endless loop instead of an overflow error, in the presence of multiple requirements that would overflow.

[moshevds]

I tried this code:

pub trait Foo {
    type Bar<'a>;
    type Baz<'a>;
}
impl Foo for ()
where
    T: for<'a> Foo<
        Bar<'a> = Self::Bar<'a>,
        Baz<'a> = Self::Baz<'a>,
    >
{}

I expected to see this happen:

An "overflow evaluating the requirement" error message should be reported. Probably it should generate:

error[E0275]: overflow evaluating the requirement `for<'a> <() as Foo>::Bar<'a> == _`

or

error[E0275]: overflow evaluating the requirement `for<'a> <() as Foo>::Baz<'a> == _`

Or perhaps another specific overflow error for the specific combined requirement it is evaluating.

Instead, this happened:
The compiler reached an endless loop.

Meta

rustc --version --verbose:

rustc 1.89.0-nightly (ce7e97f73 2025-05-11)
binary: rustc
commit-hash: ce7e97f7371af47e0786f74aa169f6ac9473ff4e
commit-date: 2025-05-11
host: x86_64-unknown-linux-gnu
release: 1.89.0-nightly
LLVM version: 20.1.4

[moxian]

apparent stack trace

rustc_driver-06b6b669ca1390ef.dll!RNvXs_NtNtCs9NfXz6qDjCy_21rustc_trait_selection6traits9normalizeNtB4_19AssocTypeNormalizerINtNtCs1KkNgFnBCUn_13rustc_type_ir4fold10TypeFolderNtNtNtCs2vqUEYxsi4E_12rustc_middle2ty7context6TyCtxtE7fold_ty+0x2d3
rustc_driver-06b6b669ca1390ef.dll!RNvXs_NtNtNtCs9NfXz6qDjCy_21rustc_trait_selection6traits5query9normalizeNtB4_15QueryNormalizerINtNtCs1KkNgFnBCUn_13rustc_type_ir4fold18FallibleTypeFolderNtNtNtCs2vqUEYxsi4E_12rustc_middle2ty7context6TyCtxtE11try_fold_ty+0
rustc_driver-06b6b669ca1390ef.dll!RNvXs_NtNtNtCs9NfXz6qDjCy_21rustc_trait_selection6traits5query9normalizeNtB4_15QueryNormalizerINtNtCs1KkNgFnBCUn_13rustc_type_ir4fold18FallibleTypeFolderNtNtNtCs2vqUEYxsi4E_12rustc_middle2ty7context6TyCtxtE11try_fold_ty+0
rustc_driver-06b6b669ca1390ef.dll!RNvNtNtCs9NfXz6qDjCy_21rustc_trait_selection6traits15outlives_bounds23implied_outlives_bounds+0x7b02
rustc_driver-06b6b669ca1390ef.dll!RNvXs_NtNtCs9NfXz6qDjCy_21rustc_trait_selection6traits9normalizeNtB4_19AssocTypeNormalizerINtNtCs1KkNgFnBCUn_13rustc_type_ir4fold10TypeFolderNtNtNtCs2vqUEYxsi4E_12rustc_middle2ty7context6TyCtxtE7fold_ty+0x2d3
rustc_driver-06b6b669ca1390ef.dll!RNvXs_NtNtNtCs9NfXz6qDjCy_21rustc_trait_selection6traits5query9normalizeNtB4_15QueryNormalizerINtNtCs1KkNgFnBCUn_13rustc_type_ir4fold18FallibleTypeFolderNtNtNtCs2vqUEYxsi4E_12rustc_middle2ty7context6TyCtxtE11try_fold_ty+0
