Parsing issues with Box'ed unboxed closures

[bitonic]

The following code works:

    pub fn new(delay: uint, callback: Box<FnMut() -> uint + 'a>, remove_on_drop: bool) -> Timer<'a> {
        ...
    }

But this doesn't:

    pub fn new(delay: uint, callback: Box<(FnMut() -> uint) + 'a>, remove_on_drop: bool) -> Timer<'a> {
        ...
    }

rustc complains that

src/sdl2/timer.rs:33:43: 33:60 error: expected a path on the left-hand side of `+`, not `(FnMut() -> uint)` [E0178]
src/sdl2/timer.rs:33     pub fn new(delay: uint, callback: Box<(FnMut() -> uint) + 'a>, remove_on_drop: bool) -> Timer<'a> {
                                                               ^~~~~~~~~~~~~~~~~
src/sdl2/timer.rs:33:43: 33:60 note: perhaps you forgot parentheses? (per RFC 438)
src/sdl2/timer.rs:33     pub fn new(delay: uint, callback: Box<(FnMut() -> uint) + 'a>, remove_on_drop: bool) -> Timer<'a> {
                                                               ^~~~~~~~~~~~~~~~~

If I understand correctly the syntax is trait + lifetime, so I'd expect those parentheses to be fine (and imho they make the code more clear). The note about forgetting parentheses is also puzzling.

[steveklabnik]

Full example:

struct Timer<'a> {
    x: &'a i32,
}

impl<'b> Timer<'b> {
    pub fn new<'a>(delay: usize, callback: Box<(FnMut() -> usize) + 'a>, remove_on_drop: bool) -> Timer<'a> {
        unimplemented!()
    }
}

fn main() {}

Same error still happens today.

[pmarcelll]

@petrochenkov if I understand it correctly, this is accepted today, but #40043 fixes the parser, so rejecting the parentheses is the correct behavior, and this is not a bug after all (and can be closed).

[petrochenkov]

@pmarcelll
Indeed.
I wasn't aware of this issue.
Confusing diagnostics perhaps you forgot parentheses? are fixed in #40043 as well.
Closing as a duplicate of #39318.