return_of_invalid_type

A value of type '{0}' can't be returned from the constructor '{1}' because it has a return type of '{2}'.

A value of type '{0}' can't be returned from the function '{1}' because it has a return type of '{2}'.

A value of type '{0}' can't be returned from the method '{1}' because it has a return type of '{2}'.

The analyzer produces this diagnostic when a method or function returns a value whose type isn't assignable to the declared return type.

The following code produces this diagnostic because f has a return type of String but is returning an int:

String f() => 3;

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

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

If the value is correct, then change the return type to match:

int f() => 3;