Backport "Fix healAmbiguous to compareAlternatives with disambiguate = true" to 3.5.1 #21372
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Before the changes, if `isAsGoodValueType` was called with an extension and a given conversion, it would prefer the conversion over the extension, because only the former yielded true in `isGiven`. Which contradicted the logic from searchImplicit which preferred extension over conversions for member selection.
We now use a left-biased scheme, as follows. From 3.6 on: - A given x: X is better than a given or implicit y: Y if y can be instantiated/widened to X. - An implicit x: X is better than a given or implicit y: Y if y can be instantiated to a supertype of X. - Use owner score for givens as a tie breaker if after all other tests we still have an ambiguity. This is not transitive, so we need a separate scheme to work around that. Other change: - Drop special handling of NotGiven in prioritization. The previous logic pretended to do so, but was ineffective.
We only have transitivity between givens or between implicits. To cope with that - We tank first all implicits, giving a best implicit search result. - Then we rank all givens startign with the implicit result. If there is a given that is better than the best implicit, the best given will be chosen. Otherwise we will stick with the best implicit.
Warnings from 3.6, change in 3.7. This is one version later than originally planned.
Make the wording of a priority change warning message stable under different orders of eligibles. We now always report the previously chosen alternative first and the new one second. Note: We can still get ambiguities by fallging different pairs of alternatives depending on initial order.
On the final result, compared with all the ambiguous candidates we are trying to recover from. We should still use `disambiguate = false` when filtering the `pending` candidates for the purpose of warnings, as in the other cases. Before the changes, it was possible for an ambiguous SearchFailure to be healed by a candidate which was considered better (possibly only) under a prioritization scheme different from the current one. As an optimization, we can avoid redoing compareAlternatives in versions which could have only used the new prioritization scheme to begin with. Also restores behaviour avoiding false positive warnings. Specifically, in cases where we could report a change in prioritization, despite having not yet done `tryImplicit` on the alternative, i.e. it was only compared as part of an early filtering See scala#21045 for related changes
This reverts commit 8a41389.
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Blocked by MiMa check, requires #21376 |
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Backports #21344 to Scala 3.5.1-RC2 using 3.5.0-RC7 backport #21344