Erroneous E0367 on struct containing a closure returning an array

[carado]

The following code fails in the playground, on the latest stable (1.32.0), latest beta (1.33.0-beta.6) and latest nightly (2019-02-07):

struct S<F: Fn() -> [u8; 1]>(F);

impl<F: Fn() -> [u8; 1]> Drop for S<F> {
    fn drop(&mut self) {}
}

fn main() {}

The error is the following:

error[E0367]: The requirement `<F as std::ops::FnOnce<()>>::Output == [u8; _]` is added only by the Drop impl.
 --> src/main.rs:3:1
  |
3 | / impl<F: Fn() -> [u8; 1]> Drop for S<F> {
4 | |     fn drop(&mut self) {}
5 | | }
  | |_^
  |
note: The same requirement must be part of the struct/enum definition
 --> src/main.rs:1:1
  |
1 | struct S<F: Fn() -> [u8; 1]>(F);
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The error seems to be the same regardless of rust version, and regardless of whether the function is an Fn, an FnMut, or an FnOnce (in particular, the error mentions std::ops::FnOnce<()>>::Output even when F isn't an FnOnce, if that's relevant).

The error does not occur when the return type [u8; 1] is replaced with, for example, u8 or (u8, u8), but still occurs with a more complex type still containing an array such as (u8, [u8; 1], u8);

Have a nice day.

[carado]

As a workaround (and in case that helps), I've found that using a type alias works.

type Array = [u8; 1];

struct S<F: Fn() -> Array>(F);

impl<F: Fn() -> Array> Drop for S<F> {
    fn drop(&mut self) {}
}

fn main() {}

Compiles as expected.