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Fix infinite loop in Mirror synthesis of unreducible match type #20133

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merged 2 commits into from
Apr 8, 2024
Merged

Fix infinite loop in Mirror synthesis of unreducible match type #20133

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5d0c47a
Fix infinite loop in Mirror synthesis of unreducible match type
smarter Apr 8, 2024
4699140
Do not assume the superType of a match type gives us a sound mirror
smarter Apr 8, 2024
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4 changes: 3 additions & 1 deletion compiler/src/dotty/tools/dotc/typer/Synthesizer.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -379,7 +379,9 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
// avoid type aliases for tuples
Right(MirrorSource.GenericTuple(types))
case _ => reduce(tp.underlying)
case tp: MatchType => reduce(tp.normalized)
case tp: MatchType =>
val n = tp.tryNormalize
if n.exists then reduce(n) else Left(i"its subpart `$tp` is an unreducible match type.")
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@EugeneFlesselle EugeneFlesselle Apr 8, 2024 •
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I think we should return the unreduced match type instead.
An instance could have been provided manually for it.

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reduce requires that we return a class symbol it seems. Also the Synthesizer logic should only be used as a fallback if an implicit wasn't found by regular search.

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Ah good point, then an error sounds fine to me.

case _ => reduce(tp.superType)
case tp @ AndType(l, r) =>
for
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13 changes: 13 additions & 0 deletions tests/neg/i19198.scala
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@@ -0,0 +1,13 @@
import deriving.Mirror
import compiletime.summonInline

type DoesNotReduce[T] = T match
case String => Any

type DoesNotReduce2[T] <: T = T match
case String => T

class Foo
@main def Test: Unit =
summonInline[Mirror.Of[DoesNotReduce[Option[Int]]]] // error
summonInline[Mirror.Of[DoesNotReduce2[Option[Int]]]] // error