ICE: Encountered errors `[FulfillmentError(Obligation(predicate=TraitPredicate(<Five as TypeVal<[type error]>>), depth=1),Ambiguity)]` resolving bounds after type-checking

[chengniansun]

Code

fn main() {
    [1; <Multiply<Five, Five>>::VAL];
}
trait TypeVal<T> {
    const VAL: T;
}
struct Five;
struct Multiply<N, M> {
    _n: PhantomData,
}
impl<N, M> TypeVal<usize> for Multiply<N, M> where N: TypeVal<VAL> {}

Meta

rustc --version --verbose:

rustc 1.49.0-nightly (8dae8cdcc 2020-10-12)

binary: rustc

commit-hash: 8dae8cdcc8fa879cea6a4bbbfa5b32e97be4c306

commit-date: 2020-10-12

host: x86_64-unknown-linux-gnu

release: 1.49.0-nightly

LLVM version: 11.0

Error output

error[E0412]: cannot find type `PhantomData` in this scope
 --> reduced_mutant.rs:9:9
  |
9 |     _n: PhantomData,
  |         ^^^^^^^^^^^ not found in this scope
  |
help: consider importing this struct
  |
1 | use std::marker::PhantomData;
  |

error[E0412]: cannot find type `VAL` in this scope
  --> reduced_mutant.rs:11:63
   |
11 | impl<N, M> TypeVal<usize> for Multiply<N, M> where N: TypeVal<VAL> {}
   |          -                                                    ^^^ not found in this scope
   |          |
   |          help: you might be missing a type parameter: `, VAL`

error[E0046]: not all trait items implemented, missing: `VAL`
  --> reduced_mutant.rs:11:1
   |
5  |     const VAL: T;
   |     ------------- `VAL` from trait
...
11 | impl<N, M> TypeVal<usize> for Multiply<N, M> where N: TypeVal<VAL> {}
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `VAL` in implementation

error: internal compiler error: compiler/rustc_trait_selection/src/traits/codegen/mod.rs:121:9: Encountered errors `[FulfillmentError(Obligation(predicate=TraitPredicate(<Five as TypeVal<[type error]>>), depth=1),Ambiguity)]` resolving bounds after type-checking

thread 'rustc' panicked at 'Box<Any>', compiler/rustc_errors/src/lib.rs:945:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

note: 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: rustc 1.49.0-nightly (8dae8cdcc 2020-10-12) running on x86_64-unknown-linux-gnu

error: aborting due to 4 previous errors

Some errors have detailed explanations: E0046, E0412.
For more information about an error, try `rustc --explain E0046`.

