[PATCH] Finish implementing P0426R1 "Constexpr for std::char_traits" for C++17

[Jonathan Wakely]

On 23/04/17 19:33 +0100, Pedro Alves wrote:
>On 04/23/2017 06:54 PM, Pedro Alves wrote:
>> Hi!
>>
>> As I had suggested in PR c++/80265, here's a patch that uses
>> __builtin_constant_p to tell whether we can defer to a constexpr
>> algorithm, which avoids having to wait for compiler support.
>>
>> Unfortunately I ran out of cycles today to run a full bootstrap/regtest
>> cycle, but constexpr_functions_c++17.cc passes cleanly on
>> x86_64 GNU/Linux at least.
>>
>> If this looks like a reasonable approach, I can maybe try running
>> full tests on the gcc compile farm next week.
>>
>> WDYT?
>
>Reading the patch via the list made me spot something
>that was incorrect.
>
>>
>> -      static char_type*
>> +      static _GLIBCXX17_CONSTEXPR char_type*
>>        assign(char_type* __s, size_t __n, char_type __a)
>>        {
>> +#if __cplusplus > 201402
>> +	if (__constant_string_p(__s)
>
>This should probably have been __constant_char_array_p, but now
>that I look again at the paper, I see I got myself confused -- this
>assign overload is not meant to be constexpr in the first place.
>
>So here's an updated patch that drops that bit.  (Same testing
>as before.)

Pedro, this is OK for trunk now we're in stage 1. Please go ahead and
commit it - thanks.

It's probably safe for gcc-7-branch too, but let's leave it on trunk
for a while first.