argument_type_not_assignable

The argument type '{0}' can't be assigned to the parameter type '{1}'. {2}

The analyzer produces this diagnostic when the static type of an argument can't be assigned to the static type of the corresponding parameter.

The following code produces this diagnostic because a num can't be assigned to a String:

String f(String x) => x;
String g(num y) => f(y);

If possible, rewrite the code so that the static type is assignable. In the example above you might be able to change the type of the parameter y:

String f(String x) => x;
String g(String y) => f(y);

If that fix isn't possible, then add code to handle the case where the argument value isn't the required type. One approach is to coerce other types to the required type:

String f(String x) => x;
String g(num y) => f(y.toString());

Another approach is to add explicit type tests and fallback code:

String f(String x) => x;
String g(Object y) => f(y is String ? y : '');

If you believe that the runtime type of the argument will always be the same as the static type of the parameter, and you're willing to risk having an exception thrown at runtime if you're wrong, then add an explicit cast:

String f(String x) => x;
String g(num y) => f(y as String);

Unless stated otherwise, the documentation on this site reflects Dart 3.8.0. Page last updated on 2025-05-08. View source or report an issue.