You can't set a fixed array to nil.

R=r
DELTA=12  (12 added, 0 deleted, 0 changed)
OCL=31915
CL=31936

test/bugs/bug170.go

+// errchk $G $D/$F.go
+
+// 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
+var v1 = ([10]int)(nil)		// ERROR "illegal|nil|invalid"
+var v2 [10]int = nil		// ERROR "illegal|nil|incompatible"
+var v3 [10]int
+func main() {
+	v3 = nil;		// ERROR "illegal|nil|incompatible"
+}

test/golden.out

@@ -109,6 +109,9 @@ BUG: should fail
 =========== bugs/bug169.go
 BUG: errchk: command succeeded unexpectedly:  6g bugs/bug169.go
 
+=========== bugs/bug170.go
+BUG: errchk: command succeeded unexpectedly:  6g bugs/bug170.go
+
 =========== fixedbugs/bug016.go
 fixedbugs/bug016.go:7: constant -3 overflows uint