return_of_invalid_type_from_closure

The returned type '{0}' isn't returnable from a '{1}' function, as required by the closure's context.

The analyzer produces this diagnostic when the static type of a returned expression isn't assignable to the return type that the closure is required to have.

The following code produces this diagnostic because f is defined to be a function that returns a String, but the closure assigned to it returns an int:

String Function(String) f = (s) => 3;

If the return type is correct, then replace the returned value with a value of the correct type, possibly by converting the existing value:

String Function(String) f = (s) => 3.toString();