issue-53121.rs

// NB You must compile with `--crate-type lib` to see the failure!

#![feature(nll)]

pub trait Accessor: Sized {
}

impl Accessor for () { }

pub enum AccessorCow<'a, 'b, T>
where
    AccessorTy<'a, T>: 'b,
    T: System<'a> + ?Sized,
    'a: 'b,
{
    /// A reference to an accessor.
    Ref(&'b AccessorTy<'a, T>),
    /// An owned accessor.
    Owned(AccessorTy<'a, T>),
}

type AccessorTy<'a, T> = <<T as System<'a>>::SystemData as DynamicSystemData<'a>>::Accessor;

pub trait System<'a> {
    type SystemData: DynamicSystemData<'a>;
}

impl<'a> System<'a> for () {
    type SystemData = ();
}

pub trait SystemData<'a> {
}

impl<'a> SystemData<'a> for () {
}

pub trait DynamicSystemData<'a> {
    /// The accessor of the `SystemData`, which specifies the read and write dependencies and does
    /// the fetching.
    type Accessor: Accessor;
}

impl<'a> DynamicSystemData<'a> for () {
    type Accessor = ();
}

fn main() {
    let x : fn(_) -> _ = AccessorCow::Ref::<()>;
}