[PATCH v2] libstdc++: Make dynamic format string checking work with Clang [PR127220]
Tomasz Kaminski
tkaminsk@redhat.com
Mon Sep 14 09:45:23 GMT 2026
On Mon, Sep 14, 2026 at 10:46 AM Jonathan Wakely <jwakely@redhat.com> wrote:
> On Mon, 14 Sept 2026 at 08:10, Tomasz Kamiński <tkaminsk@redhat.com>
> wrote:
> >
> > From: Jonathan Wakely <jwakely@redhat.com>
> >
> > 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 <tkaminsk@redhat.com>
> > (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.
>
if consteval was already used on 15 branch, I was introducing using enum,
at least to this file.
>
>
> > 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
> >
>
>
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