SI-2712 Add support for higher order unification

1 files changed, 39 insertions, 0 deletions

diff --git a/test/files/pos/t6895b.scala b/test/files/pos/t6895b.scala
new file mode 100644
index 0000000000..c465065011
--- /dev/null
+++ b/test/files/pos/t6895b.scala
@@ -0,0 +1,39 @@
+trait Foo[F[_]]
+trait Bar[F[_], A]
+
+trait Or[A, B]
+
+class Test {
+  implicit def orFoo[A]: Foo[({type L[X] = Or[A, X]})#L] = ???
+  implicit def barFoo[F[_]](implicit f: Foo[F]): Foo[({type L[X] = Bar[F, X]})#L] = ???
+
+  // Now we can define a couple of type aliases:
+  type StringOr[X] = Or[String, X]
+  type BarStringOr[X] = Bar[StringOr, X]
+
+  // ok
+  implicitly[Foo[BarStringOr]]
+  barFoo[StringOr](null) : Foo[BarStringOr]
+  barFoo(null) : Foo[BarStringOr]
+
+  // nok
+  implicitly[Foo[({type L[X] = Bar[StringOr, X]})#L]]
+  // Let's write the application explicitly, and then
+  // compile with just this line enabled and -explaintypes.
+  barFoo(null) : Foo[({type L[X] = Bar[StringOr, X]})#L]
+
+  // Foo[[X]Bar[F,X]] <: Foo[[X]Bar[[X]Or[String,X],X]]?
+  //   Bar[[X]Or[String,X],X] <: Bar[F,X]?
+  //     F[_] <: Or[String,_]?
+  //     false
+  //   false
+  // false
+
+  // Note that the type annotation above is typechecked as
+  // Foo[[X]Bar[[X]Or[String,X],X]], ie the type alias `L`
+  // is eta expanded.
+  //
+  // This is done so that it does not escape its defining scope.
+  // However, one this is done, higher kinded inference
+  // no longer is able to unify F with `StringOr` (SI-2712)
+}