Backtrace

thread 'rustc' panicked at 'Box<Any>', compiler/rustc_errors/src/lib.rs:945:9
stack backtrace:
   0: std::panicking::begin_panic
   1: rustc_errors::HandlerInner::bug
   2: rustc_errors::Handler::bug
   3: rustc_middle::util::bug::opt_span_bug_fmt::{{closure}}
   4: rustc_middle::ty::context::tls::with_opt::{{closure}}
   5: rustc_middle::ty::context::tls::with_opt
   6: rustc_middle::util::bug::opt_span_bug_fmt
   7: rustc_middle::util::bug::bug_fmt
   8: rustc_trait_selection::traits::codegen::drain_fulfillment_cx_or_panic
   9: rustc_infer::infer::InferCtxtBuilder::enter
  10: rustc_trait_selection::traits::codegen::codegen_fulfill_obligation
  11: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::codegen_fulfill_obligation>::compute
  12: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  13: rustc_data_structures::stack::ensure_sufficient_stack
  14: rustc_query_system::query::plumbing::get_query_impl
  15: rustc_ty::instance::inner_resolve_instance
  16: rustc_ty::instance::resolve_instance
  17: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::resolve_instance>::compute
  18: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  19: rustc_data_structures::stack::ensure_sufficient_stack
  20: rustc_query_system::query::plumbing::get_query_impl
  21: rustc_middle::ty::instance::Instance::resolve_opt_const_arg
  22: rustc_middle::mir::interpret::queries::<impl rustc_middle::ty::context::TyCtxt>::const_eval_resolve
  23: rustc_middle::ty::consts::Const::eval
  24: <rustc_infer::infer::at::At as rustc_trait_selection::traits::query::normalize::AtExt>::normalize
  25: rustc_infer::infer::InferCtxtBuilder::enter
  26: rustc_traits::normalize_erasing_regions::normalize_generic_arg_after_erasing_regions
  27: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  28: rustc_data_structures::stack::ensure_sufficient_stack
  29: rustc_query_system::query::plumbing::get_query_impl
  30: <rustc_middle::ty::normalize_erasing_regions::NormalizeAfterErasingRegionsFolder as rustc_middle::ty::fold::TypeFolder>::fold_const
  31: rustc_middle::ty::normalize_erasing_regions::<impl rustc_middle::ty::context::TyCtxt>::subst_and_normalize_erasing_regions
  32: rustc_mir::interpret::eval_context::InterpCx<M>::push_stack_frame
  33: rustc_mir::const_eval::eval_queries::eval_to_allocation_raw_provider
  34: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::eval_to_allocation_raw>::compute
  35: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  36: rustc_data_structures::stack::ensure_sufficient_stack
  37: rustc_query_system::query::plumbing::get_query_impl
  38: rustc_mir::const_eval::eval_queries::eval_to_const_value_raw_provider
  39: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::eval_to_const_value_raw>::compute
  40: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  41: rustc_data_structures::stack::ensure_sufficient_stack
  42: rustc_query_system::query::plumbing::get_query_impl
  43: rustc_middle::mir::interpret::queries::<impl rustc_middle::ty::context::TyCtxt>::const_eval_global_id
  44: rustc_middle::mir::interpret::queries::<impl rustc_middle::ty::context::TyCtxt>::const_eval_resolve
  45: rustc_infer::infer::InferCtxt::const_eval_resolve
  46: rustc_trait_selection::traits::const_evaluatable::is_const_evaluatable
  47: rustc_trait_selection::traits::fulfill::FulfillProcessor::progress_changed_obligations
  48: rustc_data_structures::obligation_forest::ObligationForest<O>::process_obligations
  49: <rustc_trait_selection::traits::fulfill::FulfillmentContext as rustc_infer::traits::engine::TraitEngine>::select_where_possible
  50: rustc_typeck::check::fn_ctxt::FnCtxt::select_obligations_where_possible
  51: rustc_infer::infer::InferCtxtBuilder::enter
  52: rustc_typeck::check::typeck
  53: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::typeck>::compute
  54: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  55: rustc_data_structures::stack::ensure_sufficient_stack
  56: rustc_query_system::query::plumbing::get_query_impl
  57: rustc_query_system::query::plumbing::ensure_query_impl
  58: rustc_typeck::check::typeck_item_bodies
  59: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::typeck_item_bodies>::compute
  60: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  61: rustc_data_structures::stack::ensure_sufficient_stack
  62: rustc_query_system::query::plumbing::get_query_impl
  63: rustc_typeck::check_crate
  64: rustc_interface::passes::analysis
  65: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::analysis>::compute
  66: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  67: rustc_data_structures::stack::ensure_sufficient_stack
  68: rustc_query_system::query::plumbing::get_query_impl
  69: rustc_interface::passes::QueryContext::enter
  70: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  71: rustc_span::with_source_map
  72: scoped_tls::ScopedKey<T>::set
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/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.49.0-nightly (8dae8cdcc 2020-10-12) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [codegen_fulfill_obligation] checking if `TypeVal` fulfills its obligations
#1 [resolve_instance] resolving instance `<Multiply<Five, Five> as TypeVal<usize>>::VAL`
#2 [normalize_generic_arg_after_erasing_regions] normalizing `<Multiply<Five, Five> as TypeVal<usize>>::VAL`
#3 [eval_to_allocation_raw] const-evaluating + checking `main::{constant#0}`
#4 [eval_to_const_value_raw] simplifying constant for the type system `main::{constant#0}`
#5 [typeck] type-checking `main`
#6 [typeck_item_bodies] type-checking all item bodies
#7 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 4 previous errors

Some errors have detailed explanations: E0046, E0412.
For more information about an error, try `rustc --explain E0046`.

NOTE: The bug is found by our work-in-progress compiler testing tool Kira, and the test program is reduced/minimized by Perses

[camelid]

Regression occurred in Rust 1.36.0 per testing in Godbolt.

[tesuji]

bisection

Regression in nightly-2019-04-24
Range: 6d59933...e938c2b
found 9 bors merge commits in the specified range
commit[0] 2019-04-22UTC: Auto merge of #60168 - varkor:tidy-leading-newline, r=alexcrichton
commit[1] 2019-04-22UTC: Auto merge of #60126 - estebank:continue-eval, r=oli-obk
commit[2] 2019-04-23UTC: Auto merge of #60140 - euclio:pulldown-cmark, r=GuillaumeGomez
commit[3] 2019-04-23UTC: Auto merge of #60121 - davazp:fix-sync-all-macos, r=KodrAus
commit[4] 2019-04-23UTC: Auto merge of #60172 - varkor:tidy-double-trailing-newline, r=kennytm
commit[5] 2019-04-23UTC: Auto merge of #60125 - estebank:continue-evaluating, r=oli-obk
commit[6] 2019-04-23UTC: Auto merge of #60152 - stepnivlk:visit_subpats-removal, r=varkor
commit[7] 2019-04-23UTC: Auto merge of #60155 - davidtwco:issue-59819, r=oli-obk
commit[8] 2019-04-23UTC: Auto merge of #60211 - Centril:rollup-akw4r85, r=Centril

---

#60126, #60125, #60152 are likely the culprit.

[JohnTitor]

Assigning P-medium as discussed as part of the Prioritization Working Group procedure and removing I-prioritize.