Repository: rust
Issue: alias bound candidates for opaques allow cyclic reasoning
Labels: A-type-system,T-compiler,I-unsound,A-impl-trait,C-bug,F-type_alias_impl_trait,requires-nightly,T-types

```rust
#![feature(type_alias_impl_trait)]

trait Trait<T> {
    type Assoc: Default;
}

type Ty<T> = impl Trait<T>;
    
impl<T> Trait<T> for T {
    type Assoc = <Ty<T> as Trait<T>>::Assoc;
}

#[allow(unused)]
fn define<T>(x: T) -> Ty<T> { x }

fn main() {
    let _ = <() as Trait<()>>::Assoc::default();
}
```

<details>
<summary>passes cargo check and results in an ICE</summary>

```
error: internal compiler error: compiler/rustc_middle/src/ty/normalize_erasing_regions.rs:195:90: Failed to normalize fn() -> <Ty<()> as Trait<()>>::Assoc {<<Ty<()> as Trait<()>>::Assoc as std::default::Default>::default}, maybe try to call `try_normalize_erasing_regions` instead

thread 'rustc' panicked at 'Box<dyn Any>', /rustc/da7c50c089d5db2d3ebaf227fe075bb1346bfaec/compiler/rustc_errors/src/lib.rs:1644:9
stack backtrace:
   0:     0x7fccbf4e4c2a - std::backtrace_rs::backtrace::libunwind::trace::h2515acc71f4b7930
                               at /rustc/da7c50c089d5db2d3ebaf227fe075bb1346bfaec/library/std/src/../../backtrace/src/backtrace/libunwind.rs:93:5
   1:     0x7fccbf4e4c2a - std::backtrace_rs::backtrace::trace_unsynchronized::he6766fb71a792b8f
                               at /rustc/da7c50c089d5db2d3ebaf227fe075bb1346bfaec/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7fccbf4e4c2a - std::sys_common::backtrace::_print_fmt::h66095118e41b05ee
                               at /rustc/da7c50c089d5db2d3ebaf227fe075bb1346bfaec/library/std/src/sys_common/backtrace.rs:65:5
   3:     0x7fccbf4e4c2a - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h7b02ddcbc50194ff
                               at /rustc/da7c50c089d5db2d3ebaf227fe075bb1346bfaec/library/std/src/sys_common/backtrace.rs:44:22
   4:     0x7fccbf54824e - core::fmt::write::h0189aceee7302cd0
                               at /rustc/da7c50c089d5db2d3ebaf227fe075bb1346bfaec/library/core/src/fmt/mod.rs:1254:17
   5:     0x7fccbf4d79b5 - std::io::Write::write_fmt::h918fabbbac2260c4
                               at /rustc/da7c50c089d5db2d3ebaf227fe075bb1346bfaec/library/std/src/io/mod.rs:1698:15
   6:     0x7fccbf4e49f5 - std::sys_common::backtrace::_print::h41f5e13e972dc45f
                               at /rustc/da7c50c089d5db2d3ebaf227fe075bb1346bfaec/library/std/src/sys_common/backtrace.rs:47:5
   7:     0x7fccbf4e49f5 - std::sys_common::backtrace::print::h1c3c6b0fe82cc982
                               at /rustc/da7c50c089d5db2d3ebaf227fe075bb1346bfaec/library/std/src/sys_common/backtrace.rs:34:9
   8:     0x7fccbf4e776f - std::panicking::default_hook::{{closure}}::hce27f1b648cb1527
                               at /rustc/da7c50c089d5db2d3ebaf227fe075bb1346bfaec/library/std/src/panicking.rs:271:22
   9:     0x7fccbf4e74ab - std::panicking::default_hook::hadee4be0a52a9df6
                               at /rustc/da7c50c089d5db2d3ebaf227fe075bb1346bfaec/library/std/src/panicking.rs:290:9
  10:     0x7fccc273d565 - <rustc_driver_impl[f5bb7a311e11caf0]::DEFAULT_HOOK::{closure#0}::{closure#0} as core[ef02eb5763a621f4]::ops::function::FnOnce<(&core[ef02eb5763a621f4]::panic::panic_info::PanicInfo,)>>::call_once::{shim:vtable#0}
