cmd/gc: complain about invalid whitespace chars
Fixes #4405.
R=rsc, bradfitz
CC=golang-dev
https://golang.org/cl/6855060
diff --git a/test/fixedbugs/issue4405.go b/test/fixedbugs/issue4405.go
new file mode 100644
index 0000000..c0d8085
--- /dev/null
+++ b/test/fixedbugs/issue4405.go
@@ -0,0 +1,15 @@
+// errorcheck
+
+// Copyright 2012 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 p
+
+const (
+ _ = iota
+ _ // ERROR "illegal character"
+ _ // ERROR "illegal character"
+ _ // ERROR "illegal character"
+ _ // ERROR "illegal character"
+)
