Canonicalize capture variable subtype comparisons #22299
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Fixes scala#22103 Subtype problems where at least one side is a type variable representing a capture variable are canonicalized to capturing type comparisons on the special `CapSet` for the universe capture sets. For example, `C <: CapSet^{C^}` becomes `CapSet^{C^} <: CapSet^{C^}`, and `A <: B` becomes `CapSet^{A^} <: CapSet^{B^}` if both `A` and `B` are capture variables.
This was referenced Jan 2, 2025
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Moved to dotty @odersky |
This change now makes both the tests cc-poly-varargs and capturesubtyping pass. - Move CapSet-related methods to CaptureOps - Let NamedType case in secondTry of TypeComparer handle capture variables as well - Added a case in capturesubtyping test to test forgotten NamedType case above
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Fixes #22103
Subtype problems where at least one side is a type variable representing a capture variable are canonicalized to capturing type comparisons on the special
CapSetfor the universe capture sets. For example,C <: CapSet^{C^}becomesCapSet^{C^} <: CapSet^{C^}, andA <: BbecomesCapSet^{A^} <: CapSet^{B^}if bothAandBare capture variables.Supersedes #22183 and #22289. This solution is overall cleaner and does not require adding a new bit to the TypeComparer's ApproxState.
TODOs/Issues/Questions:
D <: Efails, but its canonicalized formCapSet^{D^} <: CapSet^{E^}now succeeds. Potential problem in the subcapturing implementation.Extend to intersection/unionsLacking good uses cases, not planned right now.def f[C^, D^, E <: C | D, F <: C & D](...) = ...etc.If we haveWill be addressed by a new scheme for declaring capture variables using context bounds.C^declared in the current context, should there be a difference betweenCvs.C^for subsequent mentions? We currently do, but seems a bit too subtle for users.