Repository: rust
Issue: Compiler not resolving opaque types with associated const equality in return type
Labels: T-compiler,A-impl-trait,C-bug,F-generic_const_exprs,F-associated_const_equality

I tried this code:

```rust
#![feature(generic_const_exprs)]
#![feature(associated_const_equality)]

use std::marker::PhantomData;

trait Foo {
    const ASSOC: usize;
}

struct Bar<const S: usize> {
    a: PhantomData<[u8; S]>,
}

impl<const S: usize> Foo for Bar<S> {
    const ASSOC: usize = S;
}

fn func<const S: usize>() -> impl Foo<ASSOC = {<Bar<S> as Foo>::ASSOC}> {
    Bar::<S> { a: PhantomData }
}
```

I expected to see this happen: The opaque type to be coerced to the return type as the constants are equal

Instead, this happened: The compiler is saying that the types are not equal. If the associated const in the return type is removed, the code compiles without issue. 

### Meta

`rustc --version --verbose`:
```
rustc 1.76.0-nightly (2c1b65ee1 2023-11-11)
binary: rustc
commit-hash: 2c1b65ee1431f8d3fe2142e821eb13c623bbf8a0
commit-date: 2023-11-11
host: aarch64-apple-darwin
release: 1.76.0-nightly
LLVM version: 17.0.4
```

<!--
Include a backtrace in the code block by setting `RUST_BACKTRACE=1` in your
environment. E.g. `RUST_BACKTRACE=1 cargo build`.
-->
<details><summary>Backtrace</summary>
<p>

```
wutter@Williams-MacBook-Pro src % RUST_BACKTRACE=1 RUSTC_LOG=log rustc +nightly main.rs -Z treat-err-as-bug
warning: some trace filter directives would enable traces that are disabled statically
note: `log=trace` would enable the TRACE level for the `log` target
note: the static max level is `info`
help: to enable DEBUG logging, remove the `max_level_info` feature from the `tracing` crate
warning: the feature `generic_const_exprs` is incomplete and may not be safe to use and/or cause compiler crashes
 --> main.rs:1:12
  |
1 | #![feature(generic_const_exprs)]
  |            ^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #76560 <https://github.com/rust-lang/rust/issues/76560> for more information
  = note: `#[warn(incomplete_features)]` on by default

error[E0308]: mismatched types
  --> main.rs:18:30
   |
18 | fn func<const S: usize>() -> impl Foo<ASSOC = {<Bar<S> as Foo>::ASSOC}> {
   |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `<Bar<S> as Foo>::ASSOC`, found `{<Bar<S> as Foo>::ASSOC}`
   |
   = note: expected constant `<Bar<S> as Foo>::ASSOC`
              found constant `{<Bar<S> as Foo>::ASSOC}`

thread 'rustc' panicked at compiler/rustc_errors/src/lib.rs:1738:30:
aborting due to `-Z treat-err-as-bug=1`
stack backtrace:
   0: _rust_begin_unwind
   1: core::panicking::panic_fmt
   2: <rustc_errors::HandlerInner>::panic_if_treat_err_as_bug
   3: <rustc_errors::HandlerInner>::emit_diagnostic::{closure#2}
   4: rustc_interface::callbacks::track_diagnostic
   5: <rustc_errors::HandlerInner>::emit_diagnostic
   6: <rustc_errors::Handler>::emit_diagnostic
   7: <rustc_span::ErrorGuaranteed as rustc_errors::diagnostic_builder::EmissionGuarantee>::diagnostic_builder_emit_producing_guarantee
   8: <rustc_infer::infer::error_reporting::TypeErrCtxt as rustc_trait_selection::traits::error_reporting::type_err_ctxt_ext::InferCtxtPrivExt>::report_fulfillment_error
   9: <rustc_infer::infer::error_reporting::TypeErrCtxt as rustc_trait_selection::traits::error_reporting::type_err_ctxt_ext::TypeErrCtxtExt>::report_fulfillment_errors
  10: <rustc_hir_typeck::fn_ctxt::FnCtxt>::check_return_expr
  11: rustc_hir_typeck::check::check_fn
  12: rustc_hir_typeck::typeck
      [... omitted 2 frames ...]
