Fix regression in provablyDisjoint

[OlivierBlanvillain]

#12549 introduced a regression in provablyDisjoint. The problem comes from the following case:

case (tp1: TermRef, tp2: TypeRef) if isEnumValueOrModule(tp1)
  && !tp1.classSymbols.exists(_.derivesFrom(tp2.classSymbol)) =>
    true

The !tp1.classSymbols.exists(...) is not sufficient, for instance if tp2 is unbounded type parameter that code can wrongly conclude that a TermRef doesn't belong to that type.

[liufengyun]

#12549 introduced a regression in provablyDisjoint. The problem comes from the following case:

case (tp1: TermRef, tp2: TypeRef) if isEnumValueOrModule(tp1)
  && !tp1.classSymbols.exists(_.derivesFrom(tp2.classSymbol)) =>
    true

The !tp1.classSymbols.exists(...) is not sufficient, for instance if tp2 is unbounded type parameter that code can wrongly conclude that a TermRef doesn't belong to that type.

Good catch, I think the following will work (use tp2.symbol instead of tp2.classSymbol):

case (tp1: TermRef, tp2: TypeRef) if isEnumValueOrModule(tp1) && !tp1.classSymbols.exists(_.derivesFrom(tp2.symbol)) =>
 true

[OlivierBlanvillain]

That also doesn't work (Not[B] still reduces to "unreachable" in the added test case).

In general I don't think a test of the form !exists can be used to conclude that two types are disjoints given that one of those types could be abstract in the current context but become concrete in another context...

[OlivierBlanvillain]

It should be possible to conclude disjointness between an enum case and a class, that should be analogous to disjointness between a final class and a class which we handle in case (tp1: TypeRef, tp2: TypeRef) if tp1.symbol.isClass && tp2.symbol.isClass =>.

[anatoliykmetyuk]

@OlivierBlanvillain is this one good for a review? Or is the solution subject for discussion?

[liufengyun]

LGTM

[liufengyun]

What is the consideration of using isEnumValue instead of isEnumValueOrModule here?

[OlivierBlanvillain]

So far I haven't found a need for it, the existing infrastructure doesn't need special treatment to handle modules...