Purely-diverging functions cannot return `impl Trait`.

[kennytm]

Originally reported in #34511 (comment), separated out to make it easier to track. #43869 and #44731 are examples caused by this issue.

If a function is purely diverging, any part of the return type cannot contain impl Trait, e.g.

fn do_it_later_but_cannot() -> impl Iterator<Item=u8> { //~ ERROR E0227
    unimplemented!()
}

this fails because ! does not implement Iterator. But I think this should be allowed even without the explicit impl, since ! can be coerced to any concrete type.

[malbarbo]

@kennytm Do you know where the issue of general diverging functions cannot return impl Trait are being tracked?

[kennytm]

@malbarbo there’s no problem returning “general diverging functions”, if you mean something like this:

fn foo() -> impl Bar {
    if bar() {
        Baz::new()
    } else {
        panic!(“diverge”)
    }
}

[malbarbo]

@kennytm Yes, I mean that. But I asked because I remembered vaguely some limitations that I had encountered in the past. Looking at my old code, the limitation was not that... sorry. The limitation is this:

fn g<T>(t: T) -> Result<impl Iterator<Item=u32>, T> {
    Err(t)
}

fails to compile with error:

fn g<T>(t: T) -> Result<impl Iterator<Item=u32>, T>
                        ^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type for `_`

If we change the type to impl Clone, the error changes to:

fn g<T>(t: T) -> Result<impl Clone, T>
                        ^^^^^^^^^^
note: the return type of a function must have a statically known size

It this a know issue? Is this expected?

[Aaron1011]

I'd like to work on this.

[hanna-kruppe]

this and #36375 are duplicates of each other

[ishitatsuyuki]

I think we can close this as duplicate of #36375?