Function rustc_hir_analysis::check::compare_impl_item::check_type_bounds

pub(super) fn check_type_bounds<'tcx>(
    tcx: TyCtxt<'tcx>,
    trait_ty: AssocItem,
    impl_ty: AssocItem,
    impl_trait_ref: TraitRef<'tcx>,
) -> Result<(), ErrorGuaranteed>

Validate that ProjectionCandidates created for this associated type will be valid.

Usually given

trait X { type Y: Copy } impl X for T { type Y = S; }

We are able to normalize <T as X>::Y to S, and so when we check the impl is well-formed we have to prove S: Copy.

For default associated types the normalization is not possible (the value from the impl could be overridden). We also can’t normalize generic associated types (yet) because they contain bound parameters.