Odd error involving a closure that references itself

help
1

I have a trait Foo<'a>: 'a, and a closure F: FnOnce(&'a Bar) -> R where R: Foo<'a>. I'm trying to convert it to a boxed one which looks like this:

Box<dyn for<'a> FnOnce(&'a Bar) -> Box<dyn Foo<'a> + 'a>>

The problem is, it is not possible to use HRTBs for both F and R, so I have to use a workaround. Here is my try:

trait Foo<'a>: 'a {}
struct Bar;

trait Helper<'a> {
    type R: Foo<'a>;
}

impl<'a, F, R> Helper<'a> for F
where
    F: FnOnce(&'a Bar) -> R,
    R: Foo<'a>,
{
    type R = R;
}

fn poly<F>(f: F) -> Box<dyn for<'a> FnOnce(&'a Bar) -> Box<dyn Foo<'a> + 'a>>
where
    for<'a> F: 'static + Helper<'a> + FnOnce(&'a Bar) -> <F as Helper<'a>>::R,
{
    Box::new(move |b| Box::new(f(b)))
}

fn main() {
    struct FooEx<'a>(&'a Bar);
    impl<'a> Foo<'a> for FooEx<'a> {}
    let _ = poly(|x| FooEx(x));
}

I get a weird error:

error[E0644]: closure/generator type that references itself
  --> src/main.rs:26:18
   |
26 |     let _ = poly(|x| FooEx(x));
   |                  ^^^^^^^^^^^^ cyclic type of infinite size
   |
   = note: closures cannot capture themselves or take themselves as argument;
           this error may be the result of a recent compiler bug-fix,
           see issue #46062 <https://github.com/rust-lang/rust/issues/46062>
           for more information

Clearly the closure doesn't reference itself. Adding let () = x shows that x is in fact &Bar as expected. So the error message seems as a compiler bug (I've tried both stable and nightly).

The question is, how do I implement this poly function in a way that works and keeps the lifetime relationship between the argument and the return value.

2

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