Add user operator test cases

test/libsolidity/syntaxTests/operators/custom/operator_all_types_bound_inside_contract.sol

@@ -0,0 +1,12 @@
+type Int is int;
+
+function add(Int, Int) returns (Int) {
+    return Int.wrap(0);
+}
+
+contract C {
+    using {add as +} for *;
+}
+
+// ----
+// SyntaxError 3349: (101-124): The type has to be specified explicitly when attaching specific functions.

test/libsolidity/syntaxTests/operators/custom/operator_bound_on_contract_level_to_contract_level_function.sol

@@ -0,0 +1,12 @@
+type Int is int;
+
+contract C {
+    using {add as +} for Int;
+
+    function add(Int, Int) public returns (Int) {
+        return 0;
+    }
+}
+
+// ----
+// TypeError 4167: (42-45): Only file-level functions and library functions can be bound to a type in a "using" statement

test/libsolidity/syntaxTests/operators/custom/operator_bound_to_all_types.sol

@@ -0,0 +1,8 @@
+using {add as +} for *;
+
+function add(int, int) returns (int) {
+    return 0;
+}
+
+// ----
+// SyntaxError 8118: (0-23): The type has to be specified explicitly at file level (cannot use '*').

test/libsolidity/syntaxTests/operators/custom/operator_bound_to_contract_level_function.sol

@@ -0,0 +1,12 @@
+type Int is int;
+
+using {C.add as +} for Int;
+
+contract C {
+    function add(Int, Int) public returns (Int) {
+        return 0;
+    }
+}
+
+// ----
+// TypeError 4167: (25-30): Only file-level functions and library functions can be bound to a type in a "using" statement