Restrict unary negation to signed integers.

Changelog.md

@@ -1,5 +1,7 @@
 ### 0.8.0 (unreleased)
 
+Breaking Changes:
+ * Type System: Unary negation can only be used on signed integers, not on unsigned integers.
 
 
 ### 0.7.2 (unreleased)

docs/types/value-types.rst

@@ -38,7 +38,7 @@ Operators:
 * Comparisons: ``<=``, ``<``, ``==``, ``!=``, ``>=``, ``>`` (evaluate to ``bool``)
 * Bit operators: ``&``, ``|``, ``^`` (bitwise exclusive or), ``~`` (bitwise negation)
 * Shift operators: ``<<`` (left shift), ``>>`` (right shift)
-* Arithmetic operators: ``+``, ``-``, unary ``-``, ``*``, ``/``, ``%`` (modulo), ``**`` (exponentiation)
+* Arithmetic operators: ``+``, ``-``, unary ``-`` (only for signed integers), ``*``, ``/``, ``%`` (modulo), ``**`` (exponentiation)
 
 For an integer type ``X``, you can use ``type(X).min`` and ``type(X).max`` to
 access the minimum and maximum value representable by the type.
@@ -83,8 +83,8 @@ for example ``uint256(0) - uint256(1) == 2**256 - 1``. You have to take these ov
 into account when designing safe smart contracts.
 
 The expression ``-x`` is equivalent to ``(T(0) - x)`` where
-``T`` is the type of ``x``. This means that ``-x`` will not be negative
-if the type of ``x`` is an unsigned integer type. Also, ``-x`` can be
+``T`` is the type of ``x``. It can only be applied to signed types.
+The value of ``-x`` can be
 positive if ``x`` is negative. There is another caveat also resulting
 from two's complement representation::
 

libsolidity/ast/Types.cpp

@@ -627,9 +627,10 @@ TypeResult IntegerType::unaryOperatorResult(Token _operator) const
 	// "delete" is ok for all integer types
 	if (_operator == Token::Delete)
 		return TypeResult{TypeProvider::emptyTuple()};
-	// we allow -, ++ and --
-	else if (_operator == Token::Sub || _operator == Token::Inc ||
-		_operator == Token::Dec || _operator == Token::BitNot)
+	// unary negation only on signed types
+	else if (_operator == Token::Sub)
+		return isSigned() ? TypeResult{this} : TypeResult::err("Unary negation is only allowed for signed integers.");
+	else if (_operator == Token::Inc || _operator == Token::Dec || _operator == Token::BitNot)
 		return TypeResult{this};
 	else
 		return TypeResult::err("");

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