Type inference resolves to <Diagnosticable> about sdk

[lukehutch]

Sorry, I meant to report this in the language repository... will do.

[lrhn]

This world the way it does because the upper bound operation on StateKey<_AState> and StateKey<_BState> is StateKey<U> where U is the upper bound of _AState and _BState.
Those two are different classes, so the algorithm falls through to the part that tries to give a common super-interface.

And since State<A> and State<B> are different interfaces, they are not the answer. Instead it seems Diagnosticable is the nearest unique shared superinterface.

[lukehutch]

OK, so it's WAI. Thanks for explaining!

[eernstg]

I'll add a comment here, just to check out the behavior of a proposed update to the underlying algorithm.

The proposal is about modifying the UP algorithm in dart-lang/language#3282 such that it finds the least common superclass rather than supertype. (OK, it's "standard" rather than "least", as usual).

With that proposal we do get the desired result:

UP(StateKey<_AState>, StateKey<_BState>) ==
StateKey<UP(_AState, _BState)> == 
// Standard common superclass of `_AState` and `_Bstate` is `State`,
// superinterface of `_AState`/`_BState` at `State` is `State<A>`/`State<B>`.
StateKey<UP(State<A>, State<B>)> ==
StateKey<State<UP(A, B)>> ==
StateKey<State<StatefulWidget>>

[lukehutch]

Yes, the issue you identify at the beginning of that report is what I was seeing here. So I will re-open this issue. Thanks!

[eernstg]

Hi @lukehutch!

I'm afraid it is still correct to describe this issue as reporting on a behavior which is specified. That is, there is nothing to fix here from the perspective of the SDK repository.

The issue dart-lang/language#3282 in the language repository is a proposal about changing the behavior of the UP algorithm. In other words, a future version of Dart could have a specification that allows it to behave in the way I described, if that proposal is accepted.

So we could move this issue to the language repository and consider it to be a request for a change similar to the one in dart-lang/language#3282, but it would then basically be a duplicate of that issue.

Perhaps you could write a comment on dart-lang/language#3282 referring to this issue as an example case for that change, and then close this one again?