ICE on unmonomorphized const generics in const eval: eval_const_to_op: Unexpected ConstKind

[oli-obk]

Amusingly the following ICEs while https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=67ee1b8218865042e272d1c2c1aa22fe has a compiler error that hints at the const generic actually getting resolved.

#![feature(const_generics)]
#![allow(incomplete_features)]
struct Arr<const N: usize>
where Assert::<{N < usize::max_value() / 2}>: IsTrue,
{
}

enum Assert<const CHECK: bool> {}

trait IsTrue {}

impl IsTrue for Assert<true> {}

fn main() {
    let x: Arr<{usize::max_value()}> = Arr {};
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error: internal compiler error: src/librustc_mir/interpret/operand.rs:548: eval_const_to_op: Unexpected ConstKind Const { ty: usize, val: Bound(DebruijnIndex(0), 0) }

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:907:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

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 1.45.0-nightly (74e804683 2020-05-30) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2 --crate-type bin

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

error: aborting due to previous error

error: could not compile `playground`.

To learn more, run the command again with --verbose.

[jonas-schievink]

Also happens on stable without the feature gate

[JohnTitor]

Triage: This is no longer ICE and emits an error correctly by #70107. Marking as E-needs-test.