Merge pull request #11239 from dotty-staging/fix-#9239

Fix #9239: Add regression test

Author: nicolasstucki

tests/pos/9239.scala

@@ -0,0 +1,28 @@
+object ABug:
+  sealed trait Bit
+  sealed trait B0 extends Bit
+  sealed trait B1 extends Bit
+
+  sealed trait Bin
+  sealed trait Nil extends Bin
+  sealed trait ::[U <: Bit, D <: Bin] extends Bin
+
+  type Zero = B0 :: Nil
+  type One  = B1 :: Nil
+
+  type --[B <: Bin] =
+    B match
+      case B1 :: d => B0 :: d
+      case B0 :: B1 :: Nil => B1 :: Nil
+      case B0 :: d => B1 :: --[d]
+
+  type ×[N <: Bin, M <: Bin] =
+    (N, M) match
+      case (Zero, ?) => Zero
+
+  type ![N <: Bin] =
+    N match
+      case Zero => One
+      case One => One
+      case ? => ![--[N]] × (N)
+      case ? :: ? => ![--[N]] × (N)