On Sun, 11 Jan 2026 at 12:46, Frederick Virchanza Gotham
<[email protected]> wrote:
>
> Currently the definition of the C++ template "__is_specialization_of"
> is inside the header file "std/format".
>
> I propose two reasons to move this to "bits/utility.h":
>
> (1) To avoid code duplication, as other parts of libstdc++ may need it
> in the future. Currently my implementation of "std::chimeric_ptr"
> requires it.
We don't need to make changes for out-of-tree patches, the change can
live in your branch.
> (2) At some point, "std::is_specialization_of" will be added to
> libstdc++, so again we want to avoid code duplication.
std::is_specialization_of is unlikely to happen until it can handle
non-type template parameters, and our __is_specialization_of won't be
compatible with it.
>
> Note that when it's moved from 'std/format' to 'bits/utility.h', it
> needs to be protected with "if __cplusplus >= 201402L".
>
> Here is the patch:
>
> From 76712db0fd1c7ab8c9067b344e6e3398ae3a053c Mon Sep 17 00:00:00 2001
> From: Thomas PK Healy
> Date: Thu, 8 Jan 2026 10:34:03 +0000
> Subject: [PATCH] Relocate __is_specialization_of to bits/utility.h header
> ---
> libstdc++-v3/include/bits/utility.h | 7 +++++++
> libstdc++-v3/include/std/format | 6 ------
> 2 files changed, 7 insertions(+), 6 deletions(-)
>
> diff --git a/libstdc++-v3/include/bits/utility.h
> b/libstdc++-v3/include/bits/utility.h
> index bd6b18d54dd3f..ec70d495587ea 100644
> --- a/libstdc++-v3/include/bits/utility.h
> +++ b/libstdc++-v3/include/bits/utility.h
> @@ -329,6 +329,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> inline constexpr bool __is_nontype_v<nontype_t<__val>> = true;
> #endif
>
> +#if __cplusplus >= 201402L
> + template<typename _Tp, template<typename...> class _Class>
> + constexpr bool __is_specialization_of = false;
> + template<template<typename...> class _Class, typename... _Args>
> + constexpr bool __is_specialization_of<_Class<_Args...>, _Class> = true;
> +#endif
> +
> _GLIBCXX_END_NAMESPACE_VERSION
> } // namespace
>
> diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
> index 2e4463c65969b..50315c6e9a778 100644
> --- a/libstdc++-v3/include/std/format
> +++ b/libstdc++-v3/include/std/format
> @@ -427,12 +427,6 @@ namespace __format
> size_t _M_num_args = 0;
> };
>
> -/// @cond undocumented
> - template<typename _Tp, template<typename...> class _Class>
> - constexpr bool __is_specialization_of = false;
> - template<template<typename...> class _Class, typename... _Args>
> - constexpr bool __is_specialization_of<_Class<_Args...>, _Class> = true;
> -
> namespace __format
> {
> // pre: first != last
>
