Allow explicit conversion from address to address payable

test/libsolidity/syntaxTests/conversion/explicit_conversion_address_to_payable.sol

@@ -0,0 +1,11 @@
+contract C {
+	function g(address payable _p) internal pure returns (uint) {
+		return 1;
+	}
+	function f(address _a) public pure {
+		uint x = g(payable(_a));
+		uint y = g(_a);
+	}
+}
+// ----
+// TypeError: (169-171): Invalid type for argument in function call. Invalid implicit conversion from address to address payable requested.

test/libsolidity/syntaxTests/conversion/explicit_conversion_sender_to_payable.sol

@@ -0,0 +1,9 @@
+contract C {
+	function f() public view {
+		address payable p = payable(msg.sender);
+		address payable q = payable(address(msg.sender));
+	}
+}
+// ----
+// Warning: (43-60): Unused local variable.
+// Warning: (86-103): Unused local variable.

test/libsolidity/syntaxTests/conversion/explicit_conversion_this_to_payable.sol

@@ -0,0 +1,8 @@
+contract C {
+	function f() public pure {
+		address payable p = payable(this);
+		address payable q = payable(address(this));
+	}
+}
+// ----
+// TypeError: (63-76): Explicit type conversion not allowed from "contract C" to "address payable".

test/libsolidity/syntaxTests/parsing/payable_without_arguments.sol

@@ -0,0 +1,7 @@
+contract C {
+	function f() public pure {
+		address payable q = payable;
+	}
+}
+// ----
+// ParserError: (70-71): Expected '(' but got ';'