Disallow defining operators with non-pure functions

test/libsolidity/syntaxTests/operators/userDefined/calling_operator_that_makes_external_call.sol

@@ -0,0 +1,24 @@
+type Int is int16;
+
+using {add as +} for Int;
+
+function add(Int, Int) returns (Int) {
+    B b = new B();
+    return b.f();
+}
+
+contract B {
+    Int s;
+    function f() external returns (Int) {
+        s = Int.wrap(3);
+        return s;
+    }
+}
+
+contract C {
+    function test() public returns (Int) {
+        return Int.wrap(0) + Int.wrap(0);
+    }
+}
+// ----
+// TypeError 7775: (27-30): Only pure functions can be used to define operators.

test/libsolidity/syntaxTests/operators/userDefined/implementing_operator_with_non_pure_function.sol

@@ -0,0 +1,12 @@
+using {add as +, sub as -, mul as *} for A;
+
+function add(A, A) view returns (A) {}
+function sub(A, A) returns (A) {}
+function mul(A, A) payable returns (A) {}
+
+type A is address payable;
+// ----
+// TypeError 7775: (7-10): Only pure functions can be used to define operators.
+// TypeError 7775: (17-20): Only pure functions can be used to define operators.
+// TypeError 7775: (27-30): Only pure functions can be used to define operators.
+// TypeError 9559: (118-159): Free functions cannot be payable.