Built-in super-trait doesn't imply trait object implements that super-trait

[huonw]

trait Trait: Send {}

#[cfg(not(working))]
struct Struct { x: Box<Trait> }

#[cfg(working)]
struct Struct { x: Box<Trait + Send> }

impl Trait for Struct {}

fn main() {}

Compiling with --cfg working compiles fine, then without that --cfg gives:

send2.rs:9:16: 9:22 error: the type `Struct', which does not fulfill `Send`, cannot implement this trait
send2.rs:9 impl Trait for Struct {}
                          ^~~~~~
send2.rs:9:16: 9:22 note: types implementing this trait must fulfill `Send+Sized`
send2.rs:9 impl Trait for Struct {}
                          ^~~~~~
error: aborting due to previous error

[alexcrichton]

cc #13155, quite similar, not sure if they're exactly the same though