go/types: add a test case for issue 45985

The fix for #45985 is a little subtle. Start by committing the (bad)
test case.

For #45985

Change-Id: Ia6625818e9b1c5e869b2c2f724f817c13c9944d1
Reviewed-on: https://go-review.googlesource.com/c/go/+/317471
Trust: Robert Findley <[email protected]>
Run-TryBot: Robert Findley <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>

Author: findleyr

src/go/types/infer.go

@@ -398,7 +398,7 @@ func (check *Checker) inferB(tparams []*TypeName, targs []Type, report bool) (ty
 		if sbound != nil {
 			if !u.unify(typ, sbound) {
 				if report {
-					check.errorf(tpar, 0, "%s does not match %s", tpar, sbound)
+					check.errorf(tpar, _Todo, "%s does not match %s", tpar, sbound)
 				}
 				return nil, 0
 			}
@@ -412,7 +412,7 @@ 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/go/types/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(rFindley): 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]
+}