SI-9542 Unify different reprs. of module type refs

The new unit test shows failures in transitivity of subtyping
and type equivalence, which boil down the the inconsistent
handling of the semantically equivalent:

   ThisType(pre, ModuleClass)
   ModuleTypeRef(pre, ModuleClass)
   SingleType(pre, Module)

This commit:

   - adds a case to `normalizePlus` to unwrap a `ThisType` to
     a `ModuleTypeRef`
   - Use `normalizePlus` more widely during subtype comparison
   - refactor `fourthTry` (part of `isSubType`) to remove code
     that becomes obviated by the use of `normalizePlus`.

This fixes the regression in the extension methods phase which
was triggered by https://github.com/scala/scala/pull/4749.
We can also fix that regression by tweaking the extension methods
phase itself to emit the `ThisType` representation of the owner
of the value class, as before.

I plan to demonstrate the two approaches to fixing the regression
on separate branches, and the propose that the merged result of these
two is useds.

0 files changed, 0 insertions, 0 deletions