Unexpected behaviour with untagged enums and numeric keys

[jsouto18]

When using an untagged enum to handle generic Serialization + Deserialization the deserialization will always fail and the cause seems to be the numeric keys of BTreeMap. If using the struct directly everything works as expected.

Can this be related to #1183?
Also not sure if this is a serde or serde_json issue so apologies if this is the wrong repo.

Minimum Reproducible Example:

use std::collections::BTreeMap;

use serde::{Deserialize, Serialize};

#[derive(Debug, Serialize, Deserialize)]
#[serde(tag = "form", content = "data")]
enum StoredSerializationForm<'a, T> {
    Compressed(Vec<u8>),
    #[serde(untagged)]
    #[serde(bound(deserialize = "Ptr<'a,T>: Deserialize<'de>"))]
    Uncompressed(Ptr<'a, T>),
}

#[derive(Debug, Serialize)]
#[serde(untagged)]
enum Ptr<'a, T> {
    Ref(&'a T),
    Owned(T),
}

impl<'de, 'a, T> Deserialize<'de> for Ptr<'a, T>
where
    T: Deserialize<'de>,
{
    fn deserialize<D>(deserializer: D) -> std::prelude::v1::Result<Self, D::Error>
    where
        D: serde::Deserializer<'de>,
    {
        Deserialize::deserialize(deserializer).map(Ptr::Owned)
    }
}

#[derive(Debug, Serialize, Deserialize)]
struct TestStruct {
    ctx: BTreeMap<u64, u64>,
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_fails() {
        let mut ctx = BTreeMap::new();
        ctx.insert(1, 42);
        ctx.insert(2, 42);
        let test_struct = TestStruct { ctx };

        let data = StoredSerializationForm::Uncompressed(Ptr::Ref(&test_struct));
        let serialized = serde_json::to_vec(&data).unwrap();

        let _: StoredSerializationForm<TestStruct> = serde_json::from_slice(&serialized).unwrap();
    }

    #[test]
    fn test_works() {
        let mut ctx = BTreeMap::new();
        ctx.insert(1, 42);
        ctx.insert(2, 42);
        let test_struct = TestStruct { ctx };

        let serialized = serde_json::to_vec(&test_struct).unwrap();

        let _: TestStruct = serde_json::from_slice(&serialized).unwrap();
    }
}