Merge pull request #12624 from dotty-staging/fix-9623

Fix #9623: Add regression test

Author: liufengyun

tests/pos/9623.scala

@@ -0,0 +1,22 @@
+object A {
+  sealed trait TList
+  sealed trait TNil extends TList
+  sealed trait ++:[H, T <: TList] extends TList
+
+  type :--[R <: TList, A] <: TList = R match {
+    case (A ++: t) => t
+    case (h ++: t) => h ++: (t :-- A)
+  }
+}
+
+object B {
+  import A.*
+
+  type X = (Int ++: String ++: Double ++: TNil) :-- String
+
+  class T[A]
+
+  def f(ta: T[X]) = ()
+
+  f(new T[Int ++: Double ++: TNil])
+}