Match types: fix wildcard captures handling to match intent. #77
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As demonstrated by scala/scala3#19607, picking `H` does *not* in fact always work. It was always the intent of the spec to pick an instantiation that would make subtyping work, but the particular choice was wrong. IMO this is therefore clearly a spec bug, and we can address it irrespective of compatibility concerns.
odersky
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Feb 15, 2024
…ds. (#19627) When matching in a match type, if we encounter a `TypeBounds` scrutinee and we have a wildcard capture on the right, we used to pick the `hi` bound "because anything between between `lo` and `hi` would work". It turns out that *nothing* between `lo` and `hi` works when the type constructor is invariant. Instead, we must be keep the type bounds, and instantiate the wildcard capture to a wildcard type argument. This is fine because a wildcard capture can never be referred to in the body of the case. However, previously this could never happen in successful cases, and we therefore used the presence of a `TypeBounds` in the `instances` as the canonical signal for "fail as not specific". We now use a separate `noInstances` list to be that signal. This change departs from the letter of the spec but not from its spirit. As evidenced by the wording, the spec always *intended* for "the pick" to one that would always succeed. We wrongly assumed `hi` was always working. --- Companion PR to fix the spec/SIP: scala/improvement-proposals#77
anatoliykmetyuk
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Feb 20, 2024
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As demonstrated by scala/scala3#19607, picking
Hdoes not in fact always work. It was always the intent of the spec to pick an instantiation that would make subtyping work, but the particular choice was wrong.IMO this is therefore clearly a spec bug, and we can address it irrespective of compatibility concerns.
Companion PR to fix it in the compiler: scala/scala3#19627