Compilation error with Box<FnOnce(uint) -> uint>

[carllerche]

fn action(cb: Box<FnOnce(uint) -> uint>) -> uint {
    cb(1)
}

pub fn main() {
    println!("num: {}", action(|u| u));
}

output:

$ rustc fnonce.rs
fnonce.rs:1:25: 1:26 error: expected `,`, found `(`
fnonce.rs:1 fn action(cb: Box<FnOnce(uint) -> uint>) -> uint {

/cc @pcwalton

[ftxqxd]

Looks like a dupe of #17703.