Nightly regression (expected a path on the left-hand side of `+`)

[dpc]

$ rustc --version
rustc 1.16.0-nightly (c07a6ae77 2017-01-17)
$ cargo --version
cargo 0.17.0-nightly (493abf5 2017-01-18)

Daily travis builds of slog are failing for todays nightly: https://travis-ci.org/slog-rs/slog/jobs/192913808

error[E0178]: expected a path on the left-hand side of `+`, not `(Fn(D::Error) -> E)`
   --> src/_drain.rs:119:17
    |
119 |     map_fn: Box<(Fn(D::Error) -> E) + 'static+ Send+Sync>,
    |                 ^^^^^^^^^^^^^^^^^^^ expected a path
    |
    = help: perhaps you forgot parentheses? (per RFC 438)

[petrochenkov]

This is a regression from #39110

Strictly speaking, the syntax is incorrect - parens are not permitted in bounds.
The correct variant is Box<Fn(D::Error) -> E + 'static + Send + Sync>.
I'll submit a fix ASAP though, so we could start a warning period or something later.

[dpc]

Thank you for information. I'll fix it and release patch version ASAP. I don't think many projects could be affected.