cmd/compile: fix value range check for complex constants

Fixes #11590.

Change-Id: I4144107334604a2cc98c7984df3b5d4cde3d30af
Reviewed-on: https://go-review.googlesource.com/16920Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/compile/internal/gc/const.go

@@ -259,7 +259,6 @@ func convlit1(np **Node, t *Type, explicit bool) {
 				n.SetVal(toint(n.Val()))
 				fallthrough
 
-				// flowthrough
 			case CTINT:
 				overflow(n.Val(), t)
 			}
@@ -272,7 +271,6 @@ func convlit1(np **Node, t *Type, explicit bool) {
 				n.SetVal(toflt(n.Val()))
 				fallthrough
 
-				// flowthrough
 			case CTFLT:
 				n.SetVal(Val{truncfltlit(n.Val().U.(*Mpflt), t)})
 			}
@@ -283,6 +281,7 @@ func convlit1(np **Node, t *Type, explicit bool) {
 
 			case CTFLT, CTINT, CTRUNE:
 				n.SetVal(tocplx(n.Val()))
+				fallthrough
 
 			case CTCPLX:
 				overflow(n.Val(), t)

test/fixedbugs/issue11590.go

+// errorcheck
+
+// Copyright 2015 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
+
+var _ = int8(4) * 300         // ERROR "constant overflows int8"
+var _ = complex64(1) * 1e200  // ERROR "constant overflows complex64"
+var _ = complex128(1) * 1e500 // ERROR "constant overflows complex128"