[PATCH 2/2] libstdc++: Always treat __float128 as a floating-point type

[Patrick Palka]

On Wed, 9 Jul 2025, Jonathan Wakely wrote:

> Similar to the previous commit that made is_integral_v<__int128>
> unconditionally true, this makes is_floating_point_v<__float128>
> unconditionally true. With the new extended floating-point types in
> C++23 (std::float64_t etc.) it seems unhelpful for is_floating_point_v
> to be true for them, but not for __float128. Especially as it is true on
> some targets, because __float128 is just a typedef for long double.
> 
> This change makes is_floating_point_v<__float128> true whenever the type
> is defined, giving less surprising and more portable behaviour.

LGTM

> 
> libstdc++-v3/ChangeLog:
> 
> 	* include/bits/cpp_type_traits.h (__is_floating<__float128>):
> 	Do not depend on __STRICT_ANSI__.
> 	* include/bits/stl_algobase.h (__size_to_integer(__float128)):
> 	Likewise.
> 	* include/std/type_traits (__is_floating_point_helper<__float128>):
> 	Likewise.
> ---
> 
> Tested x86_64-linux and powerpc64le-linux.
> 
> I don't _think_ this affects how <format> handles __float128, because
> there are std::formatter specializations for each floating-point type,
> rather than a partial specialization that uses std::floating_point.
> 
>  libstdc++-v3/include/bits/cpp_type_traits.h | 9 +++++++++
>  libstdc++-v3/include/bits/stl_algobase.h    | 2 +-
>  libstdc++-v3/include/std/type_traits        | 2 +-
>  3 files changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/libstdc++-v3/include/bits/cpp_type_traits.h b/libstdc++-v3/include/bits/cpp_type_traits.h
> index 770ad94b3b4d..38cea4c67b76 100644
> --- a/libstdc++-v3/include/bits/cpp_type_traits.h
> +++ b/libstdc++-v3/include/bits/cpp_type_traits.h
> @@ -313,6 +313,15 @@ __INT_N(__int128)
>        typedef __true_type __type;
>      };
>  
> +#ifdef _GLIBCXX_USE_FLOAT128
> +  template<>
> +    struct __is_floating<__float128>
> +    {
> +      enum { __value = 1 };
> +      typedef __true_type __type;
> +    };
> +#endif
> +
>  #ifdef __STDCPP_FLOAT16_T__
>    template<>
>      struct __is_floating<_Float16>
> diff --git a/libstdc++-v3/include/bits/stl_algobase.h b/libstdc++-v3/include/bits/stl_algobase.h
> index 71ef2335a311..b104ec2536a0 100644
> --- a/libstdc++-v3/include/bits/stl_algobase.h
> +++ b/libstdc++-v3/include/bits/stl_algobase.h
> @@ -1065,7 +1065,7 @@ _GLIBCXX_END_NAMESPACE_CONTAINER
>    __size_to_integer(double __n) { return (long long)__n; }
>    inline _GLIBCXX_CONSTEXPR long long
>    __size_to_integer(long double __n) { return (long long)__n; }
> -#if !defined(__STRICT_ANSI__) && defined(_GLIBCXX_USE_FLOAT128)
> +#ifdef _GLIBCXX_USE_FLOAT128
>    __extension__ inline _GLIBCXX_CONSTEXPR long long
>    __size_to_integer(__float128 __n) { return (long long)__n; }
>  #endif
> diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
> index e88d04e44d76..78a5ee8c0eb4 100644
> --- a/libstdc++-v3/include/std/type_traits
> +++ b/libstdc++-v3/include/std/type_traits
> @@ -532,7 +532,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>      : public true_type { };
>  #endif
>  
> -#if !defined(__STRICT_ANSI__) && defined(_GLIBCXX_USE_FLOAT128)
> +#ifdef _GLIBCXX_USE_FLOAT128
>    template<>
>      struct __is_floating_point_helper<__float128>
>      : public true_type { };
> -- 
> 2.50.0
> 
>