ICE: bad_placeholder_type

[Bryysen]

First time filing a bug report, apologies if i missed anything or if this is a duplicate. Encountered this bug while writing some non-sensical associated function types. Searched around for any similar issues and the closest i found was #70291, seems closely related?

Minimal example:

pub trait MyTrait {
    type AssocFn: std::ops::Fn(_);
}

fn main() {}

Playground link:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=02cf6fb6aed04c3058b258f5f41200d2

The ICE happens on all channels it seems

Backtrace

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', src/librustc_errors/lib.rs:366:17
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.aaakk.us.kg-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.aaakk.us.kg-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:78
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:59
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1076
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1537
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:62
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:49
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:198
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:218
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:490
  12: std::panicking::begin_panic
  13: <rustc_errors::HandlerInner as core::ops::drop::Drop>::drop
  14: core::ptr::drop_in_place
  15: <alloc::rc::Rc<T> as core::ops::drop::Drop>::drop
  16: core::ptr::drop_in_place
  17: rustc_interface::interface::run_compiler_in_existing_thread_pool
  18: rustc_ast::attr::with_globals

note: rustc 1.45.0 (5c1f21c3b 2020-07-13) running on x86_64-unknown-linux-gnu
note: compiler flags: -C opt-level=3 -C embed-bitcode=no -C codegen-units=1 --crate-type bin

[JohnTitor]

Searched around for any similar issues and the closest i found was #70291, seems closely related?

Yes, I think so too.
The snippet is rejected as expected on 1.42 but ICE occurs instead since 1.43.

[spastorino]

Assigning P-medium as discussed as part of the Prioritization Working Group procedure and removing I-prioritize.