Merge pull request #13489 from dotty-staging/i13487

Fix #13487: Unification check in deriving is incorrect
scala · Sep 27, 2021 · ee8d2ab · ee8d2ab
2 parents 164b5d8 + 6fd5bed
commit ee8d2ab
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Showing 4 changed files with 17 additions and 1 deletion.
diff --git a/compiler/src/dotty/tools/dotc/typer/Deriving.scala b/compiler/src/dotty/tools/dotc/typer/Deriving.scala
@@ -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)) {

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