unused_element_parameter

A value for optional parameter '{0}' isn't ever given.

The analyzer produces this diagnostic when a value is never passed for an optional parameter declared within a private declaration.

Assuming that no code in the library passes a value for y in any invocation of _m, the following code produces this diagnostic:

class C {
  void _m(int x, [int? y]) {}

  void n() => _m(0);
}

If the declaration isn't needed, then remove it:

class C {
  void _m(int x) {}

  void n() => _m(0);
}

If the declaration is intended to be used, then add the code to use it.