extension_override_argument_not_assignable

The type of the argument to the extension override '{0}' isn't assignable to the extended type '{1}'.

The analyzer produces this diagnostic when the argument to an extension override isn't assignable to the type being extended by the extension.

The following code produces this diagnostic because 3 isn't a String:

extension E on String {
  void method() {}
}
​
void f() {
  E(3).method();
}

If you're using the correct extension, then update the argument to have the correct type:

extension E on String {
  void method() {}
}
​
void f() {
  E(3.toString()).method();
}

If there's a different extension that's valid for the type of the argument, then either replace the name of the extension or unwrap the argument so that the correct extension is found.