Fix #1490: type test of union types via type alias

[liufengyun]

Make type erasure accept one additional parameter:

• eraseOrType: Whether erase OrType to its lowest uppper bound

[smarter]

I would write Whether OrType should be erased to its lowest upper bound

[liufengyun]

Thanks, will address in the next commit.

[odersky]

I am missing the context. Why do we need to make this distinction? The way it is done it adds a significant cost to compiler complexity. Adding a new boolean parameter to a core method should not be taken lightly, because you just have doubled all possible error scenarios!

[smarter]

transformIsInstanceOf takes erased arguments but sometimes you don't actually want to erase things because you're going to do transformations on the isInstanceOf checks like for union types and singleton types. The alternative would be to rewrite transformIsInstanceOf to expect non-erased arguments and only erase them after doing the rewritings it needs to do, that might be better indeed.

[smarter]

By the way, the same thing is true in https://github.com/lampepfl/dotty/blob/master/src/dotty/tools/dotc/transform/IsInstanceOfEvaluator.scala which deals with erased values and so might end up doing an incorrect rewriting.

[liufengyun]

The alternative would be to rewrite transformIsInstanceOf to expect non-erased arguments and only erase them after doing the rewritings it needs to do, that might be better indeed.

@smarter That is how the previous implementation was done, but it doesn't handle the case in #1490 . A proper handling of #1490 would require us to recursively get rid of all type alias, that's part of the logic of type erasure. Either we need to find a way to reuse the type erasure code, or we have to duplicate some of its code.

[odersky]

Question: What should the imlementation

xs.isInstanceOf[List[A | B]] 

be? Erase | or not? I would assume erase. In which case, the whole thing does belong in isInstanceOfEvaluator. Can #1490 not be addressed by dealiasing?

[smarter]

I think that in your example, A | B is not erased if eraseOrType is false, but List[A | B] is erased to a raw List anyway so it doesn't matter

[liufengyun]

I agree it's better to make the core simpler. Will follow the original approach by dealiasing.

[smarter]

LGTM