Re: [PATCH] c++: Define built-in for std::tuple_element [PR100157]

[Marek Polacek <polacek@redhat.com>]

On Thu, Jul 07, 2022 at 06:14:36PM +0100, Jonathan Wakely wrote:
> This adds a new built-in to replace the recursive class template
> instantiations done by traits such as std::tuple_element and
> std::variant_alternative. The purpose is to select the Nth type from a
> list of types, e.g. __builtin_type_pack_element(1, char, int, float) is
> int.
> 
> For a pathological example tuple_element_t<1000, tuple<2000 types...>>
> the compilation time is reduced by more than 90% and the memory  used by
> the compiler is reduced by 97%. In realistic examples the gains will be
> much smaller, but still relevant.
> 
> Clang has a similar built-in, __type_pack_element<N, T...>, but that's a
> "magic template" built-in using <> syntax, which GCC doesn't support. So
> this provides an equivalent feature, but as a built-in function using
> parens instead of <>. I don't really like the name "type pack element"
> (it gives you an element from a pack of types) but the semi-consistency
> with Clang seems like a reasonable argument in favour of keeping the
> name. I'd be open to alternative names though, e.g. __builtin_nth_type
> or __builtin_type_at_index.
> 
> 
> The patch has some problems though ...
> 
> FIXME 1: Marek pointed out that this this ICEs:
> template<class... T> using type = __builtin_type_pack_element(sizeof(T), T...);
> type<int, char> c;
> 
> The sizeof(T) expression is invalid, because T is an unexpanded pack,
> but it's not rejected and instead crashes:

I think this could be fixed by

  if (check_for_bare_parameter_packs (n))
    return error_mark_node;

in finish_type_pack_element.

(I haven't looked at the rest of the patch yet.)

Marek