Value types are not properly compared in `TypeComparer`

[felixmulder]

The following subtyping test fails Ycheck after erasure:

class Test1(val x: Int) extends AnyVal
class Test2(val y: Test1) extends AnyVal

In tree checker:

tree.tpe <:< pt // tree.tpe = ErasedValueType(Test2, ErasedValueType(Test1, Int))($this)
                //       pt = Int

This is what causes erasure not to be Ycheckable. The next phase, ElimErasedValueType, removes the ErasedValueTypes and casts so that it is again, Ycheckable.

ping @odersky

[felixmulder]

After this issue is resolved, the work from #2415 makes erasure Ycheckable

[smarter]

ErasedValueType(A, B) is intentionally not a subtype or supertype of either A or B to trigger boxing/unboxing in Erasure, but Erasure should add the necessary casts (evt2u to convert an ErasedValueType to its underlying value: https://github.com/lampepfl/dotty/blob/master/compiler/src/dotty/tools/dotc/transform/Erasure.scala#L251). If after Erasure you have a tree with type ErasedValueType whose expected type is Int, then something is wrong, because the cast should have been inserted.

[odersky]

I agree with @smarter. This is intentional, and done so that erasure is forced to insert the proper casts.

[smarter]

I talked with @felixmulder and we did find one issue: when a value class underlying type is also a value class, you may need to wrap/unwrap using evt2u/u2evt multiple times, but this is apparently not always done, I haven't investigated more but @felixmulder is on it.