ICE with const_generic + associated type + double return statement

[simon-auch]

I tried this code:

#![feature(const_generics)]
#![feature(const_generic_impls_guard)]
use core::array::LengthAtMost32;

struct Foo<const D: usize> {
    state: Option<[u8;D]>,
}
impl<const D: usize> Iterator for Foo<{D}> where
    [u8;D]: LengthAtMost32,
{
    type Item = [u8; D];
    fn next(&mut self) -> Option<Self::Item> {
        return Some(self.state.unwrap().clone());
        return Some(self.state.unwrap().clone());
    }
}

I expected to see this happen:
It should build.

Instead, this happened:
rustc crashed with:
error: internal compiler error: src/librustc/infer/unify_key.rs:143: equating two const variables, both of which have known values

Possible workaround:
replace Self::Item with [u8;D].

Meta

playground: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=849c7ac992950ce15a2fcf075fd61828

rustc version: 1.39.0-nightly 2019-09-15 96d07e0

[mati865]

cc @varkor: b872e63

[hellow554]

Also ICEs without explicit feature gate enabled :/

// #![feature(const_generics)]

trait F {
    type Item;
    fn next() -> Self::Item;
}

struct Foo<const D: usize>;
impl<const D: usize> F for Foo<{D}>
{
    type Item = [u8; D];
    fn next() -> Self::Item {
        return [0; D];
        return [0; D];
    }
}