Add corresponding regression test

Author: danielhenrymantilla

src/test/ui/deriving/deriving-default-enum.rs

@@ -12,6 +12,16 @@ enum Foo {
     Beta(NotDefault),
 }
 
+// #[default] on a generic enum does not add `Default` bounds to the type params.
+#[derive(Default)]
+enum MyOption<T> {
+    #[default]
+    None,
+    #[allow(dead_code)]
+    Some(T),
+}
+
 fn main() {
     assert_eq!(Foo::default(), Foo::Alpha);
+    assert!(matches!(MyOption::<NotDefault>::default(), MyOption::None));
 }