Use user defined function return type as user defined operator type

libsolidity/analysis/TypeChecker.cpp

@@ -1835,7 +1835,7 @@ void TypeChecker::endVisit(BinaryOperation const& _operation)
 			*userDefinedFunctionType->parameterTypes().at(0) ==
 			*userDefinedFunctionType->parameterTypes().at(1)
 		);
-		commonType = userDefinedFunctionType->parameterTypes().at(0);
+		commonType = userDefinedFunctionType->returnParameterTypes().at(0);
 	}
 
 	_operation.annotation().commonType = commonType;

libsolidity/codegen/ir/IRGeneratorForStatements.cpp

@@ -775,7 +775,7 @@ bool IRGeneratorForStatements::visit(BinaryOperation const& _binOp)
 {
 	setLocation(_binOp);
 
-	// TOOD make this nicer
+	// TODO: make this nicer
 	if (_binOp.annotation().userDefinedFunction)
 	{
 		_binOp.leftExpression().accept(*this);

libsolidity/parsing/Parser.cpp

@@ -990,7 +990,7 @@ ASTPointer<UsingForDirective> Parser::parseUsingDirective()
 				};
 				if (!util::contains(overridable, operator_))
 					parserError(
-						1885_error,
+						4403_error,
 						("The operator " + string{TokenTraits::toString(operator_)} + " cannot be user-implemented. This is only possible for the following operators: ") +
 						util::joinHumanReadable(overridable | ranges::views::transform([](Token _t) { return string{TokenTraits::toString(_t)}; }))
 					);