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Dealias type when type parameters inference occurs #23242

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Dealias type when type parameters inference occurs #23242

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ba564fa
dealias type when type inference occurs
Alex1005a May 22, 2025
4805297
add test case for dealias during hk type parameter inference
Alex1005a May 23, 2025
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13 changes: 10 additions & 3 deletions compiler/src/dotty/tools/dotc/core/TypeComparer.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -1208,8 +1208,15 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
def directionalRecur(tp1: Type, tp2: Type): Boolean =
if fromBelow then recur(tp2, tp1) else recur(tp1, tp2)

val otherTycon = other.tycon
val otherArgs = other.args
// If the arity of the type constructors does not match, dealias to avoid creating type lambdas.
val otherDealiased =
if other.args.hasSameLengthAs(args) then other
else other.dealias match
case dealiasedType: AppliedType if dealiasedType.args.hasSameLengthAs(args) =>
dealiasedType
case _ => other
val otherTycon = otherDealiased.tycon
val otherArgs = otherDealiased.args

val d = otherArgs.length - args.length
d >= 0 && {
Expand All @@ -1234,7 +1241,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
otherTycon
rollbackConstraintsUnless:
(assumedTrue(tycon) || directionalIsSubType(tycon, adaptedTycon))
&& directionalRecur(adaptedTycon.appliedTo(args), other)
&& directionalRecur(adaptedTycon.appliedTo(args), otherDealiased)
}
}
end compareAppliedTypeParamRef
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8 changes: 8 additions & 0 deletions tests/pos/dealias-hk-typeparam.scala
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@@ -0,0 +1,8 @@
type Result[F[_], Err, Res] = F[Either[Err, Res]]

trait TypeClass[F[_]]

def id[F[_]: TypeClass, A](x: F[A]) = x

def test[F[_]: TypeClass](x: Result[F, String, Int]) =
id(x)
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