Fix #6314: match type unsoundness #6319
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OlivierBlanvillain
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Seams to be a leftover from scalac old pattern matcher and has always been dead code in dotty.
Singleton is not part of the normal type hierarchy, special treatment in requiered in intersecting to avoid making incorrect assumptions.
It timed out on my machine...
This breaks tests/run/typeclass-derivation2c.scala There is no way to statically know that the type passed to constValue reduces.
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OlivierBlanvillain
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odersky
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May 7, 2019
SeklomType are already mapped over correctly, that is, as .info.
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This PR fixes several match issues
The first issue was that the algorithm assumes types in an
AndTypealways came in the same order, so it wrongly concluded thatX & YandY & Typeare non intersecting from the fact thatXandYare non intersecting. The updated logic checks both permutations, and simply get stuck whenTypeis abstract, which fixes the unsoundness.The second issue is related to the
Singleton. This marker type needs to be taken it into account by the intersection algorithm, see the added test case.Finally the previous handling of abstract type was problematic a showed by several of the examples in #6314. The logic is updated to do a sub-type check where all the abstract types (members or parameters) are replace by
WildcardType. If the scrutinee is not a subtype of a pattern, but it is with wildcards, then the type shouldn't reduce.