Using associated constant as array length: without (funny error) and with (ICE) const generics

[birkenfeld]

Given the following test and nightly 2020-01-12:

#![allow(unused)]
#![feature(const_generics)]

trait Child {
    const N: usize;
}

struct C1;
impl Child for C1 {
    const N: usize = 2;
}

struct Main<C: Child> {
    c: C
}

impl<C: Child> Main<C> {
    fn test() -> [u8; C::N] {
        [0; C::N]
    }
}

I get that this is not supposed to work without const generics, but the error message suggestion is very funny:

error[E0599]: no associated item named `N` found for type parameter `C` in the current scope
  --> src/main.rs:22:27
   |
22 |     fn test2() -> [u8; C::N] {
   |                           ^ associated item not found in `C`
   |
   = help: items from traits can only be used if the type parameter is bounded by the trait
help: the following trait defines an item `N`, perhaps you need to restrict type parameter `C` with it:
   |
17 | impl<C: Child + Child> Main<C> {
   |      ^^^^^^^^^^

With const generics:

warning: Error finalizing incremental compilation session directory `/home/gbr/devel/rust-test/target/debug/incremental/rust_test-xwudz02f6bay/s-fjpbjlpyxj-1ihvwt0-working`: No such file or directory (os error 2)

error: internal compiler error: cat_expr Errd
  --> src/main.rs:22:30
   |
22 |       fn test2() -> [u8; C::N] {
   |  ______________________________^
23 | |         [0; C::N]
24 | |     }
   | |_____^

error: internal compiler error: cat_expr Errd
  --> src/main.rs:23:9
   |
23 |         [0; C::N]
   |         ^^^^^^^^^

error: internal compiler error: PromoteTemps: MIR had errors
  --> src/main.rs:22:5
   |
22 | /     fn test2() -> [u8; C::N] {
23 | |         [0; C::N]
24 | |     }
   | |_____^

error: internal compiler error: broken MIR in DefId(0:16 ~ rust_test[3df1]::{{impl}}[1]::test2[0]) ("return type"): bad type [type error]
  --> src/main.rs:22:5
   |
22 | /     fn test2() -> [u8; C::N] {
23 | |         [0; C::N]
24 | |     }
   | |_____^

error: internal compiler error: broken MIR in DefId(0:16 ~ rust_test[3df1]::{{impl}}[1]::test2[0]) (LocalDecl { mutability: Mut, local_info: Other, internal: false, is_block_tail: None, ty: [type error], user_ty: UserTypeProjections { contents: [] }, source_info: SourceInfo { span: src/main.rs:22:5: 24:6, scope: scope[0] } }): bad type [type error]
  --> src/main.rs:22:5
   |
22 | /     fn test2() -> [u8; C::N] {
23 | |         [0; C::N]
24 | |     }
   | |_____^

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', src/librustc_errors/lib.rs:346:17
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.42.0-nightly (3ebcfa145 2020-01-12) running on x86_64-unknown-linux-gnu

note: compiler flags: -C debuginfo=2 -C incremental --crate-type bin

note: some of the compiler flags provided by cargo are hidden

[Shadlock0133]

Weird suggestion is on current stable as well.

[Alexendoo]

The ICE component is fixed by #68388, the diagnostics issue remains (Child + Child)

[JohnTitor]

I think it's another example of #68695 now.