Equivalent types have different behavior with old trait solver #145286
Description
I tried this code:
pub trait Mirror { type Output; } impl<T> Mirror for T { type Output = T; } // Refl<X> = X pub type Refl<X> = <X as Mirror>::Output; pub trait Trait { type Assoc; } fn works<T>() where T: Trait, <Refl<T> as Trait>::Assoc: Sized, { } fn fails<T>() where Refl<T>: Trait, <Refl<T> as Trait>::Assoc: Sized, { }I expected both works and fails to compile. Instead, fails doesn't compile, producing the following error:
error[E0277]: the trait bound `T: Trait` is not satisfied --> src/lib.rs:22:1 | 22 | / fn fails<T>() 23 | | where 24 | | Refl<T>: Trait, 25 | | <Refl<T> as Trait>::Assoc: Sized, | |_____________________________________^ the trait `Trait` is not implemented for `T` | help: consider further restricting type parameter `T` with trait `Trait` | 25 | <Refl<T> as Trait>::Assoc: Sized, T: Trait | ++++++++ error[E0277]: the trait bound `T: Trait` is not satisfied --> src/lib.rs:25:32 | 25 | <Refl<T> as Trait>::Assoc: Sized, | ^^^^^ the trait `Trait` is not implemented for `T` | help: consider further restricting type parameter `T` with trait `Trait` | 25 | <Refl<T> as Trait>::Assoc: Sized, T: Trait | ++++++++ For more information about this error, try `rustc --explain E0277`. Minimized from code written by backslash_phi on the rust community discord
@rustbot labels +A-trait-system +T-types
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Reproducible on the playground with version 1.91.0-nightly (2025-08-10 c8ca44c98eade864824a)