Function rustc_typeck::collect::item_bounds::opaque_type_bounds

pub(in collect::item_bounds) fn opaque_type_bounds<'tcx>(
    tcx: TyCtxt<'tcx>, 
    opaque_def_id: DefId, 
    bounds: &'tcx [GenericBound<'tcx>], 
    span: Span
) -> &'tcx [(Predicate<'tcx>, Span)]

Opaque types don't inherit bounds from their parent: for return position impl trait it isn't possible to write a suitable predicate on the containing function and for type-alias impl trait we don't have a backwards compatibility issue.