https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87106
--- Comment #8 from Marc Glisse <glisse at gcc dot gnu.org> ---
(In reply to Arthur O'Dwyer from comment #7)
> 2) the conditionally trivial stuff is not very convenient, it seems to
> involve a lot of code duplication. People regularly suggest attributes of the
> form [[trivially_relocatable(condition)]] which might reduce the noise but
> are harder to specify.
>
> Agreed on all counts. Particularly for this attribute, I worry that the
> common use-case would be something really ugly like
>
> template<class T, class A>
> class [[trivially_relocatable(
> is_trivially_relocatable_v<A> &&
> is_trivially_relocatable_v<typename allocator_traits<A>::pointer>
> )]] vector {
> };
>
> which I hope you'd agree would be even more ridiculous than the current
> metaprogramming/duplication.
We already have a number of similar constructs, like noexcept. Still better
than duplicating the whole class IMO.
> And then on top of that, parsing an
> attribute-list would become as hard as parsing all of C++, which I don't
> think we want.
The code is already there in the parser, calling it should not be too hard.
However, we end up with questions like: when is the expression evaluated?
Instantiating some member classes too eagerly could break some code.
