RFR: JDK-8299386: Refactor metaprogramming to use <type_traits>

[Kim Barrett]

On Thu, 29 Dec 2022 06:48:44 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> I started out intending to suggest replacing some of these changes by instead
>> updating the uses of a couple that are rarely used, and getting rid of the
>> associated pre-C++11 synonyms entirely. But it turns out most are rarely used,
>> leading me to question this change almost in its entirety.
>> 
>> I think better would be a series of changes to replace uses of the synonyms
>> with the corresponding element from <type_traits> and delete the synonyms.  I
>> say "a series of changes" because I would prefer it not be done all in one
>> changeset, for easier reviewing.
>> 
>> I suggest making an exception for EnableIf, because of the number of uses and
>> because I'm hoping we'll be able to revise them in a better way in the not too
>> distant future. Many of the uses are for function SFINAE, where the syntax is
>> pretty horrible. We have the newer form of function SFINAE available (see
>> ENABLE_IF), but I think in many/most cases even better would be to use C++17
>> constexpr if. That's why I haven't proposed rewriting any of them using
>> ENABLE_IF. I'm hopeful that using C++17 isn't too far away, now that there
>> seems to be a compiler available for the aix-ppc port.
>
>> @kimbarrett Though once its just those three files, it begs the question of whether having `metaprogramming/` itself is necessary. Maybe it makes more sense to have a single header under `utilities/`?
> 
> I'm hopeful that we'll soon have C++17, which will similarly remove the need for logical.hpp.  There aren't many uses of those metafunctions either.  So I think there's no point in moving it.
> 
> I've already described what I think should be done with EnableIf, so again no point in moving it.
> 
> PrimitiveConversions::cast<T> => std::bit_cast<T>, but that's in C++20, so likely still some way out for us.
> And the PrimitiveConversions::Translate is our own thing.
> So it might make sense to move that someday, but I don't see any need to hurry.

> @kimbarrett I created 6 separate issues and associated PRs to deal with parts of them in isolation, so far.
> 
> JDK-8299399, JDK-8299396, JDK-8299395, JDK-8299398, JDK-8299402, and JDK-8299397.
> 
> Once those are in I'll re-visit this and see how we can chop the remaining work and remove most of what remains of `metaprogramming/`, minus `enableIf.hpp`, `logic.hpp`, and `primitiveConversions.hpp`.

Thanks.  Those PRs were very easy to review.

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PR: https://git.openjdk.org/jdk/pull/11794