Am Do., 15. Mai 2025 um 13:00 Uhr schrieb Jonathan Wakely <
[email protected]>:
> 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'");
?
- Daniel
