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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.
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