Properly recover from nested ambiguous implicit search failures #9937
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This commit does not change the compiler behavior but replaces `compareCandidate` by a more generic `compareAlternatives` that will be useful in the next commit, this required introducing a base trait `RefAndLevel` (not a great name, but I couldn't think of anything better...).
From http://dotty.epfl.ch/docs/reference/changed-features/implicit-resolution.html: > The treatment of ambiguity errors has changed. If an ambiguity is > encountered in some recursive step of an implicit search, the ambiguity > is propagated to the caller. This is supposed to be handled by `healAmbiguous` but the implementation was incorrect: it compared pending candidates against the two ambiguous results from the search, but this doesn't make sense when the failure happened in a nested search: for example in `i9793`, the failure is: Applied.bazApplied[F](/* ambiguous: both value baz and value bar match type Foo[F] */summon[Foo[F]) We should be able to recover from this failure because `barApplied` is more specific than `bazApplied`, but before this commit we ended up comparing `barApplied` to `baz` and `bar` which isn't meaningful. Fixes scala#9793.
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From
http://dotty.epfl.ch/docs/reference/changed-features/implicit-resolution.html:
This is supposed to be handled by
healAmbiguousbut the implementationwas incorrect: it compared pending candidates against the two ambiguous
results from the search, but this doesn't make sense when the failure
happened in a nested search: for example in
i9793, the failure is:We should be able to recover from this failure because
barAppliedismore specific than
bazApplied, but before this commit we ended upcomparing
barAppliedtobazandbarwhich isn't meaningful.Fixes #9793.