Even though Box<Reader> is a Reader, Box<Reader + Send> is not a Reader.

[reem]

use std::io::{MemReader, MemWriter};
use std::io::util::copy;

fn main() {
    let mut rain = box MemReader::new(vec![1, 2, 3]) as Box<Reader>;
    let mut pool = MemWriter::new();
    copy(&mut rain, &mut pool);
}

Compiles.

use std::io::{MemReader, MemWriter};
use std::io::util::copy;

fn main() {
    let mut rain = box MemReader::new(vec![1, 2, 3]) as Box<Reader + Send>;
    let mut pool = MemWriter::new();
    copy(&mut rain, &mut pool);
}

Fails with:

<anon>:7:5: 7:9 error: failed to find an implementation of trait std::io::Reader for Box<std::io::Reader:Send>
<anon>:7     copy(&mut rain, &mut pool);
             ^~~~
playpen: application terminated with error code 101

This seems like it should work and is very non-ergonomic if it doesn't work.

[alexcrichton]

cc @nikomatsakis, @pcwalton, I think this is fallout of more strict bounds matching recently.

[ghost]

Closing as a dupe of #18524.