Fix #10510: Add regression test

Author: OlivierBlanvillain

tests/pos/10510.scala

@@ -0,0 +1,17 @@
+sealed trait Bool
+case object True extends Bool
+case object False extends Bool
+
+sealed trait SBool[B <: Bool]
+case object STrue extends SBool[True.type]
+case object SFalse extends SBool[False.type]
+
+type Not[B <: Bool] <: Bool = B match {
+  case True.type => False.type
+  case False.type => True.type
+}
+
+def not[B <: Bool](b: SBool[B]): SBool[Not[B]] = b match {
+  case STrue => SFalse
+  case SFalse => STrue
+}