On Wed, Oct 8, 2025 at 6:55 PM Jonathan Wakely <[email protected]> wrote:
> We can use diagnostic pragmas to allow the constexpr if version of this
> function to be used for C++14 as well. We still need the __and_ version
> for C++11 because we can't use 'if' in constexpr functions at all before
> C++14.
>
> Also use the integral_constant::value static data member instead of
> invoking integral_constant::operator(), as it's a tiny bit cheaper to
> compiler.
>
> libstdc++-v3/ChangeLog:
>
> * include/bits/hashtable.h (_Hashtable::_S_nothrow_move): Use
> diagnostic pragmas to allow constexpr if in C++14. Use value
> member instead of operator().
> ---
>
> Tested x86_64-linux.
>
LGTM.
>
> libstdc++-v3/include/bits/hashtable.h | 15 +++++++++------
> 1 file changed, 9 insertions(+), 6 deletions(-)
>
> diff --git a/libstdc++-v3/include/bits/hashtable.h
> b/libstdc++-v3/include/bits/hashtable.h
> index da6a1e19a636..06cc51ac4a0f 100644
> --- a/libstdc++-v3/include/bits/hashtable.h
> +++ b/libstdc++-v3/include/bits/hashtable.h
> @@ -477,15 +477,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> static constexpr bool
> _S_nothrow_move()
> {
> -#if __cplusplus <= 201402L
> +#if __cpp_constexpr >= 201304 // >= C++14
> +# pragma GCC diagnostic push
> +# pragma GCC diagnostic ignored "-Wc++17-extensions" // if constexpr
> + if constexpr (_No_realloc)
> + if constexpr (is_nothrow_copy_constructible<_Hash>::value)
> + return is_nothrow_copy_constructible<_Equal>::value;
> + return false;
> +# pragma GCC diagnostic pop
> +#else // In C++11 a constexpr function must be a single statement.
> return __and_<__bool_constant<_No_realloc>,
> is_nothrow_copy_constructible<_Hash>,
> is_nothrow_copy_constructible<_Equal>>::value;
> -#else
> - if constexpr (_No_realloc)
> - if constexpr (is_nothrow_copy_constructible<_Hash>())
> - return is_nothrow_copy_constructible<_Equal>();
> - return false;
> #endif
> }
>
> --
> 2.51.0
>
>
