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Backport "Fix infinite loop in Mirror synthesis of unreducible match type" to LTS #21042

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Merged
merged 2 commits into from
Jul 5, 2024
Merged

Backport "Fix infinite loop in Mirror synthesis of unreducible match type" to LTS #21042

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c278af7
Fix infinite loop in Mirror synthesis of unreducible match type
smarter Apr 8, 2024
f73c990
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 @@ -378,7 +378,9 @@ 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.")
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