On Mon, 14 Sept 2026 at 08:10, Tomasz Kamiński <[email protected]> wrote:
>
> From: Jonathan Wakely <[email protected]>
>
> Clang cannot compile format_parse_context::check_dynamic_spec due to a
> longstanding bug[1] with out-of-line function templates called during
> constant evaluation. This patch refactors check_dynamic_spec and related
> functions, so that the definitions are inline in the class body.
>
> To make this work we move the definition of __format::_Arg_t earlier,
> and redefine _Scanner<C>::_Parse_context earlier by making it a nested
> class within basic_format_parse_context<C>, called _Scan_parse_context.
> The __check_dynamic_spec function is changed to take a span<_Arg_t>
> instead of being instantiated with a template parameter pack of arg
> types. The callers that have fixed arg types can pass a span that refers
> to an array of the fixed integer or string enumerators, reducing the
> template instantiations needed for those common cases. The generic form
> (which is rarely needed in user code) uses the __to_arg_t_enum to
> convert arbitrary types to _Arg_t enums, and then passes a span
> referring to that.
>
> As well as simplifying the code a little, this allows checking dynamic
> spec to work with Clang because it no longer needs to instantiate an
> out-of-line function template.
>
> [1]: https://github.com/llvm/llvm-project/issues/73232
>
> libstdc++-v3/ChangeLog:
>
> PR libstdc++/127220
> * include/std/format (__format::_Arg_t): Move definition
> earlier.
> (basic_format_parse_context::check_dynamic_spec): Construct
> array of _Arg_t from template parameter list.
> (basic_format_parse_context::check_dynamic_spec_integral):
> Construct array of _Arg_t based on supported types.
> (basic_format_parse_context::check_dynamic_spec_string):
> Likewise.
> (basic_format_parse_context::__check_dynamic_spec): Replace
> template parameter list with span<const _Arg_t> function
> parameter and simplify accordingly. Define in class body.
> (basic_format_parse_context::_Scan_parse_context): New struct
> the replaces _Scanner::_Parse_context.
> (__format::_Scanner::_M_pc): Change type to _Scan_parse_context.
>
> Reviewed-by: Tomasz Kamiński <[email protected]>
> (cherry picked from commit 3a26070979bcd3f8d2594a9c08b02addd9369cde)
> ---
> I have changed _Arg_t to enum class in r16-760-g67584acb5906eb, and
> this patch moves the defintion from 15 up, instead of effectivelly
> backporting that change.
>
> I have checked that using enum is supported since clang 12 from 2021,
> so it is safe to user here.
And 'if consteval' since Clang 14.
> Tested on x86_64-linux. *format* tested in all standard modes.
> Ok for 15?
OK thanks.
>
> libstdc++-v3/include/std/format | 127 +++++++++++++++++---------------
> 1 file changed, 68 insertions(+), 59 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
> index 3373eadea2d..589be4ec506 100644
> --- a/libstdc++-v3/include/std/format
> +++ b/libstdc++-v3/include/std/format
> @@ -266,6 +266,25 @@ namespace __format
>
> template<typename _CharT> class _Scanner;
>
> + enum _Arg_t : unsigned char {
> + _Arg_none, _Arg_bool, _Arg_c, _Arg_i, _Arg_u, _Arg_ll, _Arg_ull,
> + _Arg_flt, _Arg_dbl, _Arg_ldbl, _Arg_str, _Arg_sv, _Arg_ptr, _Arg_handle,
> + _Arg_i128, _Arg_u128,
> + _Arg_bf16, _Arg_f16, _Arg_f32, _Arg_f64, // These are unused.
> +#ifdef _GLIBCXX_LONG_DOUBLE_ALT128_COMPAT
> + _Arg_next_value_,
> + _Arg_f128 = _Arg_ldbl,
> + _Arg_ibm128 = _Arg_next_value_,
> +#else
> + _Arg_f128,
> +#endif
> + _Arg_max_
> + };
> +
> + template<typename _CharT, typename _Tp>
> + consteval _Arg_t
> + __to_arg_t_enum() noexcept;
> +
> } // namespace __format
> /// @endcond
>
> @@ -326,7 +345,7 @@ namespace __format
> __format::__invalid_arg_id_in_format_string();
> }
>
> -#if __cpp_lib_format >= 202305L
> +#if __cpp_lib_format >= 202305L // >= C++26
> template<typename... _Ts>
> constexpr void
> check_dynamic_spec(size_t __id) noexcept
> @@ -334,17 +353,22 @@ namespace __format
> static_assert(__valid_types_for_check_dynamic_spec<_Ts...>(),
> "template arguments for check_dynamic_spec<Ts...>(id)
> "
> "must be unique and must be one of the allowed
> types");
> - if consteval {
> - __check_dynamic_spec<_Ts...>(__id);
> - }
> + if constexpr (sizeof...(_Ts))
> + if consteval {
> + const __format::_Arg_t __t[] = {
> + __format::__to_arg_t_enum<_CharT, _Ts>()...
