A comparison of two values with a type equivalent to string

gives an type mismatch error, although both values appear to
have the same type.

R=ken,rsc
DELTA=23  (23 added, 0 deleted, 0 changed)
OCL=28786
CL=28805

test/bugs/bug151.go

+// $G $D/$F.go || echo BUG: bug151
+
+// Copyright 2009 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 S string
+
+type Empty interface {}
+
+func (v S) Less(e Empty) bool {
+	return v < e.(S);
+}
+
+/*
+bugs/bug151.go:10: illegal types for operand: CALL
+	string
+	S
+*/

test/golden.out

@@ -109,6 +109,12 @@ BUG: should compile
 bugs/bug150.go:13: reorder2: too many funcation calls evaluating parameters
 BUG: bug150
 
+=========== bugs/bug151.go
+bugs/bug151.go:10: illegal types for operand: CALL
+	string
+	S
+BUG: bug151
+
 =========== fixedbugs/bug016.go
 fixedbugs/bug016.go:7: constant -3 overflows uint