Interface inheritance and override rules are unclear

[johnniwinther]

The paragraph the defines 'inherited(J, K)' for interfaces (§8.1.1) uses the unclear wording 'm is a member of A' instead of for instance the well-defined 'A declares m'.

This makes is unclear how to handle the synthetic members introduced in case of multiple inherited members.

Consider this:

class A {
  method(int a) => null;
}
class B {
  method(num a) => null;
}
abstract class C implements A, B {
  // C has a 'method' of type '(dynamic) -> dynamic'.
}
class Class implements C {
  method(double a) => null;
}

If the synthetic '(dynamic) -> dynamic' method in C is 'a member of C' the checking of 'method' declared in 'Class' should be done against the synthetic member, thus causing no warning since '(double) -> dynamic' is a subtype of '(dynamic) -> dynamic'.

If the synthetic '(dynamic) -> dynamic' method in C is not 'a member of C' the checking of 'method' declared in 'Class' should be done against the members declared in A and B, thus causing a warning for the override of the method declared in A since '(double) -> dynamic' is a not subtype of '(int) -> dynamic'.

I think the latter should be the case, since it would be consistent with the hierarchy in which 'Class' directly overrides methods in 'A' and 'B':

class A {
  method(int a) => null;
}
class B {
  method(num a) => null;
}
class Class implements A, B {
  method(double a) => null; /// warning for the override of A.method.
}

[jwren]

Marked this as blocking #16498.

[gbracha]

The text should be clarified to state: 'declares a member" as that is the original intent.

---

Added Accepted label.

[eernstg]

No Dart 2 milestone: This is now considered a subtask of #31228, which has a Dart 2 milestone.

[eernstg]

This CL contains a proposal for a feature specification about conflict resolution regarding inheritance and overriding which would resolve this issue in a feature spec (remaining: integrate that into dartLangSpec.tex).

[eernstg]

Said feature specification was completed, and then 614ae7f integrated this topic into dartLangSpec.tex.