  13: rustc_middle::query::plumbing::query_get_at::<rustc_query_system::query::caches::VecCache<rustc_span::def_id::LocalDefId, rustc_middle::query::erase::Erased<[u8; 8]>>>
  14: rustc_hir_analysis::collect::type_of::opaque::find_opaque_ty_constraints_for_rpit
      [... omitted 2 frames ...]
  15: rustc_middle::query::plumbing::query_get_at::<rustc_query_system::query::caches::DefaultCache<rustc_span::def_id::DefId, rustc_middle::query::erase::Erased<[u8; 8]>>>
  16: rustc_hir_analysis::collect::type_of::type_of
      [... omitted 2 frames ...]
  17: rustc_middle::query::plumbing::query_get_at::<rustc_query_system::query::caches::DefaultCache<rustc_span::def_id::DefId, rustc_middle::query::erase::Erased<[u8; 8]>>>
  18: rustc_hir_analysis::check::check::check_item_type
  19: rustc_hir_analysis::check::check::check_mod_item_types
      [... omitted 2 frames ...]
  20: <rustc_middle::hir::map::Map>::for_each_module::<rustc_hir_analysis::check_crate::{closure#5}::{closure#0}>
  21: rustc_hir_analysis::check_crate
  22: rustc_interface::passes::analysis
      [... omitted 2 frames ...]
  23: <rustc_middle::ty::context::GlobalCtxt>::enter::<rustc_driver_impl::run_compiler::{closure#1}::{closure#2}::{closure#6}, core::result::Result<(), rustc_span::ErrorGuaranteed>>
  24: <rustc_interface::interface::Compiler>::enter::<rustc_driver_impl::run_compiler::{closure#1}::{closure#2}, core::result::Result<core::option::Option<rustc_interface::queries::Linker>, rustc_span::ErrorGuaranteed>>
  25: rustc_span::set_source_map::<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_interface::interface::run_compiler<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure#1}>::{closure#0}::{closure#0}>
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

error: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: please attach the file at `/Users/wutter/dev/c_test/src/rustc-ice-2023-11-15T01_14_45-74879.txt` to your bug report

note: compiler flags: -Z treat-err-as-bug

query stack during panic:
#0 [typeck] type-checking `func`
#1 [type_of_opaque] computing type of opaque `func::{opaque#0}`
#2 [type_of] computing type of `func::{opaque#0}`
#3 [check_mod_item_types] checking item types in top-level module
#4 [analysis] running analysis passes on this crate
end of query stack
error: internal compiler error: {OpaqueTypeKey { def_id: DefId(0:19 ~ main[2f94]::func::{opaque#0}), args: [S/#0: usize] }: OpaqueTypeDecl { hidden_type: OpaqueHiddenType { span: main.rs:18:30: 18:72 (#4), ty: ?0t } }}

thread 'rustc' panicked at compiler/rustc_errors/src/lib.rs:1746:25:
aborting after 2 errors due to `-Z treat-err-as-bug=1`
stack backtrace:
   0:        0x103201314 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::hb9ebedcbc23b10d0
   1:        0x1032432d8 - core::fmt::write::he0b2e8e8c929d646
   2:        0x1031f7a48 - std::io::Write::write_fmt::h0960618879d4216c
   3:        0x103201150 - std::sys_common::backtrace::print::h68d22bbf1485ed7a
   4:        0x103203b50 - std::panicking::default_hook::{{closure}}::h323a3c9be995dd8d
