cmd/compile/internal/types2: add test case for issue 45985

This is a port of https://golang.org/cl/317471.

Updates #45985.

Change-Id: I539d0fa6f9a77eb2fc169a21c5db9921f8eceb03
Reviewed-on: https://go-review.googlesource.com/c/go/+/317732
Trust: Robert Griesemer <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: griesemer

src/cmd/compile/internal/types2/infer.go

@@ -417,7 +417,7 @@ func (check *Checker) inferB(tparams []*TypeName, targs []Type, report bool) (ty
 	// was given, unification produced the type list [int, []C, *A]. We eliminate the
 	// remaining type parameters by substituting the type parameters in this type list
 	// until nothing changes anymore.
-	types, index = u.x.types()
+	types, _ = u.x.types()
 	if debug {
 		for i, targ := range targs {
 			assert(targ == nil || types[i] == targ)

src/cmd/compile/internal/types2/testdata/fixedbugs/issue45985.go2

@@ -0,0 +1,14 @@
+// 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 issue45985
+
+// TODO(gri): this error should be on app[int] below.
+func app[S /* ERROR "type S = S does not match" */ interface{ type []T }, T any](s S, e T) S {
+    return append(s, e)
+}
+
+func _() {
+	_ = app[int]
+}