On Thu, 15 May 2025 at 12:15, Daniel Krügler <daniel.krueg...@gmail.com> wrote:
>
> Am Do., 15. Mai 2025 um 13:00 Uhr schrieb Jonathan Wakely
> <jwak...@redhat.com>:
>>
>> Although Clang trunk has been adjusted to handle our std::format_kind
>> definition (because they need to be able to compile the GCC 15.1.0
>> release), it's probably better to not rely on something that they might
>> start diagnosing again in future.
>>
>> Define the primary template in terms of an immediately invoked function
>> expression, so that we can put a static_assert(false) in the body.
>>
>> The diagnostic that users will see looks like this:
>>
>> include/c++/16.0.0/format:5470:21: error: static assertion failed
>> 5470 | static_assert(false); // Cannot use primary template of
>> std::format_kind
>> | ^~~~~
>>
>> libstdc++-v3/ChangeLog:
>>
>> PR libstdc++/120190
>> * include/std/format (format_kind): Adjust primary template to
>> not depend on itself.
>> * testsuite/std/format/ranges/format_kind_neg.cc: Adjust
>> expected errors. Check more invalid specializations.
>> ---
>>
>> Tested x86_64-linux.
>>
>> I haven't tested it fully with Clang, because the version of Clang in
>> Fedora is older than the change which started rejecting the previous
>> definition of std::format_kind, and the build in Compiler Explorer is
>> newer than the revert of that change.
>>
>> libstdc++-v3/include/std/format | 19 ++++++++++++++-----
>> .../std/format/ranges/format_kind_neg.cc | 15 ++++++++++-----
>> 2 files changed, 24 insertions(+), 10 deletions(-)
>>
>> diff --git a/libstdc++-v3/include/std/format
>> b/libstdc++-v3/include/std/format
>> index bfda5895e0c0..887e891f2096 100644
>> --- a/libstdc++-v3/include/std/format
>> +++ b/libstdc++-v3/include/std/format
>> @@ -5464,13 +5464,22 @@ namespace __format
>> debug_string
>> };
>>
>> - /// @cond undocumented
>> + /** @brief A constant determining how a range should be formatted.
>> + *
>> + * The primary template of `std::format_kind` cannot be instantiated.
>> + * There is a partial specialization for input ranges and you can
>> + * specialize the variable template for your own cv-unqualified types
>> + * that satisfy the `ranges::input_range` concept.
>> + *
>> + * @since C++23
>> + */
>> template<typename _Rg>
>> - constexpr auto format_kind =
>> - __primary_template_not_defined(
>> - format_kind<_Rg> // you can specialize this for non-const input ranges
>> - );
>> + constexpr auto format_kind = []{
>> + static_assert(false); // Cannot use primary template of
>> 'std::format_kind'
>
>
> Shouldn't the comment actually be used as static_assert message
>
> static_assert(false, "Cannot use primary template of 'std::format_kind'");
>
> ?
Ha, yes, obviously that would be better! :-)
