Nightly associated types regression E0284

@rustbot

Code

I tried this code:

pub trait Matrix {
    type Coordinates;
}

pub trait Matrix2D: Matrix<Coordinates = <Self as Matrix2D>::RowIndex> {
    type RowIndex;
}

pub trait Transposable<T: Matrix2D<RowIndex = Self::RowIndex>>: Matrix2D {
    fn transpose(&self) -> T;
}

Should compile, parse cargo check as it currently does on stable.

Currently, it rejects code that works nominally in stable.

error[E0284]: type annotations needed
  --> src/lib.rs:10:5
   |
10 |     fn transpose(&self) -> T;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type
   |
   = note: cannot satisfy `<T as Matrix2D>::RowIndex == _`

For more information about this error, try `rustc --explain E0284`.

Version it worked on

rustc 1.101.0-nightly (c1070d693 2026-09-28)
binary: rustc
commit-hash: c1070d69382b8d2f2eb65119c738a77d9e324c9e
commit-date: 2026-09-28
host: x86_64-unknown-linux-gnu
release: 1.101.0-nightly
LLVM version: 23.1.1

Version with regression

rustc 1.101.0-nightly (5c543b0b8 2026-09-29)
binary: rustc
commit-hash: 5c543b0b8c73c7b72bc8284ced4fb22ead15734d
commit-date: 2026-09-29
host: x86_64-unknown-linux-gnu
release: 1.101.0-nightly
LLVM version: 23.1.1

searched nightlies: from nightly-2026-09-29 to nightly-2026-09-30
regressed nightly: nightly-2026-09-30
searched commit range: c1070d6...5c543b0
regressed commit: b46c90c

bisected with cargo-bisect-rustc v0.6.11

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --start 2026-09-29 --end 2026-09-30 --regress error -- check

Backtrace

RUST_BACKTRACE=1 rustc +nightly-2026-09-30 --crate-type=lib -Ztreat-err-as-bug=1 src/lib.rs

stack backtrace:
   0: __rustc::rust_begin_unwind
   1: core::panicking::panic_fmt
   2: <rustc_errors::DiagCtxtInner>::panic_if_treat_err_as_bug
   3: <rustc_errors::DiagCtxtInner>::emit_diagnostic::{closure#3}
   4: rustc_interface::callbacks::track_diagnostic::<core::option::Option<rustc_span::ErrorGuaranteed>>
   5: <rustc_errors::DiagCtxtInner>::emit_diagnostic
   6: <rustc_errors::DiagCtxtHandle>::emit_diagnostic
   7: <rustc_errors::diagnostic::Diag>::emit_err
   8: <rustc_trait_selection::error_reporting::TypeErrCtxt>::maybe_report_ambiguity
   9: <rustc_trait_selection::error_reporting::TypeErrCtxt>::report_fulfillment_error
  10: <rustc_trait_selection::error_reporting::TypeErrCtxt>::report_fulfillment_errors
  11: rustc_trait_selection::traits::do_normalize_clauses
  12: rustc_trait_selection::traits::normalize_param_env_or_error
  13: rustc_trait_selection::traits::dyn_compatibility::virtual_call_violations_for_method
  14: rustc_trait_selection::traits::dyn_compatibility::dyn_compatibility_violations_for_assoc_item
  [15-16: iterator adapters in dyn_compatibility_violations_for_trait]
  17: rustc_trait_selection::traits::dyn_compatibility::dyn_compatibility_violations
  18: rustc_trait_selection::traits::dyn_compatibility::is_dyn_compatible
  19: <rustc_lint::BuiltinCombinedLateLintModPass as rustc_lint::passes::LateLintPass>::check_item
  20: <rustc_lint::late::LateContextAndPass<rustc_lint::BuiltinCombinedLateLintModPass> as rustc_hir::intravisit::Visitor>::visit_nested_item
  21: rustc_lint::lint_mod
  [22-31: query system and driver]

query stack during panic:
#0 [dyn_compatibility_violations] determining dyn-compatibility of trait `Transposable`
#1 [is_dyn_compatible] checking if trait `Transposable` is dyn-compatible
#2 [lint_mod] linting top-level module
#3 [analysis] running analysis passes on crate `lib`
end of query stack

@rustbot modify labels: +regression-from-stable-to-nightly -regression-untriaged

@rustbot

Oh wow, fresh from the tap.

This one also happens with -Znext-solver=coherence, so it's a regression independent from the switch to next-solver!

@rustbot label S-has-bisection A-inference T-types T-compiler

~~Probably #162655.~~

The return type on transpose(&self) is not necessary for the error.

@dianne

This should be from #163200 cc @dianne

Oh yeah that makes more sense ^^

Assigning P-high (discussion on Zulip).

Rollup merge of rust-lang#163958 - dianne:fix-rustaceansareawesome-ta…

Unrolled build for #163958 in rollup 164005

Rollup merge of #163958 - dianne:fix-rustaceansareawesome-take-2-part…

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Nightly associated types regression E0284 #163550

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Nightly associated types regression E0284 #163550

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maxdexh commented on Sep 30, 2026

maxdexh commented on Sep 30, 2026

lqd commented on Sep 30, 2026

maxdexh commented on Sep 30, 2026

rustbot commented on Oct 7, 2026

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