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df11395a55
Fix `#[derive(Default)]` on a generic `#[default]` enum adding unnecessary `Default` bounds
That is, given something like:
```rs
// #[default] on a generic enum does not add `Default` bounds to the type params.
#[derive(Default)]
enum MyOption<T> {
#[default]
None,
Some(T),
}
```
then `MyOption<T> : Default`_as currently implemented_ only holds when `T : Default`, as reported by ```@5225225``` [over Zulip](https://rust-lang.zulipchat.com/#narrow/stream/122651-general/topic/.23.5Bderive.28Default.29.5D.20for.20enums.20with.20fields).
This is contrary to [what the accepted RFC proposes](https://rust-lang.github.io/rfcs/3107-derive-default-enum.html#generated-bounds) (_i.e._, that `T` be allowed not to be itself `Default`), and indeed seems to be a rather unnecessary limitation.