Fix scala#13855: Add regression test

Author: OlivierBlanvillain

tests/pos/13855.scala

@@ -0,0 +1,13 @@
+type A[X] = X match
+  case Int => Int
+  case _   => B[X]
+
+def a[X](x: X): A[X] = x match
+  case v: Int => v
+  case _      => b(x)
+
+type B[X] = X match
+  case String => String
+
+def b[X](x: X): B[X] = x match
+  case v: String => v