Ignore capture sets in certain cases of subtyping #22183
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noti0na1
reviewed
Dec 11, 2024
| @@ -12,8 +12,4 @@ def either[T1, T2, Cap^]( | |||
| src2: Source[T2, Cap]^{Cap^}): Source[Either[T1, T2], Cap]^{Cap^} = | |||
| val left = src1.transformValuesWith(Left(_)) | |||
| val right = src2.transformValuesWith(Right(_)) | |||
| race[Either[T1, T2], Cap](left, right) | |||
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The extension methods above can be tested as well. I remember they have similar issues.
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It causes an infinite regress now, looking into it.
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We should have standalone file to test the subtyping: def test[C^] =
val a: C = ???
val b: CapSet^{C^} = a
val c: C = b
val d: CapSet^{C^, ...} = a
...We also want to test nested types are still working: def test[C^, D^] =
A[C] <:< A[CapSet^{C^}] // ok
A[C] <:< A[CapSet^{D^}] // error
A[C]^{D^} <:< A[CapSet^{D^}]^{D^} // error |
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bracevac
commented
Dec 22, 2024
| val d: CapSet^{C^, c} = a | ||
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| // TODO: make "CapSet-ness" of type variables somehow contagious? | ||
| // Then we don't have to spell out the bounds explicitly... |
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We should support that. Say we have def foo[C^, D <: C, E >: C], then we should automatically constraint both D and E to be capture set variables, i.e., D is implicitly lower-bounded by CapSet, and E is implicitly upper-bounded by CapSet^
bracevac
commented
Dec 22, 2024
| val d_e_f2: CapSet^{D^,E^,F^} = e1 | ||
| val d_e_f3: CapSet^{D^,E^,F^} = f1 | ||
| val f2: F = d_e_f1 | ||
| val c3: C = d_e_f1 // error |
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This line and the three following lines are all not flagged as errors, but they should be. Investigating.
This was referenced Jan 1, 2025
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The continuation: #22299 |
bracevac
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Jan 22, 2025
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 `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. 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: - [x] Fix extension method in test [cc-poly-varargs.scala](https://github.com/dotty-staging/dotty/blob/capture-subtyping-canon/tests/pos-custom-args/captures/cc-poly-varargs.scala). Currently causes an infinite regress. - [x] Fix the aftermath * tests/neg-custom-args/captures/lazylists-exceptions.scala * tests/neg-custom-args/captures/exceptions.scala * tests/neg-custom-args/captures/real-try.scala * tests/run-custom-args/captures/colltest5 - [x] Some negative cases in test [capture-vars-subtyping.scala](https://github.com/dotty-staging/dotty/blob/capture-subtyping-canon/tests/neg-custom-args/captures/capture-vars-subtyping.scala) pass: `D <: E` fails, but its canonicalized form `CapSet^{D^} <: CapSet^{E^}` now succeeds. Potential problem in the subcapturing implementation. - [x] ~Extend to intersection/unions `def f[C^, D^, E <: C | D, F <: C & D](...) = ...` etc.~ Lacking good uses cases, not planned right now. - [X] ~If we have `C^` declared in the current context, should there be a difference between `C` vs. `C^` for subsequent mentions? We currently do, but seems a bit too subtle for users.~ Will be addressed by a new scheme for declaring capture variables using context bounds.
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Fixes #22103
If
Cis a capture-set variable, then a comparisonC <: CapSet^{C^}would previously fail in theTypeComparerdue to a level mismatch resulting in the testCapSet^ <: CapSet.For such cases, we now ignore the LHS capture annotation. This is controlled by a new bit in the
ApproxStateof theTypeComparer.