test: add in regression test for #7445

This adds in a regression test for the minimized issue, but the original
one now has a different issue with pickling. I've added this in as well,
but added it to the excludes.

Author: ckipp01

compiler/test/dotc/pos-test-recheck.excludes

@@ -18,3 +18,6 @@ gadt-cast-singleton.scala
 i11247.scala
 i6199b.scala
 t3440.scala
+
+# pickling related issue
+i7445b.scala

tests/pos/i7445a.scala

@@ -0,0 +1,10 @@
+// https://github.com/lampepfl/dotty/issues/7445
+
+object Main {
+  type O1[A] = {
+    type OutInner[X] = Unit
+    type Out = OutInner[A]
+  }
+
+  def f1: O1[Int]#Out = ???
+}

tests/pos/i7445b.scala

@@ -0,0 +1,12 @@
+// https://github.com/lampepfl/dotty/issues/7445
+
+type O1[A] = {
+  type OutInner[Ts] <: Tuple = Ts match {
+    case EmptyTuple => EmptyTuple
+    case h *: t => h *: OutInner[t]
+  }
+
+  type Out = OutInner[A]
+}
+
+def f1: O1[(Int, Int)]#Out = ???