

test/typeparam/issue46591.go

@@ -0,0 +1,22 @@
+// run -gcflags=-G=3
+
+// Copyright 2021 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
+
+type T[_ any] struct{}
+
+var m = map[interface{}]int{
+	T[struct{ int }]{}: 0,
+	T[struct {
+		int "x"
+	}]{}: 0,
+}
+
+func main() {
+	if len(m) != 2 {
+		panic(len(m))
+	}
+}