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Fix specifity comparison for extensions in polymorphic givens #13509

Merged
merged 3 commits into from Sep 16, 2021
Merged

Fix specifity comparison for extensions in polymorphic givens #13509

merged 3 commits into from Sep 16, 2021

Commits on Sep 11, 2021

  1. Small refactorings related to TyperState 

    - Add Context#withTyperState
- Add a committable parameter to TyperState#fresh
    @smarter
    smarter committed Sep 11, 2021
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  2. TyperState#mergeConstraintWith: use the correct context 

    Previously, the code in the closure passed to `comparing` could have
been using the wrong context. I don't think this happened in practice so
far because comparingCtx was always equal to ctx but it will matter in the
next commit.
    @smarter
    smarter committed Sep 11, 2021
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  3. Fix specifity comparison for extensions in polymorphic givens 

    When we compare polymorphic methods for specificity, we replace their
type parameters by type variables constrained in the current
context (see isAsSpecific), but for extension methods in polymorphic
givens, the comparison was done with a constraint set that does not
include the type parameters of the givens, this lead to ambiguity errors
as experienced in
typelevel/spotted-leopards#2.

We fix this by ensuring the TyperState we use for the comparison
contains any additional constraint specific to the TyperState of either
alternative. This required generalizing TyperState#mergeConstraintWith
to handle `this` not being committable (because in that case `this` does
not need to take ownership of the type variables owned by `that`,
therefore `that` itself is allowed to be committable).
    @smarter
    smarter committed Sep 11, 2021
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