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Fix #13487: Unification check in deriving is incorrect #13489

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Merged
merged 1 commit into from
Sep 27, 2021
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Fix #13487: Unification check in deriving is incorrect #13489

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2 changes: 1 addition & 1 deletion compiler/src/dotty/tools/dotc/typer/Deriving.scala
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Expand Up @@ -160,7 +160,7 @@ trait Deriving {
val clsParamInfos = clsType.typeParams
val clsArity = clsParamInfos.length
val alignedClsParamInfos = clsParamInfos.takeRight(instanceArity)
val alignedTypeClassParamInfos = typeClassParamInfos.take(alignedClsParamInfos.length)
val alignedTypeClassParamInfos = typeClassParamInfos.takeRight(alignedClsParamInfos.length)


if ((instanceArity == clsArity || instanceArity > 0) && sameParamKinds(alignedClsParamInfos, alignedTypeClassParamInfos)) {
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4 changes: 4 additions & 0 deletions tests/neg/i13487.check
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@@ -0,0 +1,4 @@
-- Error: tests/neg/i13487.scala:6:32 ----------------------------------------------------------------------------------
6 |case class Foo[A](a: A) derives TC // error
| ^^
| Foo cannot be unified with the type argument of TC
6 changes: 6 additions & 0 deletions tests/neg/i13487.scala
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@@ -0,0 +1,6 @@
trait TC[F[_, _[_]]]
object TC {
def derived[F[_, _[_]]]: TC[F] = ???
}

case class Foo[A](a: A) derives TC // error
6 changes: 6 additions & 0 deletions tests/pos/i13487.scala
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@@ -0,0 +1,6 @@
trait TC[F[_[_], _]]
object TC {
def derived[F[_[_], _]]: TC[F] = ???
}

case class Foo[A](a: A) derives TC