ICE w/ default generic type parameter and const parameter #67858

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

pub trait Trait {
    type Associated;
}

pub struct Foo<T, A = <T as Trait>::Associated, const N: u8>(T, A);

impl<T: Trait, const N: u8> Foo<T, N> {
}

fn main() {
}

error: internal compiler error: potentially unconstrained type parameters weren't evaluated
  --> potentially-unconstrained.rs:10:1
   |
10 | / impl<T: Trait, const N: u8> Foo<T, N> {
11 | | }
   | |_^

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', src/librustc_errors/lib.rs:345:17
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 (c5840f9d2 2020-01-03) running on x86_64-unknown-linux-gnu