False positive for "lifetime must be declared" error in 2018 preview

[alexcrichton]

This code:

#![feature(rust_2018_preview)]

trait A {
    
}

impl<F> A for F where F: FnOnce(&i32) {}

fn main() {}

yields:

error[E0687]: lifetimes used in `fn` or `Fn` syntax must be explicitly declared using `<...>` binders
 --> src/main.rs:7:33
  |
7 | impl<F> A for F where F: FnOnce(&i32) {}
  |                                 ^ in-band lifetime definition

error: aborting due to previous error

For more information about this error, try `rustc --explain E0687`.

[nikomatsakis]

cc @cramertj =)

[nikomatsakis]

(I'm investigating a bit)

[nikomatsakis]

Got a fix coming

[nikomatsakis]

#51220