Library function can be used as user operator

test/libsolidity/semanticTests/operators/custom/operator_bound_to_library_function.sol

@@ -0,0 +1,33 @@
+==== Source: a.sol ====
+library L {
+    type Int is int128;
+
+    function add(Int, Int) pure public returns (Int) {
+        return Int.wrap(7);
+    }
+
+    function sub(Int) pure public returns (Int) {
+        return Int.wrap(5);
+    }
+}
+==== Source: b.sol ====
+import "a.sol" as a;
+
+contract C {
+    using {a.L.add as +} for a.L.Int;
+    using {a.L.sub as -} for a.L.Int;
+
+    function f() pure public returns (a.L.Int) {
+	return a.L.Int.wrap(0) + a.L.Int.wrap(0);
+    }
+
+    function g() pure public returns (a.L.Int) {
+	return - a.L.Int.wrap(0);
+    }
+}
+
+// ====
+// compileViaYul: also
+// ----
+// f() -> 7
+// g() -> 5