> + };
> + __check_dynamic_spec(__id, __t);
> + }
> }
>
> constexpr void
> check_dynamic_spec_integral(size_t __id) noexcept
> {
> if consteval {
> - __check_dynamic_spec<int, unsigned, long long,
> - unsigned long long>(__id);
> + using enum __format::_Arg_t;
> + const __format::_Arg_t __t[] = { _Arg_i, _Arg_u, _Arg_ll, _Arg_ull
> };
> + __check_dynamic_spec(__id, __t);
> }
> }
>
> @@ -352,7 +376,9 @@ namespace __format
> check_dynamic_spec_string(size_t __id) noexcept
> {
> if consteval {
> - __check_dynamic_spec<const _CharT*,
> basic_string_view<_CharT>>(__id);
> + using enum __format::_Arg_t;
> + const __format::_Arg_t __t[] = { _Arg_str, _Arg_sv };
> + __check_dynamic_spec(__id, __t);
> }
> }
>
> @@ -392,14 +418,31 @@ namespace __format
> }
> }
>
> - template<typename... _Ts>
> - consteval void
> - __check_dynamic_spec(size_t __id) noexcept;
> + // Common implementation of check_dynamic_spec{,_string,_integral}
> + consteval void
> + __check_dynamic_spec(size_t __id,
> + span<const __format::_Arg_t> __types) noexcept
> + {
> + if (__id >= _M_num_args)
> + __format::__invalid_arg_id_in_format_string();
> +
> + // This static_cast is safe because all parse contexts created by the
> + // library are _Scan_parse_context (or a type derived from that).
> + // User code can only create basic_format_parse_context objects with
> + // _M_num_args == 0 and those will be rejected by the condition above.
> + if (auto* __args = static_cast<_Scan_parse_context*>(this)->_M_types)
> + {
> + for (auto __t : __types)
> + if (__args[__id] == __t)
> + return;
> +
> + __invalid_dynamic_spec("arg(id) type does not match");
> + }
> + // else this is a formatting scanner, do not do any type checks.
> + }
>
> // This must not be constexpr.
> static void __invalid_dynamic_spec(const char*);
> -
> - friend __format::_Scanner<_CharT>;
> #endif
>
> // This constructor should only be used by the implementation.
> @@ -416,6 +459,18 @@ namespace __format
> _Indexing _M_indexing = _Unknown;
> size_t _M_next_arg_id = 0;
> size_t _M_num_args = 0;
> +
> + // Derived parse context used by Scanner when checking format strings.
> + struct _Scan_parse_context;
> + friend __format::_Scanner<_CharT>;
> + };
> +
> + template<typename _CharT>
> + struct basic_format_parse_context<_CharT>::_Scan_parse_context
> + : basic_format_parse_context<_CharT>
> + {
> + using basic_format_parse_context<_CharT>::basic_format_parse_context;
> + const __format::_Arg_t* _M_types = nullptr;
> };
>
> /// @cond undocumented
> @@ -3564,21 +3619,6 @@ namespace __format
> }
> };
>
> - enum _Arg_t : unsigned char {
> - _Arg_none, _Arg_bool, _Arg_c, _Arg_i, _Arg_u, _Arg_ll, _Arg_ull,
> - _Arg_flt, _Arg_dbl, _Arg_ldbl, _Arg_str, _Arg_sv, _Arg_ptr, _Arg_handle,
> - _Arg_i128, _Arg_u128,
> - _Arg_bf16, _Arg_f16, _Arg_f32, _Arg_f64, // These are unused.
> -#ifdef _GLIBCXX_LONG_DOUBLE_ALT128_COMPAT
> - _Arg_next_value_,
> - _Arg_f128 = _Arg_ldbl,
> - _Arg_ibm128 = _Arg_next_value_,
> -#else
> - _Arg_f128,
> -#endif
> - _Arg_max_
> - };
> -
> template<typename _Context>
> struct _Arg_value
> {
> @@ -4444,11 +4484,7 @@ namespace __format
> {
> using iterator = typename basic_format_parse_context<_CharT>::iterator;
>
> - struct _Parse_context : basic_format_parse_context<_CharT>
> - {
> - using basic_format_parse_context<_CharT>::basic_format_parse_context;
> - const _Arg_t* _M_types = nullptr;
> - } _M_pc;
> + typename basic_format_parse_context<_CharT>::_Scan_parse_context _M_pc;
>
> constexpr explicit
> _Scanner(basic_string_view<_CharT> __str, size_t __nargs = (size_t)-1)
> @@ -4769,33 +4805,6 @@ namespace __format
> } // namespace __format
> /// @endcond
>
> -#if __cpp_lib_format >= 202305L // >= C++26
> - /// @cond undocumented
> - // Common implementation of check_dynamic_spec{,_string,_integral}
> - template<typename _CharT>
> - template<typename... _Ts>
> - consteval void
> - basic_format_parse_context<_CharT>::
> - __check_dynamic_spec(size_t __id) noexcept
> - {
> - if (__id >= _M_num_args)
> - __format::__invalid_arg_id_in_format_string();
> - if constexpr (sizeof...(_Ts) != 0)
> - {
> - using _Parse_ctx = __format::_Scanner<_CharT>::_Parse_context;
> - auto __arg = static_cast<_Parse_ctx*>(this)->_M_types[__id];
> - __format::_Arg_t __types[] = {
> - __format::__to_arg_t_enum<_CharT, _Ts>()...
> - };
> - for (auto __t : __types)
> - if (__arg == __t)
> - return;
> - }
> - __invalid_dynamic_spec("arg(id) type does not match");
> - }
> - /// @endcond
> -#endif
> -
> template<typename _CharT, typename... _Args>
> template<typename _Tp>
> requires convertible_to<const _Tp&, basic_string_view<_CharT>>
> --
> 2.55.0
>