  11:     0x7fccbf4e7fad - <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call::h0c02084a5fe3170c
                               at /rustc/da7c50c089d5db2d3ebaf227fe075bb1346bfaec/library/alloc/src/boxed.rs:2002:9
  12:     0x7fccbf4e7fad - std::panicking::rust_panic_with_hook::h0498acdcd60e6d9e
                               at /rustc/da7c50c089d5db2d3ebaf227fe075bb1346bfaec/library/std/src/panicking.rs:696:13
  13:     0x7fccc2c775b1 - std[d2fc22aa23273b84]::panicking::begin_panic::<rustc_errors[155c2eed68b88154]::ExplicitBug>::{closure#0}
  14:     0x7fccc2c75096 - std[d2fc22aa23273b84]::sys_common::backtrace::__rust_end_short_backtrace::<std[d2fc22aa23273b84]::panicking::begin_panic<rustc_errors[155c2eed68b88154]::ExplicitBug>::{closure#0}, !>
  15:     0x7fccc2c8a476 - std[d2fc22aa23273b84]::panicking::begin_panic::<rustc_errors[155c2eed68b88154]::ExplicitBug>
  16:     0x7fccc2cec4d6 - std[d2fc22aa23273b84]::panic::panic_any::<rustc_errors[155c2eed68b88154]::ExplicitBug>
  17:     0x7fccc2ceaf76 - <rustc_errors[155c2eed68b88154]::HandlerInner>::bug::<&alloc[146302d0966b713d]::string::String>
  18:     0x7fccc2ceac40 - <rustc_errors[155c2eed68b88154]::Handler>::bug::<&alloc[146302d0966b713d]::string::String>
  19:     0x7fccc2ce2fab - rustc_middle[44f4fa3c07a5b96d]::util::bug::opt_span_bug_fmt::<rustc_span[fc3e3761b6371eb]::span_encoding::Span>::{closure#0}
  20:     0x7fccc2ce19fa - rustc_middle[44f4fa3c07a5b96d]::ty::context::tls::with_opt::<rustc_middle[44f4fa3c07a5b96d]::util::bug::opt_span_bug_fmt<rustc_span[fc3e3761b6371eb]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  21:     0x7fccc2ce19c6 - rustc_middle[44f4fa3c07a5b96d]::ty::context::tls::with_context_opt::<rustc_middle[44f4fa3c07a5b96d]::ty::context::tls::with_opt<rustc_middle[44f4fa3c07a5b96d]::util::bug::opt_span_bug_fmt<rustc_span[fc3e3761b6371eb]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  22:     0x7fccc2ce2ef6 - rustc_middle[44f4fa3c07a5b96d]::util::bug::opt_span_bug_fmt::<rustc_span[fc3e3761b6371eb]::span_encoding::Span>
  23:     0x7fccc0dc86d3 - rustc_middle[44f4fa3c07a5b96d]::util::bug::bug_fmt
  24:     0x7fccc08b296a - <rustc_middle[44f4fa3c07a5b96d]::ty::normalize_erasing_regions::NormalizeAfterErasingRegionsFolder as rustc_type_ir[4e16d9d0ea93f861]::fold::TypeFolder<rustc_middle[44f4fa3c07a5b96d]::ty::context::TyCtxt>>::fold_ty
  25:     0x7fccc122e51a - <rustc_const_eval[7020ac6afbf43f29]::interpret::eval_context::InterpCx<rustc_mir_transform[40d85b965ee1f86e]::const_prop::ConstPropMachine>>::eval_mir_constant
  26:     0x7fccc12250f1 - <rustc_mir_transform[40d85b965ee1f86e]::const_prop_lint::ConstPropagator as rustc_middle[44f4fa3c07a5b96d]::mir::visit::Visitor>::visit_basic_block_data
  27:     0x7fccc1220c89 - <rustc_mir_transform[40d85b965ee1f86e]::const_prop_lint::ConstProp as rustc_mir_transform[40d85b965ee1f86e]::pass_manager::MirLint>::run_lint
  28:     0x7fccc11f78b9 - rustc_mir_transform[40d85b965ee1f86e]::run_analysis_to_runtime_passes
  29:     0x7fccc11f67d1 - rustc_mir_transform[40d85b965ee1f86e]::mir_drops_elaborated_and_const_checked
  30:     0x7fccc11f4c50 - rustc_query_system[fba465ff5df52802]::query::plumbing::try_execute_query::<rustc_query_impl[2a1bc161575ee7a7]::queries::mir_drops_elaborated_and_const_checked, rustc_query_impl[2a1bc161575ee7a7]::plumbing::QueryCtxt>
