[gcc r16-9653] libstdc++: Make __valid_types_for_check_dynamic_spec static [PR127219]

[Tomasz Kaminski]

https://gcc.gnu.org/g:b7c4fd69b69f6fdb4851a88be29a352c695ad875

commit r16-9653-gb7c4fd69b69f6fdb4851a88be29a352c695ad875
Author: Steven Sudit <stevensudit@gmail.com>
Date:   Tue Sep 8 21:00:00 2026 -0400

    libstdc++: Make __valid_types_for_check_dynamic_spec static [PR127219]
    
    basic_format_parse_context::check_dynamic_spec<_Ts...> checks its
    template arguments with
    
      static_assert(__valid_types_for_check_dynamic_spec<_Ts...>(), ...);
    
    where __valid_types_for_check_dynamic_spec is a non-static consteval
    member, so the operand is an implicit member access on *this evaluated
    directly by the static_assert, not inside a call to a constexpr member
    function.  P2280R4 allows that use of this (the function never reads
    through it), and GCC and MSVC accept it, but Clang (through 23 and
    trunk, llvm/llvm-project#191104) and EDG 6.9 still reject the
    instantiation:
    
      error: static assertion expression is not an integral constant
        expression
      note: implicit use of 'this' pointer is only allowed within the
        evaluation of a call to a 'constexpr' member function
    
    so any user formatter that calls check_dynamic_spec<Ts...>(id) fails to
    compile with those front ends in C++26 mode.
    
    The function uses no non-static member, so declaring it static removes
    the this and changes nothing else.  std/format/parse_ctx.cc already
    instantiates check_dynamic_spec<Ts...>, which GCC accepts with either
    spelling.
    
    Assisted-by: Claude Fable 5.1 (Anthropic)
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/127219
            * include/std/format
            (basic_format_parse_context::__valid_types_for_check_dynamic_spec):
            Make static, so that the static_assert in check_dynamic_spec does
            not use this.
    
    Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
    Signed-off-by: Steven Sudit <stevensudit@gmail.com>
    (cherry picked from commit bb591391ec98a280bb706c20029bbeada87304d0)

Diff:
---
 libstdc++-v3/include/std/format | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index a90a0b03597d..7742c3e50771 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -371,7 +371,7 @@ namespace __format
 	static constexpr bool __once = (is_same_v<_Tp, _Ts> + ...) == 1;
 
       template<typename... _Ts>
-	consteval bool
+	static consteval bool
 	__valid_types_for_check_dynamic_spec()
 	{
 	  // _GLIBCXX_RESOLVE_LIB_DEFECTS