Merge pull request #2607 from retronym/ticket/7517

SI-7517 Fix higher kinded type inference regression

2 files changed, 25 insertions, 4 deletions

diff --git a/src/reflect/scala/reflect/internal/Types.scala b/src/reflect/scala/reflect/internal/Types.scala
index ee584bed2c..0cc2b91a44 100644
--- a/src/reflect/scala/reflect/internal/Types.scala
+++ b/src/reflect/scala/reflect/internal/Types.scala
@@ -3218,10 +3218,9 @@ trait Types extends api.Types { self: SymbolTable =>
           sameLength(typeArgs, tp.typeArgs) && {
             val lhs = if (isLowerBound) tp.typeArgs else typeArgs
             val rhs = if (isLowerBound) typeArgs else tp.typeArgs
-            // this is a higher-kinded type var with same arity as tp.
-            // side effect: adds the type constructor itself as a bound
-            addBound(tp.typeConstructor)
-            isSubArgs(lhs, rhs, params, AnyDepth)
+            // This is a higher-kinded type var with same arity as tp.
+            // If so (see SI-7517), side effect: adds the type constructor itself as a bound.
+            isSubArgs(lhs, rhs, params, AnyDepth) && { addBound(tp.typeConstructor); true }
           }
         }
         // The type with which we can successfully unify can be hidden
diff --git a/test/files/pos/t7517.scala b/test/files/pos/t7517.scala
new file mode 100644
index 0000000000..7ce4c6b13e
--- /dev/null
+++ b/test/files/pos/t7517.scala
@@ -0,0 +1,22 @@
+trait Box[ K[A[x]] ]
+
+object Box {
+   // type constructor composition
+   sealed trait ∙[A[_], B[_]] { type l[T] = A[B[T]] }
+
+   // composes type constructors inside K
+   type SplitBox[K[A[x]], B[x]] = Box[ ({ type l[A[x]] = K[ (A ∙ B)#l] })#l ]
+
+   def split[ K[A[x]], B[x] ](base: Box[K]): SplitBox[K,B] = ???
+
+   class Composed[B[_], L[A[x]] ] {
+      val box: Box[L] = ???
+
+      type Split[ A[x] ] = L[ (A ∙ B)#l ]
+      val a: Box[Split] = Box.split(box)
+
+      //Either of these work:
+      val a1: Box[Split] = Box.split[L,B](box)
+      val a2: Box[ ({ type l[A[x]] = L[ (A ∙ B)#l ] })#l ] = Box.split(box)
+   }
+}
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