rustc_driver-06b6b669ca1390ef.dll!RNvXs_NtNtNtCs9NfXz6qDjCy_21rustc_trait_selection6traits5query9normalizeNtB4_15QueryNormalizerINtNtCs1KkNgFnBCUn_13rustc_type_ir4fold18FallibleTypeFolderNtNtNtCs2vqUEYxsi4E_12rustc_middle2ty7context6TyCtxtE11try_fold_ty+0
rustc_driver-06b6b669ca1390ef.dll!RNvNtNtCs9NfXz6qDjCy_21rustc_trait_selection6traits15outlives_bounds23implied_outlives_bounds+0x7b02
rustc_driver-06b6b669ca1390ef.dll!RNvXs_NtNtCs9NfXz6qDjCy_21rustc_trait_selection6traits9normalizeNtB4_19AssocTypeNormalizerINtNtCs1KkNgFnBCUn_13rustc_type_ir4fold10TypeFolderNtNtNtCs2vqUEYxsi4E_12rustc_middle2ty7context6TyCtxtE7fold_ty+0x2d3
rustc_driver-06b6b669ca1390ef.dll!RNvXs_NtNtNtCs9NfXz6qDjCy_21rustc_trait_selection6traits5query9normalizeNtB4_15QueryNormalizerINtNtCs1KkNgFnBCUn_13rustc_type_ir4fold18FallibleTypeFolderNtNtNtCs2vqUEYxsi4E_12rustc_middle2ty7context6TyCtxtE11try_fold_ty+0
rustc_driver-06b6b669ca1390ef.dll!RNvXs_NtNtNtCs9NfXz6qDjCy_21rustc_trait_selection6traits5query9normalizeNtB4_15QueryNormalizerINtNtCs1KkNgFnBCUn_13rustc_type_ir4fold18FallibleTypeFolderNtNtNtCs2vqUEYxsi4E_12rustc_middle2ty7context6TyCtxtE11try_fold_ty+0
rustc_driver-06b6b669ca1390ef.dll!RNvNtNtCs9NfXz6qDjCy_21rustc_trait_selection6traits15outlives_bounds23implied_outlives_bounds+0x7b02
rustc_driver-06b6b669ca1390ef.dll!RNvXs_NtNtCs9NfXz6qDjCy_21rustc_trait_selection6traits9normalizeNtB4_19AssocTypeNormalizerINtNtCs1KkNgFnBCUn_13rustc_type_ir4fold10TypeFolderNtNtNtCs2vqUEYxsi4E_12rustc_middle2ty7context6TyCtxtE7fold_ty+0x2d3
rustc_driver-06b6b669ca1390ef.dll!RNvXs_NtNtNtCs9NfXz6qDjCy_21rustc_trait_selection6traits5query9normalizeNtB4_15QueryNormalizerINtNtCs1KkNgFnBCUn_13rustc_type_ir4fold18FallibleTypeFolderNtNtNtCs2vqUEYxsi4E_12rustc_middle2ty7context6TyCtxtE11try_fold_ty+0
rustc_driver-06b6b669ca1390ef.dll!RNvXs_NtNtNtCs9NfXz6qDjCy_21rustc_trait_selection6traits5query9normalizeNtB4_15QueryNormalizerINtNtCs1KkNgFnBCUn_13rustc_type_ir4fold18FallibleTypeFolderNtNtNtCs2vqUEYxsi4E_12rustc_middle2ty7context6TyCtxtE11try_fold_ty+0
rustc_driver-06b6b669ca1390ef.dll!RNvNtNtCs9NfXz6qDjCy_21rustc_trait_selection6traits15outlives_bounds23implied_outlives_bounds+0x7b02
<...repeated manyfold..>

Bisection points to nightly-2023-01-13. PRs in range:

