SI-6895 Test cases to explain the limitations in tcpoly inference

I've determined that the failure in this bug report boils down
to SI-2712. Submitting my working as neg tests for posterity.

The type error disabmiguator develops a serious stutter with this
example. I'll fix that in the following commit.

1 files changed, 26 insertions, 0 deletions

diff --git a/test/files/neg/t6895.scala b/test/files/neg/t6895.scala
new file mode 100644
index 0000000000..5fb20d8c61
--- /dev/null
+++ b/test/files/neg/t6895.scala
@@ -0,0 +1,26 @@
+trait Foo[F1[F1_P]]
+trait Bar[F2[F2_P]]
+
+class Test {
+  def barFoo[F3[F3_P]]: Foo[F3] = ???
+
+  // Now we can define a couple of type aliases:
+  type M[X1] = String
+  type N[X2] = Bar[M]
+
+  // val ok1: Foo[N] = barFoo
+  // Foo[?F3] <:< Foo[Test.this.N]
+  //   [X2]Test.this.N[X2] <:< [F3_P]?F3[F3_P]
+  //      Test.this.N[X2] <:< ?F3[X2]
+  //        true, ?F3=N
+
+  // val ok2: Foo[({type L[X] = Bar[M]})#L] = barFoo[N]
+
+  val nok: Foo[({type L[X3] = Bar[M]})#L] = barFoo /* Type inference can't unify F with L */
+  // Foo[?F3] <:< Foo[[X3]Bar[[X1]String]]
+  //   [X3]Bar[[X1]String] <:< ?F3
+  //     [X3]Bar[[X1]String] <:< [F3_P]?F3[F3_P]
+  //        Bar[[X1]String] <:< ?F3[X3]
+  //          X3 <:< [X1]String
+  //            false
+}