Invalid override is generated when overridden method has covariant

[srawlins]

From @athomas :

class A {
  void foo(num n) {}
  void bar(int n) {}
}

class B extends A {
  @override
  void foo(covariant int n) {}
  @override
  void bar(int n) {}
}

// Roughly what mockito generates:
class MockitoFake extends B {
  @override
  void foo(int? n) {} // <-- invalid override
  @override
  void bar(int? n) {}
}

This foo override is invalid.

[srawlins]

I think when we encounter a covariant parameter, we must walk up inheritance and get the LUB of all the corresponding parameters. Testing considerations:

• chain of two covariant parameters which successively narrow the type
• covariant parameter in method overriding method from mixin
• positional parameters
• named parameters
• parameters with default values (the AST plays tricks here)
• covariant optional parameter with corresponding super-parameter with an overridden method which does not feature a corresponding parameter.
• covariant fields... drowning in test cases