Untagged enum containing a subtype fails to deserialize, whilst the subtype itself can directly deserialize

[elertan]

Playground: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=9e3bd4da5ac247254d3017e839bd5f39

I have a struct that contains a HashMap<u32, String>.
This type correctly deserializes when using serde_json::from_str() with a json string such as {"data":{"1":"test"}}.

Yet when I put this type into an untagged enum variant, it does not work anymore.
If I then change the key type from u32 to String, it compiles.

This does not seem like intended behaviour, otherwise the variant itself should also fail, see the playground for a better example.

Might be related to:
#1254
#1244