Dealias an applied match alias #19854
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This patch fixes a recursion situation in collectParts, by reimplementing a previous fix. Recursion is already attempted to be cutoff with `partSeen`, and `handleRecursion` is also used to prevent any unhandled recursion situations (like the one fixed here) from crashing the compiler. For context, AppliedType aren't hash-consed (i.e. the flyweight pattern) which means that every time you apply the same type arguments to the same type constructor you get a fresh AppliedType. Using i18171 as an example, the sequence of events where this matters is: 0. When typing BAZ, so much earlier than the collectParts call, because the MatchType on the rhs of BAZ reduces at definition site, the RHS is reduced to the `DFVal[BAZREC[T]]`, which means that BAZ's info is a TypeAlias rather than a MatchAlias, meaning it can dealias. 1. `BAZREC[Any]` is extracted by MatchType.InDisguise, which applies the Any to return a fresh MatchType 2. `Tuple.Map[Any, BAZ]` is also extracted by MatchType.InDisguise, which returns its own fresh MatchType 3. `BAZ[h]` dealiases to a fresh `DFVal[BAZREC[h]]` instance (see step 0) 4. `BAZREC[h]` is extracted by MatchType.InDisguise, repeating the cycle The reason that the cases with MatchType.InDisguise and MatchType were introduced is i17395. Looking back, it seems the only need is to capture any parts that are in the reduction of an applied MatchType. With this patch applied in the case of i18171, this now cuts off quickly, as `BAZREC[Any]` doesn't reduce to anything. In the case of i17395, `ExtractPart[ValuePartHolder]` reduces to `Value`, so `Value` is successfully recorded as a part.
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Let's go with #19871 |
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Fixes #19821
Builds on #19867