DeserializeOwned not work with derive macro

[sineptic]

It's not compile:

pub trait A: Serialize + DeserializeOwned {}

#[derive(Serialize, Deserialize)]
pub struct B<T: A> {
    t: T,
}

and generates 90 lines of unuseful errors

[sineptic]

It compiles:

pub trait A: Serialize + DeserializeOwned {}

#[derive(Serialize, Deserialize)]
#[serde(bound(deserialize = "T: A"))]
pub struct B<T> {
    t: T,
}

but it's no:

#[derive(Serialize, Deserialize)]
pub struct B<T: A> {
    t: T,
}

WHY???

And also why first example compiles and serializes if bound only for deserializing?