[dev.typeparams] cmd/compile: fix missing condition in usemethod

CL 330670 simplified usemethod, but dropped the previous condition to
ensure the function have 1 or 2 result. This CL restore that condition,
and also add a test for it.

Change-Id: I434e3736785b43ceea0b386d8d9d01ad78a4ccd2
Reviewed-on: https://go-review.googlesource.com/c/go/+/336609
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/walk/expr.go

@@ -957,7 +957,14 @@ func usemethod(n *ir.CallExpr) {
 	if t.NumParams() != 1 || t.Params().Field(0).Type.Kind() != pKind {
 		return
 	}
-	if t.NumResults() == 2 && t.Results().Field(1).Type.Kind() != types.TBOOL {
+	switch t.NumResults() {
+	case 1:
+		// ok
+	case 2:
+		if t.Results().Field(1).Type.Kind() != types.TBOOL {
+			return
+		}
+	default:
 		return
 	}
 

test/reflectmethod8.go

@@ -0,0 +1,26 @@
+// compile
+
+// 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.
+
+// Make sure that the compiler can analyze non-reflect
+// Type.{Method,MethodByName} calls.
+
+package p
+
+type I interface {
+	MethodByName(string)
+	Method(int)
+}
+
+type M struct{}
+
+func (M) MethodByName(string) {}
+func (M) Method(int)          {}
+
+func f() {
+	var m M
+	I.MethodByName(m, "")
+	I.Method(m, 42)
+}