error: internal compiler error: /checkout/src/librustc_const_eval/eval.rs:773: impossible case reached

[yoshiii]

Following comparison leads to an internal compiler error:
if ('x' as char) < ('y' as char)

I tried this code:

fn main()
{
    let x = 'x' as char;
    let y = 'y' as char;
    
    //if ('x' as char) < ('y' as char)
    if x < y
    {
        print!("x");
    }
    else
    {
        print!("y");
    }
}

I expected to see this happen: code compiles

Instead, this happened:

yoshiii@yoshiii-desktop:~/src/rust/bug$ cargo run
   Compiling bug v0.1.0 (file:///home/yoshiii/src/rust/bug)
error: internal compiler error: /checkout/src/librustc_const_eval/eval.rs:773: impossible case reached

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

thread 'rustc' panicked at 'Box<Any>', /checkout/src/librustc_errors/lib.rs:418
note: Run with `RUST_BACKTRACE=1` for a backtrace.

error: Could not compile `bug`.

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

Meta

rustc --version --verbose:

rustc 1.17.0 (56124baa9 2017-04-24)
binary: rustc
commit-hash: 56124baa9e73f28c0709e59e74783cf234a978cf
commit-date: 2017-04-24
host: x86_64-unknown-linux-gnu
release: 1.17.0
LLVM version: 3.9

Backtrace:

yoshiii@yoshiii-desktop:~/src/rust/bug$ RUST_BACKTRACE=1 cargo run
   Compiling bug v0.1.0 (file:///home/yoshiii/src/rust/bug)
error: internal compiler error: /checkout/src/librustc_const_eval/eval.rs:773: impossible case reached

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: run with `RUST_BACKTRACE=1` for a backtrace

thread 'rustc' panicked at 'Box<Any>', /checkout/src/librustc_errors/lib.rs:418
stack backtrace:
   0: std::sys::imp::backtrace::tracing::imp::unwind_backtrace
             at /checkout/src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::_print
             at /checkout/src/libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at /checkout/src/libstd/sys_common/backtrace.rs:60
             at /checkout/src/libstd/panicking.rs:355
   3: std::panicking::default_hook
             at /checkout/src/libstd/panicking.rs:365
   4: std::panicking::rust_panic_with_hook
             at /checkout/src/libstd/panicking.rs:549
   5: std::panicking::begin_panic
   6: rustc_errors::Handler::bug
   7: rustc::session::opt_span_bug_fmt::{{closure}}
   8: rustc::session::opt_span_bug_fmt
   9: rustc::session::bug_fmt
  10: rustc_const_eval::eval::cast_const_int
  11: rustc_const_eval::eval::eval_const_expr_partial
  12: rustc_const_eval::eval::ConstContext::eval
  13: <rustc_passes::consts::CheckCrateVisitor<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_expr
  14: <rustc_passes::consts::CheckCrateVisitor<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_expr
  15: <rustc_passes::consts::CheckCrateVisitor<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_nested_body
  16: rustc_passes::consts::check_crate
  17: rustc_driver::driver::phase_3_run_analysis_passes::{{closure}}
  18: rustc_driver::driver::phase_3_run_analysis_passes
  19: rustc_driver::driver::compile_input
  20: rustc_driver::run_compiler
  21: std::panicking::try::do_call
  22: __rust_maybe_catch_panic
             at /checkout/src/libpanic_unwind/lib.rs:98
  23: <F as alloc::boxed::FnBox<A>>::call_box
  24: std::sys::imp::thread::Thread::new::thread_start
             at /checkout/src/liballoc/boxed.rs:650
             at /checkout/src/libstd/sys_common/thread.rs:21
             at /checkout/src/libstd/sys/unix/thread.rs:84
  25: start_thread
  26: clone

error: Could not compile `bug`.

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

[Mark-Simulacrum]

This has been fixed since last stable, and works on today's beta, nightly, and master.