Add test case

Author: OlivierBlanvillain

tests/neg/12549.scala

@@ -0,0 +1,18 @@
+enum Bool {
+  case True
+  case False
+}
+
+import Bool.*
+
+type Not[B <: Bool] = B match {
+  case True.type => False.type
+  case False.type => True.type
+  case _ => "unreachable"
+}
+
+def foo[B <: Bool & Singleton]: Unit = {
+  implicitly[Not[B] =:= "unreachable"] // error
+
+  ()
+}