ICE: Returning impl Trait with placeholder type parameter #67995
Description
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
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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.