ICE: Returning impl Trait with placeholder type parameter

[riptl]

Hope this isn't a duplicate.

EDIT: Updated example, removing async/await

Minimal Example

fn main() {
    evil();
}

trait Trait<T> {}

fn evil() -> impl Trait<_> {}

Workaround

Avoid placeholders.
fn evil() -> impl Trait<()> {

Meta

rustc 1.42.0-nightly (859764425 2020-01-07) running on x86_64-unknown-linux-gnu

EDIT: No ICE on stable Rust 1.40.0, build fails with error E0121 at fn evil() ....

Compiler Log

Expand

    Checking crash_test v0.1.0 (/home/richie/prj/crash_test)
error: internal compiler error: bad placeholder type
 --> src/main.rs:7:25
  |
7 | fn evil() -> impl Trait<_> {}
  |                         ^

error: internal compiler error: errors selecting obligation during MIR typeck: [FulfillmentError(Obligation(predicate=Binder(TraitPredicate(<() as Trait<[type error]>>)), depth=0),Ambiguity)]

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', src/librustc_errors/lib.rs:347:17
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 (859764425 2020-01-07) 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

error: could not compile `crash_test`.

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

[pnkfelix]

triage: P-high, removing I-nominated label

[lqd]

bisected to 71bb0ff from #67597 cc @estebank

[riptl]

Turns out it's not async/await related. It occurs when returning any impl Trait from a function that has placeholder type parameters.

Simplified example in updated issue description, see above.