Allow obtaining the address of a library by conversion to `address`.

test/libsolidity/semanticTests/libraries/library_address.sol

@@ -0,0 +1,56 @@
+library L {
+    function f(uint256 v) external pure returns (uint) {
+        return v * v;
+    }
+    function g(uint256 v) external returns (uint) {
+        return v * v;
+    }
+}
+contract C {
+    function addr() public view returns (bool) {
+        return address(L) == address(0);
+    }
+    function g(uint256 v) public view returns (uint256) {
+        return L.f(v);
+    }
+    function h(uint256 v) public returns (uint256) {
+        (bool success, bytes memory result) = address(L).delegatecall(abi.encodeWithSignature("f(uint256)", v));
+        assert(success);
+        return abi.decode(result, (uint256));
+    }
+    function i(uint256 v) public returns (uint256) {
+        (bool success, bytes memory result) = address(L).call(abi.encodeWithSignature("f(uint256)", v));
+        assert(success);
+        return abi.decode(result, (uint256));
+    }
+    function j(uint256 v) public returns (uint256) {
+        (bool success, bytes memory result) = address(L).delegatecall(abi.encodeWithSignature("g(uint256)", v));
+        assert(success);
+        return abi.decode(result, (uint256));
+    }
+    function k(uint256 v) public returns (uint256) {
+        (bool success, bytes memory result) = address(L).call(abi.encodeWithSignature("g(uint256)", v));
+        assert(success);
+        return abi.decode(result, (uint256));
+    }
+}
+// ====
+// EVMVersion: >=byzantium
+// ----
+// library: L
+// addr() -> false
+// g(uint256): 1 -> 1
+// g(uint256): 2 -> 4
+// g(uint256): 4 -> 16
+// h(uint256): 1 -> 1
+// h(uint256): 2 -> 4
+// h(uint256): 4 -> 16
+// i(uint256): 1 -> 1
+// i(uint256): 2 -> 4
+// i(uint256): 4 -> 16
+// j(uint256): 1 -> 1
+// j(uint256): 2 -> 4
+// j(uint256): 4 -> 16
+// k(uint256): 1 -> FAILURE
+// k(uint256): 2 -> FAILURE
+// k(uint256): 4 -> FAILURE

test/libsolidity/semanticTests/libraries/library_address_homestead.sol

@@ -0,0 +1,24 @@
+library L {
+    function f(uint256 a, uint256 b) external {
+        assert(a * a == b);
+    }
+}
+contract C {
+    function addr() public view returns (bool) {
+        return address(L) == address(0);
+    }
+    function g(uint256 a, uint256 b) public returns (bool) {
+        (bool success,) = address(L).delegatecall(abi.encodeWithSignature("f(uint256,uint256)", a, b));
+        return success;
+    }
+}
+// ----
+// library: L
+// g(uint256,uint256): 1, 1 -> true
+// g(uint256,uint256): 1, 2 -> false
+// g(uint256,uint256): 2, 3 -> false
+// g(uint256,uint256): 2, 4 -> true
+// g(uint256,uint256): 2, 5 -> false
+// g(uint256,uint256): 4, 15 -> false
+// g(uint256,uint256): 4, 16 -> true
+// g(uint256,uint256): 4, 17 -> false

test/libsolidity/syntaxTests/types/library_to_address.sol

@@ -0,0 +1,8 @@
+library L {
+}
+contract C {
+    function f() public pure returns (address) {
+        return address(L);
+    }
+}
+// ----

test/libsolidity/syntaxTests/types/library_to_address_payable.sol

@@ -0,0 +1,9 @@
+library L {
+}
+contract C {
+    function f() public pure returns (address payable) {
+        return address(L);
+    }
+}
+// ----
+// TypeError: (99-109): Return argument type address is not implicitly convertible to expected type (type of first return variable) address payable.