Where-clauses on associated types are not checked for well-formedness

I tried this code:

pub struct NC<T: Copy>(pub T);

pub trait Tr {
    type A where NC<String>: Sized;
}

Expected: E0277 (String: Copy is not satisfied, required by NC). The same clause on a method in the same trait gives exactly that

error[E0277]: the trait bound `String: Copy` is not satisfied
4 |     fn a() where NC<String>: Sized;
  |                              ^^^^^ the trait `Copy` is not implemented for `String`

Actual: compiles, under both solvers, on nightly-2026-10-29 and on stable 1.99.0.

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rustc --version --verbose:

rustc 1.101.0-nightly (92ffebece 2026-10-28)
binary: rustc
commit-hash: 92ffebeceaa0a071066f233873c2438e73d70fe7
commit-date: 2026-10-28
host: x86_64-unknown-linux-gnu
release: 1.101.0-nightly
LLVM version: 23.1.4

Stable comparison: rustc 1.99.0 (b940084d7 2026-09-28).

@rustbot

@rustbot claim

Not the case for assoc fns but it's also the case for assoc consts (under GCI).

Contrary to assoc tys & assoc consts, for assoc fns we do call check_where_clauses in check_fn_or_method in check_associated_item. That's the oversight.

~~I wonder if the assoc ty case is a regression 🤔 a lot of refactorings happened in that area in the last two years moving things from HIR to middle::ty checks...~~ doesn't look like it. Well, I've only checked 1.65 which is the version that stably intro'ed GATs.

@ManasDasri

@ManasDasri, ah, sorry I forgot that you used to be assigned. Ping me in your future PR so I can perf it & issue a crater run :)

Rollup merge of rust-lang#165184 - fmease:gci-wf-assoc-const-wc, r=kh…

Unrolled build for #165184 in rollup 165243

@ManasDasri

@ManasDasri, just checking in. Are you still interested in pursuing this? If not I can just spin up a PR for this

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Where-clauses on associated types are not checked for well-formedness #165177

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Where-clauses on associated types are not checked for well-formedness #165177

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ManasDasri commented on Oct 30, 2026

fmease commented on Oct 30, 2026

fmease commented on Oct 30, 2026

theemathas commented on Oct 30, 2026

fmease commented on Oct 30, 2026

fmease commented on Oct 30, 2026

fmease commented on Oct 30, 2026

fmease commented on Nov 12, 2026

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