ICE with closure return type as impl trait type alias defining use

[Luexa]

In an attempt to make the compiler recognize my defining use of an impl trait type alias without making a separate wrapper function, I ran this code, but it resulted in an ICE:

#![feature(type_alias_impl_trait)]

pub type Closure = impl FnOnce();

fn main() {
    || -> Closure { || () };
}

Playground

Compiler Output:

   Compiling playground v0.0.1 (/playground)
error: internal compiler error: broken MIR in DefId(0:14 ~ playground[1d12]::main[0]::{{closure}}[0]) (bb0[0]): equate_normalized_input_or_output: `Closure==[closure@src/main.rs:6:21: 6:26]` failed with `NoSolution`
 --> src/main.rs:6:28
  |
6 |     || -> Closure { || () };
  |                            ^

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', src/librustc_errors/lib.rs:361:17
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.34/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.34/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:47
   3: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:36
   4: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:200
   5: std::panicking::default_hook
             at src/libstd/panicking.rs:214
   6: rustc::util::common::panic_hook
   7: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:481
   8: std::panicking::begin_panic
   9: <rustc_errors::Handler as core::ops::drop::Drop>::drop
  10: core::ptr::real_drop_in_place
  11: <alloc::rc::Rc<T> as core::ops::drop::Drop>::drop
  12: core::ptr::real_drop_in_place
  13: rustc_interface::interface::run_compiler_in_existing_thread_pool
  14: std::thread::local::LocalKey<T>::with
  15: syntax::with_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
query stack during panic:
end of query stack

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 1.38.0-nightly (d3f8a0b5d 2019-08-04) running on x86_64-unknown-linux-gnu

note: compiler flags: -C codegen-units=1 -C debuginfo=2 --crate-type bin

note: some of the compiler flags provided by cargo are hidden

error: Could not compile `playground`.

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

[hellow554]

Smaller:

#![feature(type_alias_impl_trait)]

pub type Closure = impl FnOnce();

fn main() {
    || -> Closure { || () };
}