PRs in range:
  - #106660 (Add a regression test for argument copies with DestinationPropagation) by saethlin
  - #106743 (Rollup of 6 pull requests) by matthiaskrgr
    - #106620 (Detect struct literal needing parentheses)
    - #106622 (Detect out of bounds range pattern value)
    - #106703 (Note predicate span on `ImplDerivedObligation`)
    - #106705 (Report fulfillment errors in new trait solver)
    - #106726 (Fix some typos in code comments.)
    - #106734 (Deny having src/test exisiting in tidy)
  - #106537 (Recover from where clauses placed before tuple struct bodies) by fmease
  - #106711 (Use CI LLVM in `test-various` builder) by albertlarsan68
  - #106757 (Rollup of 10 pull requests) by matthiaskrgr
    - #106167 (Fix invalid syntax and incomplete suggestion in impl Trait parameter type suggestions for E0311)
    - #106309 (Prefer non-`[type error]` candidates during selection)
    - #106532 (Allow codegen to unsize `dyn*` to `dyn`)
    - #106596 (Hide more of long types in E0271)
    - #106638 (std tests: use __OsLocalKeyInner from realstd)
    - #106676 (Test that we cannot use trait impl methods arguments as defining uses)
    - #106702 (Conserve cause of `ImplDerivedObligation` in E0599)
    - #106732 (rustc_llvm: replace llvm::makeArrayRef with ArrayRef constructors.)
    - #106733 (Revert "warn newer available version of the x tool")
    - #106748 (Clean up `OnUnimplementedFormatString::verify`)
  - #106760 (Rollup of 8 pull requests) by compiler-errors
    - #103236 (doc: rewrite doc for signed int::{carrying_add,borrowing_sub})
    - #103800 (Stabilize `::{core,std}::pin::pin!`)
    - #106097 (Migrate mir_build diagnostics 2 of 3)
    - #106170 (Move autoderef to `rustc_hir_analysis`)
    - #106323 (Stabilize f16c_target_feature)
    - #106360 (Tweak E0277 `&`-removal suggestions)
    - #106524 (Label `struct/enum constructor` instead of `fn item`, mention that it should be called on type mismatch)
    - #106739 (Remove `<dyn AstConv<'tcx>>::fun(c, ...)` calls in favour of `c.astconv().fun(...)`)
  - #105603 (Harden the pre-tyctxt query system against accidental recomputation) by oli-obk
  - #106773 (Rollup of 6 pull requests) by Nilstrieb
    - #105806 (Support eager subdiagnostics again)
    - #106322 (Handle inference variables in `CollectAllMismatches` correctly)
    - #106579 (Suggest making private tuple struct field public)
    - #106714 (remove unreachable error code `E0490`)
    - #106751 (Fix rendering 'const' in header for intrinsics)
    - #106761 (Add `WaffleLapkin` to compiler reviewers)
  - #106780 (Update Clippy) by flip1995

but that does not look very useful; nothing in particular stands out to me from the list

The hang does not reproduce with -Znext-solver.

@rustbot label: +I-hang +T-types +A-GATs +A-higher-ranked

[compiler-errors]

This is probably due to #106309; this has to do with type errors b/c T is missing in the generics list. Related to that, the error that should be reported is probably not an overflow error, but instead:

error[E0412]: cannot find type `T` in this scope
 --> <source>:7:5
  |
7 |     T: for<'a> Foo<
  |     ^ not found in this scope

[moshevds]

This is probably due to #106309; this has to do with type errors b/c T is missing in the generics list. Related to that, the error that should be reported is probably not an overflow error, but instead:

error[E0412]: cannot find type `T` in this scope
 --> <source>:7:5
  |
7 |     T: for<'a> Foo<
  |     ^ not found in this scope

The E0412 error is in fact already reported by the compiler before it hangs. While simplifying the code for a minimal reproducible example I removed the type parameter, because it had no meaningful effect on the endless loop itself.

pub trait Foo<T> {
    type Bar<'a>;
    type Baz<'a>;
}
impl<T> Foo<T> for ()
where
    T: for<'a> Foo<T,
        Bar<'a> = Self::Bar<'a>,
        Baz<'a> = Self::Baz<'a>,
    >
{
    type Bar<'a> = &'a str;
    type Baz<'a> = &'a str;
}

This example hangs as well, I presume for the same reason.

If you remove one of the GATs from this example, the error will be:

error[E0275]: overflow evaluating the requirement `(): Foo<T>`

Which is - I think - also the expected behavior when both GATs are present.