   5:        0x103203898 - std::panicking::default_hook::h3178081fe3d6696e
   6:        0x10c61fd50 - <alloc[3eec36f0695f91dd]::boxed::Box<rustc_driver_impl[b18e1ff6c233986d]::install_ice_hook::{closure#0}> as core[a3728727bc16c43f]::ops::function::Fn<(&dyn for<'a, 'b> core[a3728727bc16c43f]::ops::function::Fn<(&'a core[a3728727bc16c43f]::panic::panic_info::PanicInfo<'b>,), Output = ()> + core[a3728727bc16c43f]::marker::Sync + core[a3728727bc16c43f]::marker::Send, &core[a3728727bc16c43f]::panic::panic_info::PanicInfo)>>::call
   7:        0x1032041c8 - std::panicking::rust_panic_with_hook::hb82fa29ca1cfd15b
   8:        0x103203f90 - std::panicking::begin_panic_handler::{{closure}}::hba8a5811f2860467
   9:        0x10320177c - std::sys_common::backtrace::__rust_end_short_backtrace::h8d6f5d4f0729b511
  10:        0x103203d2c - _rust_begin_unwind
  11:        0x10325d128 - core::panicking::panic_fmt::hec9d5dd17af33c2f
  12:        0x10c6581ec - <rustc_errors[40606b26c3d6373]::HandlerInner>::panic_if_treat_err_as_bug
  13:        0x10c657710 - <rustc_errors[40606b26c3d6373]::HandlerInner>::emit_diagnostic::{closure#2}
  14:        0x10cd788ac - rustc_interface[cd87de0d474aaed5]::callbacks::track_diagnostic
  15:        0x10c657174 - <rustc_errors[40606b26c3d6373]::HandlerInner>::emit_diagnostic
  16:        0x10cc2e540 - <rustc_errors[40606b26c3d6373]::HandlerInner>::emit_diag_at_span::<rustc_span[27a5aeb524309f01]::span_encoding::Span>
  17:        0x10cc2e714 - <rustc_errors[40606b26c3d6373]::HandlerInner>::span_bug::<rustc_span[27a5aeb524309f01]::span_encoding::Span, alloc[3eec36f0695f91dd]::string::String>
  18:        0x10cc2d558 - <rustc_errors[40606b26c3d6373]::Handler>::delay_span_bug::<rustc_span[27a5aeb524309f01]::span_encoding::Span, alloc[3eec36f0695f91dd]::string::String>
  19:        0x10ca8cbe8 - core[a3728727bc16c43f]::ptr::drop_in_place::<rustc_hir_typeck[fe10be7b174d6d2f]::inherited::Inherited>
  20:        0x10ca98824 - rustc_hir_typeck[fe10be7b174d6d2f]::typeck
  21:        0x10d825910 - rustc_query_impl[7f29d27b792675dd]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[7f29d27b792675dd]::query_impl::typeck::dynamic_query::{closure#2}::{closure#0}, rustc_middle[b25dbacccc2b5bd2]::query::erase::Erased<[u8; 8usize]>>
  22:        0x10d9226f4 - <rustc_query_impl[7f29d27b792675dd]::query_impl::typeck::dynamic_query::{closure#2} as core[a3728727bc16c43f]::ops::function::FnOnce<(rustc_middle[b25dbacccc2b5bd2]::ty::context::TyCtxt, rustc_span[27a5aeb524309f01]::def_id::LocalDefId)>>::call_once
  23:        0x10d7daf74 - rustc_query_system[3f4e6b31e9532aea]::query::plumbing::try_execute_query::<rustc_query_impl[7f29d27b792675dd]::DynamicConfig<rustc_query_system[3f4e6b31e9532aea]::query::caches::VecCache<rustc_span[27a5aeb524309f01]::def_id::LocalDefId, rustc_middle[b25dbacccc2b5bd2]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[7f29d27b792675dd]::plumbing::QueryCtxt, false>
  24:        0x10d9ea35c - rustc_query_impl[7f29d27b792675dd]::query_impl::typeck::get_query_non_incr::__rust_end_short_backtrace
