Nonsensical "trait bound is not satisfied" error on trait method definition

@jnkel

I tried this code:

use std::ops::Deref;

// Imagine mapping ((T, U), V) to fn(T, U) -> V
trait SignatureToFnPtr {
    type Ptr;
}

trait DerefsToFn<Args>
where
    (Args, Self::Output): SignatureToFnPtr,
    Self: Deref<Target = <(Args, Self::Output) as SignatureToFnPtr>::Ptr>,
{
    type Output;
    fn method(&self);
}

I expected to see this happen: the code compiles correctly.

Instead, this happened:

error[E0277]: the trait bound `(Args, <RustaceansAreAwesome as DerefsToFn<Args>>::Output): SignatureToFnPtr` is not satisfied
  --> src/lib.rs:14:5
   |
14 |     fn method(&self);
   |     ^^^^^^^^^^^^^^^^^ the trait `SignatureToFnPtr` is not implemented for `(Args, <RustaceansAreAwesome as DerefsToFn<Args>>::Output)`

RustaceansAreAwesome look like an implementation detail that shouldn't have leaked.
(Args, Self::Output): SignatureToFnPtr should follow from the where bound on the trait definition.
The error is not shown if fn method(&self) is replaced with fn method(self).

This looks like two different regressions in the two trait solvers. I bisected the old solver regression to #138174, @jnkel bisected the next solver regression to #156976. Tentatively marking as regression-from-stable-to-stable, but it's old enough that this might not be useful (the last version where this worked by default is nightly-2025-03-12)...

Meta

rustc --version --verbose:

rustc 1.100.0-nightly (f7d782a3b 2026-08-19)
binary: rustc
commit-hash: f7d782a3be46d6bb4b9792fe69a61db389ba1769
commit-date: 2026-08-19
host: x86_64-unknown-linux-gnu
release: 1.100.0-nightly
LLVM version: 23.1.0

@rustbot label +A-trait-system +T-compiler +regression-from-stable-to-stable

@rustbot

@rustbot label +A-dyn-trait

Since this looks related to dynamic dispatch (thanks @jnkel)

The RustaceansAreAwesome is a synthetic type parameter which comes from here:

rust/compiler/rustc_trait_selection/src/traits/dyn_compatibility.rs

Line 686 in d287eb7

Ty::new_param(tcx, u32::MAX, rustc_span::sym::RustaceansAreAwesome);

Separately from fixing this bug, it would be useful to rename it to something that better reflects its purpose, for example, DispachCheckPlaceholder.

@purplesyringa

I found this while working on my own project, and was directed here from this zulip conversation:

trait Associator<S: ?Sized> {
    type Point;
}

trait Marker<T> {}

trait Trait<A: Associator<Self> + ?Sized>: Marker<A::Point> {
    fn method(&self);
}

and got the same sort of error:

error[E0277]: the trait bound `A: Associator<RustaceansAreAwesome>` is not satisfied
 --> repro.rs:8:5
  |
8 |     fn method(&self);
  |     ^^^^^^^^^^^^^^^^^ the trait `Associator<RustaceansAreAwesome>` is not implemented for `A`

Have reduced it a bit further than what I had in the Zulip chat: the method no longer references A::Point or even A.

Looks like almost exactly the same situation as @purplesyringa's but with the supertrait replacing the where bound.

A link to a playground with this behaviour:
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024&gist=8c1d408647e11a102db507b5a1a34b1c

Meta

rustc --version --verbose:

rustc 1.98.1 (48a229cea 2026-09-01)
binary: rustc
commit-hash: 48a229ceaefd4985c50990b14116b6d856af0985
commit-date: 2026-09-01
host: x86_64-unknown-linux-gnu
release: 1.98.1
LLVM version: 22.1.8

rustc +nightly --version --verbose:

rustc 1.100.0-nightly (923c95cdf 2026-09-16)
binary: rustc
commit-hash: 923c95cdf5ba65cea505aa2ea829f578e1506ed8
commit-date: 2026-09-16
host: x86_64-unknown-linux-gnu
release: 1.100.0-nightly
LLVM version: 23.1.1

Rollup merge of rust-lang#163200 - dianne:dyn-satisfies-bounds, r=oli…

Unrolled build for #163200 in rollup 163472

Rollup merge of #163200 - dianne:dyn-satisfies-bounds, r=oli-obk

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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Nonsensical "trait bound is not satisfied" error on trait method definition #161621

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Nonsensical "trait bound is not satisfied" error on trait method definition #161621

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purplesyringa commented on Aug 23, 2026

teor2345 commented on Sep 21, 2026

HaineSensei commented on Sep 21, 2026

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