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Normalize MatchAlias in unrollTupleTypes #19565

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Jan 30, 2024
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Normalize MatchAlias in unrollTupleTypes #19565

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3 changes: 3 additions & 0 deletions compiler/src/dotty/tools/dotc/inlines/Inlines.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -443,6 +443,9 @@ object Inlines:
unrollTupleTypes(tail).map(head :: _)
case tpe: TermRef if tpe.symbol == defn.EmptyTupleModule =>
Some(Nil)
case tpRef: TypeRef => tpRef.info match
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The fix works, but I am not sure if this is the correct way to handle MatchAliases.

@sjrd could you have a look at this one?

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It looks reasonable, although usually we would rather expect

case tpe: AppliedType if tpe.isMatchAlias => ... tpe.tryNormalize ...

It is a bit suspicious that we directly encounter the tycon here, rather than the surrounding AppliedType. What are tpe and tpe.dealias when we get here on the problematic test case?

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@EugeneFlesselle EugeneFlesselle Jan 30, 2024 •
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Yes I noticed that for instance isMatchAlias is defined on AppliedTypes.
But in this case, we have tpe := TypeRef(NoPrefix, Alias) which remains the same after dealias since the info of Alias is not a TypeAlias but a MatchAlias. The case is not covered here:
https://github.com/lampepfl/dotty/blob/997103d818ead17d528d320b01edefb8792579ac/compiler/src/dotty/tools/dotc/core/Types.scala#L1438-L1445

case MatchAlias(alias) => unrollTupleTypes(alias.tryNormalize)
case _ => None
case _ =>
None

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8 changes: 8 additions & 0 deletions tests/pos/i19385.scala
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@@ -0,0 +1,8 @@
import scala.compiletime.summonAll

inline def f[M <: Tuple]: Unit =
type Alias = Tuple.Map[M, [X] =>> Numeric[X]]
summonAll[Tuple.Map[M, [X] =>> Numeric[X]]] // compiles
summonAll[Alias] // error: Tuple element types must be known at compile time

val y1 = f[(Int, Int)]
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