  31:     0x7fccc1aa2ac2 - rustc_mir_transform[40d85b965ee1f86e]::optimized_mir
  32:     0x7fccc1aa0ccf - rustc_query_system[fba465ff5df52802]::query::plumbing::try_execute_query::<rustc_query_impl[2a1bc161575ee7a7]::queries::optimized_mir, rustc_query_impl[2a1bc161575ee7a7]::plumbing::QueryCtxt>
  33:     0x7fccc15e4eba - rustc_monomorphize[882ebeeb6d5d3a1d]::collector::collect_neighbours
  34:     0x7fccc15e13aa - rustc_monomorphize[882ebeeb6d5d3a1d]::collector::collect_items_rec
  35:     0x7fccc1bdca5a - rustc_data_structures[a666149ac6da9659]::sync::par_for_each_in::<alloc[146302d0966b713d]::vec::Vec<rustc_middle[44f4fa3c07a5b96d]::mir::mono::MonoItem>, rustc_monomorphize[882ebeeb6d5d3a1d]::collector::collect_crate_mono_items::{closure#1}::{closure#0}>
  36:     0x7fccc1bdc6f1 - <rustc_session[1ea5019974f9ef49]::session::Session>::time::<(), rustc_monomorphize[882ebeeb6d5d3a1d]::collector::collect_crate_mono_items::{closure#1}>
  37:     0x7fccc1bdc464 - rustc_monomorphize[882ebeeb6d5d3a1d]::collector::collect_crate_mono_items
  38:     0x7fccc1bda7b0 - rustc_monomorphize[882ebeeb6d5d3a1d]::partitioning::collect_and_partition_mono_items
  39:     0x7fccc1932c82 - rustc_query_system[fba465ff5df52802]::query::plumbing::try_execute_query::<rustc_query_impl[2a1bc161575ee7a7]::queries::collect_and_partition_mono_items, rustc_query_impl[2a1bc161575ee7a7]::plumbing::QueryCtxt>
  40:     0x7fccc193293d - <rustc_query_impl[2a1bc161575ee7a7]::Queries as rustc_middle[44f4fa3c07a5b96d]::ty::query::QueryEngine>::collect_and_partition_mono_items
  41:     0x7fccc1c6c013 - rustc_codegen_ssa[9a4b9ef85533b414]::base::codegen_crate::<rustc_codegen_llvm[5a6ec57f0ad6e07b]::LlvmCodegenBackend>
  42:     0x7fccc1c6bdde - <rustc_codegen_llvm[5a6ec57f0ad6e07b]::LlvmCodegenBackend as rustc_codegen_ssa[9a4b9ef85533b414]::traits::backend::CodegenBackend>::codegen_crate
  43:     0x7fccc19567b1 - <rustc_session[1ea5019974f9ef49]::session::Session>::time::<alloc[146302d0966b713d]::boxed::Box<dyn core[ef02eb5763a621f4]::any::Any>, rustc_interface[9d302fd946af7c88]::passes::start_codegen::{closure#0}>
  44:     0x7fccc19562d9 - rustc_interface[9d302fd946af7c88]::passes::start_codegen
  45:     0x7fccc1953018 - <rustc_middle[44f4fa3c07a5b96d]::ty::context::GlobalCtxt>::enter::<<rustc_interface[9d302fd946af7c88]::queries::Queries>::ongoing_codegen::{closure#0}::{closure#0}, core[ef02eb5763a621f4]::result::Result<alloc[146302d0966b713d]::boxed::Box<dyn core[ef02eb5763a621f4]::any::Any>, rustc_span[fc3e3761b6371eb]::ErrorGuaranteed>>
  46:     0x7fccc1951574 - <rustc_interface[9d302fd946af7c88]::queries::Queries>::ongoing_codegen
  47:     0x7fccc1950b41 - <rustc_interface[9d302fd946af7c88]::interface::Compiler>::enter::<rustc_driver_impl[f5bb7a311e11caf0]::run_compiler::{closure#1}::{closure#2}, core[ef02eb5763a621f4]::result::Result<core[ef02eb5763a621f4]::option::Option<rustc_interface[9d302fd946af7c88]::queries::Linker>, rustc_span[fc3e3761b6371eb]::ErrorGuaranteed>>
