Fix #7859: Add test case

#8409 also fixed this one

Author: odersky

tests/pos/i8397.scala

@@ -1,3 +1,22 @@
 given foo(using x: Int) as AnyRef:
   type T = x.type
 
+// #7859
+
+trait Lub2[A, B]:
+  type Output
+
+given [A <: C, B <: C, C] as Lub2[A, B]:
+  type Output = C
+
+trait Lub[Union]:
+  type Output
+
+given [A] as Lub[A]:
+  type Output = A
+
+given [Left, Right](
+    using lubLeft: Lub[Right], lubRight: Lub[Right])(
+    using lub2: Lub2[lubLeft.Output, lubRight.Output])
+as Lub[Left | Right]:
+  type Output = lub2.Output