Allow poly-kinded GADT vars to infer constraints

[dwijnand]

Allowing the types to flow through in thirdTryNamed requires a tweak to
baseType. Given class C[T <: Int] (from tests/pos-macros/i10864a) the
basetype of a TypeRef of just "C", with the Any class symbol, shouldn't
return Any because "C" is not a proper/value type. That is, in
baseType, we need to short-circuit TypeProxy.superType (i.e.
underlying), which is returns resultType for a HKLambda.

Fixes #19904

[dwijnand]

With this change, when typing y match { case Bx[b] => Unit }], when we constrain Bx[b] against y, we correctly infer the b goes from <: AnyKind to <: Any. With that, when we get to checkOverride in RefChecks, we end up with Any <:< Any for the lambda param infos.

[sjrd]

Was this change actually necessary? Isn't tryBaseType always going to fail anyway when tp1.hasSimpleKind is false? (given that you added the case HKTypeLambda => false in baseType, which makes sense)

[dwijnand]

Yes, because then it cascades to more "tries", including allowing a gadt variable bounded by AnyKind inferring a gadt bound.

[sjrd]

That is, in baseType, we need to short-circuit TypeProxy.superType (i.e. underlying), which is returns resultType for a HKLambda.

I have always found dubious that HKTypeLambda.superType returns Any. Would we solve this issue if we made it return AnyKind instead?

[dwijnand]

That is, in baseType, we need to short-circuit TypeProxy.superType (i.e. underlying), which is returns resultType for a HKLambda.

I have always found dubious that HKTypeLambda.superType returns Any. Would we solve this issue if we made it return AnyKind instead?

Not sure. And I wouldn't be surprised it's a battle to make that change. But we can try.

[dwijnand]

That is, in baseType, we need to short-circuit TypeProxy.superType (i.e. underlying), which is returns resultType for a HKLambda.

I have always found dubious that HKTypeLambda.superType returns Any. Would we solve this issue if we made it return AnyKind instead?

Not sure. And I wouldn't be surprised it's a battle to make that change. But we can try.

Tried and it was as battle, so I gave up.