[PATCH] libstdc++: Correct export checks for _GLIBCXX_EXTERN_TEMPLATE

[Tomasz Kaminski]

On Tue, Jun 23, 2026 at 7:59 PM Tomasz Kaminski <tkaminsk@redhat.com> wrote:

>
>
> On Tue, Jun 23, 2026 at 7:52 PM Tomasz Kamiński <tkaminsk@redhat.com>
> wrote:
>
>> Replace a basic_string specific _GLIBCXX_EXTERN_TEMPLATE > 0 with
>> simple check for _GLIBCXX_EXTERN_TEMPLATE. This make sure that
>> extern definition is also use for -O0 (default).
>> Exported declarations are also important for future extensibility,
>> as they always pick latest implementation.
>>
>> libstdc++-v3/ChangeLog:
>>
>>         * include/std/format (__do_vformat_to): Change the preprocessor
>>         check for __do_vformat_to.
>> ---
>> I haven't noticed the issue, because I was testing the build times
>> with -O2.
>>
> For the file compiled with -O0 now I see:
>  U
> _ZNSt8__format15__do_vformat_toIcLj1EEENS_10_Sink_iterIT_EES3_St17basic_string_viewIS2_St11char_traitsIS2_EERSt20basic_format_contextIS3_S2_E
>
>>
>> Testing on x86_64-linux. OK for trunk and 16 when all test passes?
>>
> All tests passed.

>
>>  libstdc++-v3/include/std/format | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/libstdc++-v3/include/std/format
>> b/libstdc++-v3/include/std/format
>> index 4a517b70889..db226ecb0c8 100644
>> --- a/libstdc++-v3/include/std/format
>> +++ b/libstdc++-v3/include/std/format
>> @@ -5381,7 +5381,7 @@ namespace __format
>>  // instantiates formatters for types stored in basic_format_arg, we can
>>  // support only single encoding, in this case unicode. This should cover
>>  // most common use cases.
>> -#if __cplusplus <= 202002L && _GLIBCXX_EXTERN_TEMPLATE > 0
>> +#if __cplusplus <= 202002L && _GLIBCXX_EXTERN_TEMPLATE
>>      extern template _Sink_iter<char>
>>        __do_vformat_to<char, 1>(_Sink_iter<char>, string_view,
>>                                format_context&);
>> --
>> 2.54.0
>>
>>
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