Restrict unary negation to signed integers.

test/libsolidity/SolidityEndToEndTest.cpp

@@ -778,7 +778,7 @@ BOOST_AUTO_TEST_CASE(sign_extension)
 			function run() public returns(uint256 y) {
 				int64 x = -int32(0xff);
 				if (x >= 0xff) return 0;
-				return -uint256(x);
+				return 0 - uint256(x);
 			}
 		}
 	)";

test/libsolidity/semanticTests/storage/packed_storage_signed.sol

@@ -9,7 +9,7 @@ contract C {
         returns (uint256 x1, uint256 x2, uint256 x3, uint256 x4)
     {
         a = -2;
-        b = -uint8(a) * 2;
+        b = (0 - uint8(a)) * 2;
         c = a * int8(120) * int8(121);
         x1 = uint256(a);
         x2 = b;

test/libsolidity/syntaxTests/negation.sol

@@ -0,0 +1,9 @@
+contract test {
+    function f() public pure {
+        int x;
+        uint y = uint(-x);
+        -y;
+    }
+}
+// ----
+// TypeError 4907: (97-99): Unary operator - cannot be applied to type uint256