pub fn unexpected_hidden_region_diagnostic<'a, 'tcx>(
    infcx: &'a InferCtxt<'tcx>,
    generic_param_scope: LocalDefId,
    span: Span,
    hidden_ty: Ty<'tcx>,
    hidden_region: Region<'tcx>,
    opaque_ty_key: OpaqueTypeKey<'tcx>,
) -> Diag<'a>