  25:        0x10c910594 - rustc_middle[b25dbacccc2b5bd2]::query::plumbing::query_get_at::<rustc_query_system[3f4e6b31e9532aea]::query::caches::VecCache<rustc_span[27a5aeb524309f01]::def_id::LocalDefId, rustc_middle[b25dbacccc2b5bd2]::query::erase::Erased<[u8; 8usize]>>>
  26:        0x10c9231f8 - rustc_hir_analysis[2e3a0b5a7c9d3afd]::collect::type_of::opaque::find_opaque_ty_constraints_for_rpit
  27:        0x10d820ed0 - rustc_query_impl[7f29d27b792675dd]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[7f29d27b792675dd]::query_impl::type_of_opaque::dynamic_query::{closure#2}::{closure#0}, rustc_middle[b25dbacccc2b5bd2]::query::erase::Erased<[u8; 8usize]>>
  28:        0x10d9f8fac - <rustc_query_impl[7f29d27b792675dd]::query_impl::type_of_opaque::dynamic_query::{closure#2} as core[a3728727bc16c43f]::ops::function::FnOnce<(rustc_middle[b25dbacccc2b5bd2]::ty::context::TyCtxt, rustc_span[27a5aeb524309f01]::def_id::DefId)>>::call_once
  29:        0x10d78ed54 - rustc_query_system[3f4e6b31e9532aea]::query::plumbing::try_execute_query::<rustc_query_impl[7f29d27b792675dd]::DynamicConfig<rustc_query_system[3f4e6b31e9532aea]::query::caches::DefaultCache<rustc_span[27a5aeb524309f01]::def_id::DefId, rustc_middle[b25dbacccc2b5bd2]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[7f29d27b792675dd]::plumbing::QueryCtxt, false>
  30:        0x10d923060 - rustc_query_impl[7f29d27b792675dd]::query_impl::type_of_opaque::get_query_non_incr::__rust_end_short_backtrace
  31:        0x10c8abd80 - rustc_middle[b25dbacccc2b5bd2]::query::plumbing::query_get_at::<rustc_query_system[3f4e6b31e9532aea]::query::caches::DefaultCache<rustc_span[27a5aeb524309f01]::def_id::DefId, rustc_middle[b25dbacccc2b5bd2]::query::erase::Erased<[u8; 8usize]>>>
  32:        0x10c8c4e70 - rustc_hir_analysis[2e3a0b5a7c9d3afd]::collect::type_of::type_of
  33:        0x10d825984 - rustc_query_impl[7f29d27b792675dd]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[7f29d27b792675dd]::query_impl::type_of::dynamic_query::{closure#2}::{closure#0}, rustc_middle[b25dbacccc2b5bd2]::query::erase::Erased<[u8; 8usize]>>
  34:        0x10d9ff924 - <rustc_query_impl[7f29d27b792675dd]::query_impl::type_of::dynamic_query::{closure#2} as core[a3728727bc16c43f]::ops::function::FnOnce<(rustc_middle[b25dbacccc2b5bd2]::ty::context::TyCtxt, rustc_span[27a5aeb524309f01]::def_id::DefId)>>::call_once
  35:        0x10d78ed54 - rustc_query_system[3f4e6b31e9532aea]::query::plumbing::try_execute_query::<rustc_query_impl[7f29d27b792675dd]::DynamicConfig<rustc_query_system[3f4e6b31e9532aea]::query::caches::DefaultCache<rustc_span[27a5aeb524309f01]::def_id::DefId, rustc_middle[b25dbacccc2b5bd2]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[7f29d27b792675dd]::plumbing::QueryCtxt, false>
  36:        0x10da00930 - rustc_query_impl[7f29d27b792675dd]::query_impl::type_of::get_query_non_incr::__rust_end_short_backtrace
  37:        0x10c8abd80 - rustc_middle[b25dbacccc2b5bd2]::query::plumbing::query_get_at::<rustc_query_system[3f4e6b31e9532aea]::query::caches::DefaultCache<rustc_span[27a5aeb524309f01]::def_id::DefId, rustc_middle[b25dbacccc2b5bd2]::query::erase::Erased<[u8; 8usize]>>>
