#![feature(lazy_normalization_consts)] struct P<T: ?Sized>([u8; 1 + 4], T); fn main() { let x: Box<P<[u8; 0]>> = Box::new(P(Default::default(), [0; 0])); let _: Box<P<[u8]>> = x; }

This works on stable but fails with the following error when using lazy_normalization_consts

error[E0308]: mismatched types --> src/main.rs:6:27 | 6 | let _: Box<P<[u8]>> = x; | ------------ ^ expected slice `[u8]`, found array `[u8; 0]` | | | expected due to this | = note: expected struct `Box<P<[u8]>>` found struct `Box<P<[u8; 0]>>` 

For this to fail the struct has to contain a type referencing an unevaluated constant, struct P<T: ?Sized>([u8; 5], T) does work.

assigning to myself, but won't get to this right away. If you are interested in working on this before that feel free to do so.

cc @nikomatsakis @varkor