  48:     0x7fccc194ec10 - rustc_span[fc3e3761b6371eb]::with_source_map::<core[ef02eb5763a621f4]::result::Result<(), rustc_span[fc3e3761b6371eb]::ErrorGuaranteed>, rustc_interface[9d302fd946af7c88]::interface::run_compiler<core[ef02eb5763a621f4]::result::Result<(), rustc_span[fc3e3761b6371eb]::ErrorGuaranteed>, rustc_driver_impl[f5bb7a311e11caf0]::run_compiler::{closure#1}>::{closure#0}::{closure#0}>
  49:     0x7fccc194e1b9 - std[d2fc22aa23273b84]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[9d302fd946af7c88]::util::run_in_thread_pool_with_globals<rustc_interface[9d302fd946af7c88]::interface::run_compiler<core[ef02eb5763a621f4]::result::Result<(), rustc_span[fc3e3761b6371eb]::ErrorGuaranteed>, rustc_driver_impl[f5bb7a311e11caf0]::run_compiler::{closure#1}>::{closure#0}, core[ef02eb5763a621f4]::result::Result<(), rustc_span[fc3e3761b6371eb]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[ef02eb5763a621f4]::result::Result<(), rustc_span[fc3e3761b6371eb]::ErrorGuaranteed>>
  50:     0x7fccc20267ba - <<std[d2fc22aa23273b84]::thread::Builder>::spawn_unchecked_<rustc_interface[9d302fd946af7c88]::util::run_in_thread_pool_with_globals<rustc_interface[9d302fd946af7c88]::interface::run_compiler<core[ef02eb5763a621f4]::result::Result<(), rustc_span[fc3e3761b6371eb]::ErrorGuaranteed>, rustc_driver_impl[f5bb7a311e11caf0]::run_compiler::{closure#1}>::{closure#0}, core[ef02eb5763a621f4]::result::Result<(), rustc_span[fc3e3761b6371eb]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[ef02eb5763a621f4]::result::Result<(), rustc_span[fc3e3761b6371eb]::ErrorGuaranteed>>::{closure#1} as core[ef02eb5763a621f4]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  51:     0x7fccbf4f1e83 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::hc115614f1f0f79d3
                               at /rustc/da7c50c089d5db2d3ebaf227fe075bb1346bfaec/library/alloc/src/boxed.rs:1988:9
  52:     0x7fccbf4f1e83 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h123e851e4d4b5225
                               at /rustc/da7c50c089d5db2d3ebaf227fe075bb1346bfaec/library/alloc/src/boxed.rs:1988:9
  53:     0x7fccbf4f1e83 - std::sys::unix::thread::Thread::new::thread_start::h823b35624605ef69
                               at /rustc/da7c50c089d5db2d3ebaf227fe075bb1346bfaec/library/std/src/sys/unix/thread.rs:108:17
  54:     0x7fccbf3b9609 - start_thread
  55:     0x7fccbf2dc133 - clone
  56:                0x0 - <unknown>

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: rustc 1.70.0-nightly (da7c50c08 2023-03-19) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type bin -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
#0 [mir_drops_elaborated_and_const_checked] elaborating drops for `main`
#1 [optimized_mir] optimizing MIR for `main`
#2 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack
error: could not compile `playground` (bin "playground")
```

</details>

This has the same underlying reason as the unsoundness for [super traits with coinduction](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=94608b51cad595eebeeb4597aff3a4a9) and [alias bound for projections](https://github.com/lcnr/solver-woes/issues/9#issuecomment-1456284249). To my knowledge none of these issues are exploitable on stable right now though.

This relies on the following unsound circular reasoning:
- the impl of `Trait<T> for T` relies on the item bound `Ty<T>: Trait<T>`
- proving the item bounds for the underlying type of `Ty<T>` (`T`) uses the impl