cmd/gc: disallow pointer constants

Fixes #7760.

LGTM=iant
R=iant, remyoudompheng
CC=golang-codereviews
https://golang.org/cl/130720043

src/cmd/gc/const.c

@@ -1566,7 +1566,6 @@ isgoconst(Node *n)
 	case ORSH:
 	case OSUB:
 	case OXOR:
-	case OCONV:
 	case OIOTA:
 	case OCOMPLEX:
 	case OREAL:
@@ -1574,7 +1573,12 @@ isgoconst(Node *n)
 		if(isgoconst(n->left) && (n->right == N || isgoconst(n->right)))
 			return 1;
 		break;
-	
+
+	case OCONV:
+		if(okforconst[n->type->etype] && isgoconst(n->left))
+			return 1;
+		break;
+
 	case OLEN:
 	case OCAP:
 		l = n->left;

test/fixedbugs/issue7760.go

@@ -0,0 +1,25 @@
+// errorcheck
+
+// Copyright 2014 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.
+
+// Verify that pointers can't be used as constants.
+
+package main
+
+import "unsafe"
+
+type myPointer unsafe.Pointer
+
+const _ = unsafe.Pointer(uintptr(1)) // ERROR "is not (a )?constant"
+const _ = myPointer(uintptr(1)) // ERROR "is not (a )?constant"
+
+const _ = (*int)(unsafe.Pointer(uintptr(1))) // ERROR "is not (a )?constant"
+const _ = (*int)(myPointer(uintptr(1))) // ERROR "is not (a )?constant"
+
+const _ = uintptr(unsafe.Pointer(uintptr(1))) // ERROR "is not (a )?constant"
+const _ = uintptr(myPointer(uintptr(1))) // ERROR "is not (a )?constant"
+
+const _ = []byte("") // ERROR "is not (a )?constant"
+const _ = []rune("") // ERROR "is not (a )?constant"