ICE when type parameter and const generic are in wrong order

[is8ac]

error: internal compiler error: src/librustc/traits/codegen/mod.rs:50: Encountered ambiguity selecting `Binder(<u8 as Foo<u16, Scalar(<ZST>): [type error]>>)` during codegen, presuming due to overflow

When a const generic not in curly braces appears before a type parameter, rustc panics.

Code

#![feature(const_generics)]

pub trait Foo<T, const X: usize> {
    fn foo(&self);
}

pub trait Bar<T, const X: usize> {
    fn bar(&self);
}

impl<const X: usize> Bar<u32, X> for u32
where
    u8: Foo<X, u16>,
    //u8: Foo<u32, X>,
    //u8: Foo<{ X }, u16>,
{
    fn bar(&self) {
        0u8.foo();
    }
}

fn main() {}

Replacing u8: Foo<X, u16> with u8: Foo<u32, X> fixes the problem.
Replacing it with u8: Foo<{ X }, u16> produces an informative error message.
Commenting out 0u8.foo(); also prevents the ICE.

This is not urgent, it is merely a matter of uninformative compiler errors.

Full Logs

error: internal compiler error: src/librustc/traits/codegen/mod.rs:50: Encountered ambiguity selecting `Binder(<u8 as Foo<u16, Scalar(<ZST>): [type error]>>)` during codegen, presuming due to overflow

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:883:9
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/git.luolix.top-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/git.luolix.top-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:77
   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:1052
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1428
   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:204
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:224
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:474
  12: std::panicking::begin_panic
  13: rustc_errors::HandlerInner::bug
  14: rustc_errors::Handler::bug
  15: rustc::util::bug::opt_span_bug_fmt::{{closure}}
  16: rustc::ty::context::tls::with_opt::{{closure}}
  17: rustc::ty::context::tls::with_opt
  18: rustc::util::bug::opt_span_bug_fmt
  19: rustc::util::bug::bug_fmt
  20: rustc::ty::context::GlobalCtxt::enter_local
  21: rustc::traits::codegen::codegen_fulfill_obligation
  22: rustc::ty::query::__query_compute::codegen_fulfill_obligation
  23: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::codegen_fulfill_obligation>::compute
  24: rustc::dep_graph::graph::DepGraph::with_task_impl
  25: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  26: rustc::ty::instance::Instance::resolve
  27: rustc_mir_build::lints::check
  28: rustc::ty::context::GlobalCtxt::enter_local
  29: rustc_mir_build::build::mir_built
  30: rustc::ty::query::__query_compute::mir_built
  31: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  32: rustc_mir::transform::check_unsafety::unsafety_check_result
  33: rustc::ty::query::__query_compute::unsafety_check_result
  34: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  35: rustc_mir::transform::mir_const
  36: rustc::ty::query::__query_compute::mir_const
  37: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  38: rustc_mir::transform::mir_validated
  39: rustc::ty::query::__query_compute::mir_validated
  40: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  41: rustc_mir::borrow_check::mir_borrowck
  42: rustc::ty::query::__query_compute::mir_borrowck
  43: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::mir_borrowck>::compute
  44: rustc::dep_graph::graph::DepGraph::with_task_impl
  45: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  46: rustc::ty::<impl rustc::ty::context::TyCtxt>::par_body_owners
  47: rustc_session::utils::<impl rustc_session::session::Session>::time
  48: rustc_interface::passes::analysis
  49: rustc::ty::query::__query_compute::analysis
  50: rustc::dep_graph::graph::DepGraph::with_task_impl
  51: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  52: rustc::ty::context::tls::enter_global
  53: rustc_interface::interface::run_compiler_in_existing_thread_pool
  54: scoped_tls::ScopedKey<T>::set
  55: syntax::attr::with_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

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 1.43.0-nightly (c9290dcee 2020-02-04) 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

query stack during panic:
#0 [codegen_fulfill_obligation] checking if `Foo` fulfills its obligations
#1 [mir_built] processing `<u32 as Bar<u32, X>>::bar`
#2 [unsafety_check_result] processing `<u32 as Bar<u32, X>>::bar`
#3 [mir_const] processing `<u32 as Bar<u32, X>>::bar`
#4 [mir_validated] processing `<u32 as Bar<u32, X>>::bar`
#5 [mir_borrowck] processing `<u32 as Bar<u32, X>>::bar`
#6 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to previous error

error: could not compile `bitnn`.

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

Meta

rustc --version --verbose
rustc 1.43.0-nightly (c9290dcee 2020-02-04)
binary: rustc
commit-hash: c9290dceee2cb6b882b26ec6e294560e51ef0853
commit-date: 2020-02-04
host: x86_64-unknown-linux-gnu
release: 1.43.0-nightly
LLVM version: 9.0

[varkor]

I think this should be fixed by #68434.

[JohnTitor]

Fixed by #68434 as varkor said, closing.