Fix #6716: Add test

Author: odersky

tests/pos/i6716.scala

@@ -0,0 +1,15 @@
+trait Monad[T]
+class Foo
+object Foo {
+  given as Monad[Foo]
+}
+
+opaque type Bar = Foo
+object Bar {
+  given as Monad[Bar] = the[Monad[Foo]]
+}
+
+object Test {
+  val mf = the[Monad[Foo]]
+  val mb = the[Monad[Bar]]
+}