[PATCH] Add constexpr to iterator adaptors, array and range access

[Jonathan Wakely]

On 16/07/16 14:55 +0200, Daniel Krügler wrote:
>2016-07-16 1:09 GMT+02:00 Jonathan Wakely <jwakely@redhat.com>:
>> This patch implements http://wg21.link/p0031 which adds constexpr to
>> most operations on std::reverse_iterator, std::move_iterator,
>> std::array, as well as std::advance, std::distance, and the
>> range-access functions std::begin, std::rbegin etc.
>>
>> Strictly speaking, those functions should only be constexpr in C++17
>> and not before, but this patch makes them constexpr whenever possible.
>> That means the changes are fully implemented for C++14 (and the
>> feature-test macro is defined) but only partially implemented for
>> C++11, because some of the functions can't be constexpr in C++11.
>>
>> My thinking is that if the committee has decided that these functions
>> *should* be constexpr, and changed them for C++17, then it doesn't
>> serve users to make them non-constexpr in C++11 and C++14 just because
>> the standard says so.
>>
>> How do other people feel about that?
>>
>> The alternative would be to define a new _GLIBCXX17_CONSTEXPR macro
>> and use it in all these places, so they're only constexpr in C++17
>> (and probably for -std=gnu++14 too, but not -std=c++14).
>>
>> How strict do we want to be about obeying the "implementors can't add
>> constexpr" rule in these cases?
>
>As much as I hate the current restrictions, I tend to suggest to
>follow them strictly in __STRICT_ANSI__ mode.

As there were objections to adding constexpr to C++11 and C++14 where
it's not meant to be, I'm only adding it for C++17.

We can relax that for gnu++14 (i.e. !__STRICT_ANSI__) later, but I
haven't done that for now.

>> Here's the patch, but I'm not committing it yet.
>
>Since I made a similar (unintentional) omission recently myself:
>Shouldn't you touch std::__debug::array as well? What about other
>functions from std::__debug?

Yes, I'll fix those shortly.

Committed to trunk.

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