  38:        0x10c8bb5b4 - rustc_hir_analysis[2e3a0b5a7c9d3afd]::check::check::check_item_type
  39:        0x10c8c1c5c - rustc_hir_analysis[2e3a0b5a7c9d3afd]::check::check::check_mod_item_types
  40:        0x10d8230a0 - rustc_query_impl[7f29d27b792675dd]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[7f29d27b792675dd]::query_impl::check_mod_item_types::dynamic_query::{closure#2}::{closure#0}, rustc_middle[b25dbacccc2b5bd2]::query::erase::Erased<[u8; 0usize]>>
  41:        0x10d843538 - <rustc_query_impl[7f29d27b792675dd]::query_impl::check_mod_item_types::dynamic_query::{closure#2} as core[a3728727bc16c43f]::ops::function::FnOnce<(rustc_middle[b25dbacccc2b5bd2]::ty::context::TyCtxt, rustc_span[27a5aeb524309f01]::def_id::LocalModDefId)>>::call_once
  42:        0x10d775aa4 - rustc_query_system[3f4e6b31e9532aea]::query::plumbing::try_execute_query::<rustc_query_impl[7f29d27b792675dd]::DynamicConfig<rustc_query_system[3f4e6b31e9532aea]::query::caches::DefaultCache<rustc_span[27a5aeb524309f01]::def_id::LocalModDefId, rustc_middle[b25dbacccc2b5bd2]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[7f29d27b792675dd]::plumbing::QueryCtxt, false>
  43:        0x10da0444c - rustc_query_impl[7f29d27b792675dd]::query_impl::check_mod_item_types::get_query_non_incr::__rust_end_short_backtrace
  44:        0x10c959404 - <rustc_middle[b25dbacccc2b5bd2]::hir::map::Map>::for_each_module::<rustc_hir_analysis[2e3a0b5a7c9d3afd]::check_crate::{closure#5}::{closure#0}>
  45:        0x10c8e31d4 - rustc_hir_analysis[2e3a0b5a7c9d3afd]::check_crate
  46:        0x10cd95d78 - rustc_interface[cd87de0d474aaed5]::passes::analysis
  47:        0x10d8259ac - rustc_query_impl[7f29d27b792675dd]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[7f29d27b792675dd]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[b25dbacccc2b5bd2]::query::erase::Erased<[u8; 1usize]>>
  48:        0x10d8eab98 - <rustc_query_impl[7f29d27b792675dd]::query_impl::analysis::dynamic_query::{closure#2} as core[a3728727bc16c43f]::ops::function::FnOnce<(rustc_middle[b25dbacccc2b5bd2]::ty::context::TyCtxt, ())>>::call_once
  49:        0x10d73e9f8 - rustc_query_system[3f4e6b31e9532aea]::query::plumbing::try_execute_query::<rustc_query_impl[7f29d27b792675dd]::DynamicConfig<rustc_query_system[3f4e6b31e9532aea]::query::caches::SingleCache<rustc_middle[b25dbacccc2b5bd2]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[7f29d27b792675dd]::plumbing::QueryCtxt, false>
  50:        0x10d8eb660 - rustc_query_impl[7f29d27b792675dd]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  51:        0x10c5d5dec - <rustc_middle[b25dbacccc2b5bd2]::ty::context::GlobalCtxt>::enter::<rustc_driver_impl[b18e1ff6c233986d]::run_compiler::{closure#1}::{closure#2}::{closure#6}, core[a3728727bc16c43f]::result::Result<(), rustc_span[27a5aeb524309f01]::ErrorGuaranteed>>
