Pattern-bound type refs should have Case flag set

[milessabin]

Fixes #6157.

[smarter]

I don't know what this does but the description of this flag is manifestly incomplete:
https://github.com/lampepfl/dotty/blob/d2499265ab2d13bf70eee17bfad67734804fecd9/compiler/src/dotty/tools/dotc/core/Flags.scala#L293-L294

Could you update it to explain what Case does on a val ?

[abgruszecki]

If we're going with what @smarter proposed, then how about the following?

  /** A symbol related to pattern matching */
  final val Case: FlagSet = commonFlag(17, "case")
  /** Case class or pattern-bound type symbol. */
  final val CaseType: FlagSet = Case.toTypeFlags
  /** Case class' companion object or pattern-bound term symbol */
  final val CaseVal: FlagSet = Case.toTermFlags

Refactoring necessary to change usages of CaseClass (which doesn't really mark solely case classes).

[smarter]

Sounds like these two usages of the flag are completely unrelated, so it'd be clearer to use two flags (we're not running out of bits for them yet).

[milessabin]

I agree that it looks like we have two distinct uses of the same flag here.

I think that using Case to represent case classes came first. Do you know who repurposed it to mean pattern bound type variable and why?

[smarter]

I think it was just done for convenience and there's no deep reason for it, you have my blessing to add a PatternBound flag :).

[odersky]

We are not that far to running out of flags, so I vote for keeping a single Case flag but documenting carefully the different meanings.

[milessabin]

@odersky I'll add a comment to this PR.

[smarter]

OK, in that case maybe call the flag PatternBoundOrCase so it's more obvious that it has a dual meaning, and add convenience methods like def isCaseClass(implicit ctx: Context) = isClass && is(PatternBoundOrCase) to SymDenotation.

[milessabin]

@smarter OK, but I'd rather do that in a followup PR.

[milessabin]

Follow on issue here, assigned to me.

Any objections if I merge this now?

Comments addressed either here or TBD in follow on issue.