ICE(broken MIR and no errors despite delay_span_bug) when existential type alias is used as closure return value causing type error

[meithecatte]

Code

#![feature(min_type_alias_impl_trait)]
#![feature(type_alias_impl_trait)]

type Test = impl Copy;

fn test() -> Test {
    let y = || -> Test { () };
    7
}

Meta

rustc --version --verbose:

rustc 1.46.0-nightly (346aec9b0 2020-07-11)
binary: rustc
commit-hash: 346aec9b02f3c74f3fce97fd6bda24709d220e49
commit-date: 2020-07-11
host: x86_64-unknown-linux-gnu
release: 1.46.0-nightly
LLVM version: 10.0

Error output

warning: unused variable: `y`
 --> rust-ice.rs:6:9
  |
6 |     let y = || -> Test { () };
  |         ^ help: if this is intentional, prefix it with an underscore: `_y`
  |
  = note: `#[warn(unused_variables)]` on by default

warning: type alias is never used: `Test`
 --> rust-ice.rs:3:1
  |
3 | type Test = impl Copy;
  | ^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(dead_code)]` on by default

warning: function is never used: `test`
 --> rust-ice.rs:5:4
  |
5 | fn test() -> Test {
  |    ^^^^

warning: 3 warnings emitted

error: internal compiler error: broken MIR in DefId(0:5 ~ rust_ice[317d]::test[0]) (bb0[0]): equate_inputs_and_outputs: `impl std::marker::Copy==i32` failed with `NoSolution`
 --> rust-ice.rs:8:2
  |
8 | }
  |  ^
  |
  = note: delayed at src/librustc_mir/borrow_check/type_check/mod.rs:258:27

Backtrace

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', src/librustc_errors/lib.rs:367:17
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-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:217
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:530
  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_span::with_source_map
  18: rustc_interface::interface::create_compiler_and_run
  19: scoped_tls::ScopedKey<T>::set
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose 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 1.46.0-nightly (346aec9b0 2020-07-11) running on x86_64-unknown-linux-gnu

query stack during panic:
end of query stack

[Alexendoo]

No longer ICEs since #87287