#1400. [Extension types] Add more superinterfaces tests

[sgrekhov]

No description provided.

[eernstg]

Looks good! However, I think it would be valuable to add some test cases where the "A<T1>, A<T2>, T1 != T2" error exists because of A<T1> and A<T2> that occur in different syntactic locations.

[sgrekhov]

Updated. But there is no case

one in the current declaration and another in a superinterface

Could you show an example?

[eernstg]

If I got the history right about where the little snippets of quoted text came from then this would be an example of 'one in the current declaration and another in a superinterface':

class A<X> {}
extension type ET1(A<Never> a) implements A<int> {}
extension type ET2(A<Never> a) implements ET1, A<String> {}

The point is that it is a conflict that ET2 implements A<int>, and ET2 implements A<String> at the same time, even in the case where the two types of the form A<...> do not occur in the same syntactic list.

[sgrekhov]

Thank you! Tests added. Please review

[eernstg]

LGTM