  52:        0x10c5941b0 - <rustc_interface[cd87de0d474aaed5]::interface::Compiler>::enter::<rustc_driver_impl[b18e1ff6c233986d]::run_compiler::{closure#1}::{closure#2}, core[a3728727bc16c43f]::result::Result<core[a3728727bc16c43f]::option::Option<rustc_interface[cd87de0d474aaed5]::queries::Linker>, rustc_span[27a5aeb524309f01]::ErrorGuaranteed>>
  53:        0x10c5d41b0 - rustc_span[27a5aeb524309f01]::set_source_map::<core[a3728727bc16c43f]::result::Result<(), rustc_span[27a5aeb524309f01]::ErrorGuaranteed>, rustc_interface[cd87de0d474aaed5]::interface::run_compiler<core[a3728727bc16c43f]::result::Result<(), rustc_span[27a5aeb524309f01]::ErrorGuaranteed>, rustc_driver_impl[b18e1ff6c233986d]::run_compiler::{closure#1}>::{closure#0}::{closure#0}>
  54:        0x10c5dc92c - std[f3dc86d6d90ad197]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[cd87de0d474aaed5]::util::run_in_thread_with_globals<rustc_interface[cd87de0d474aaed5]::util::run_in_thread_pool_with_globals<rustc_interface[cd87de0d474aaed5]::interface::run_compiler<core[a3728727bc16c43f]::result::Result<(), rustc_span[27a5aeb524309f01]::ErrorGuaranteed>, rustc_driver_impl[b18e1ff6c233986d]::run_compiler::{closure#1}>::{closure#0}, core[a3728727bc16c43f]::result::Result<(), rustc_span[27a5aeb524309f01]::ErrorGuaranteed>>::{closure#0}, core[a3728727bc16c43f]::result::Result<(), rustc_span[27a5aeb524309f01]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[a3728727bc16c43f]::result::Result<(), rustc_span[27a5aeb524309f01]::ErrorGuaranteed>>
  55:        0x10c60eae8 - <<std[f3dc86d6d90ad197]::thread::Builder>::spawn_unchecked_<rustc_interface[cd87de0d474aaed5]::util::run_in_thread_with_globals<rustc_interface[cd87de0d474aaed5]::util::run_in_thread_pool_with_globals<rustc_interface[cd87de0d474aaed5]::interface::run_compiler<core[a3728727bc16c43f]::result::Result<(), rustc_span[27a5aeb524309f01]::ErrorGuaranteed>, rustc_driver_impl[b18e1ff6c233986d]::run_compiler::{closure#1}>::{closure#0}, core[a3728727bc16c43f]::result::Result<(), rustc_span[27a5aeb524309f01]::ErrorGuaranteed>>::{closure#0}, core[a3728727bc16c43f]::result::Result<(), rustc_span[27a5aeb524309f01]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[a3728727bc16c43f]::result::Result<(), rustc_span[27a5aeb524309f01]::ErrorGuaranteed>>::{closure#1} as core[a3728727bc16c43f]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  56:        0x10320c3c4 - std::sys::unix::thread::Thread::new::thread_start::hbf1197f3e522c03d
  57:        0x1b468206c - __pthread_deallocate

error: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: please attach the file at `/Users/wutter/dev/c_test/src/rustc-ice-2023-11-15T01_14_45-74879.txt` to your bug report

note: compiler flags: -Z treat-err-as-bug

query stack during panic:
#0 [typeck] type-checking `func`
#1 [type_of_opaque] computing type of opaque `func::{opaque#0}`
#2 [type_of] computing type of `func::{opaque#0}`
#3 [check_mod_item_types] checking item types in top-level module
#4 [analysis] running analysis passes on this crate
end of query stack
thread 'rustc' panicked at library/core/src/panicking.rs:144:5:
panic in a destructor during cleanup
```

</p>
</details>