clang -std=c++26 std::format complains non constant expression for a constant expression #239965

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   clangClang issues not falling into any other categoryClang issues not falling into any other category

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   clang:frontendLanguage frontend issues, e.g. anything involving "Sema"Language frontend issues, e.g. anything involving "Sema"

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   @llvm/issue-subscribers-clang-frontend

   Author: GoblinHack (goblinhack)

   Details

   Seems like this should work with std::format

   #include <format>
   
   int main() {
     auto s1 = std::format(std::runtime_format("{:<{}}"), "x", 1); // ok
     auto s2 = std::format("{{}}", "x", 1); // ok
     auto s3 = std::format("{:<{}}", "x", 1); // fails
     return 0;
   }
   
   

   throws:

   <source>:6:25: error: call to consteval function 'std::basic_format_string<char, const char (&)[2], int>::basic_format_string<char[7]>' is not a constant expression
       6 |   auto s3 = std::format("{:<{}}", "x", 1); // fails
         |                         ^
   /opt/compiler-explorer/arm64/gcc-15.2.0/aarch64-unknown-linux-gnu/lib/gcc/aarch64-unknown-linux-gnu/15.2.0/../../../../aarch64-unknown-linux-gnu/include/c++/15.2.0/format:346:4: note: undefined function '__check_dynamic_spec<int, unsigned int, long long, unsigned long long>' cannot be used in a constant expression
     346 |           __check_dynamic_spec<int, unsigned, long long,
         |           ^
   /opt/compiler-explorer/arm64/gcc-15.2.0/aarch64-unknown-linux-gnu/lib/gcc/aarch64-unknown-linux-gnu/15.2.0/../../../../aarch64-unknown-linux-gnu/include/c++/15.2.0/format:685:6: note: in call to '__pc.check_dynamic_spec_integral(1)'
     685 |             __pc.check_dynamic_spec_integral(__val);
         |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   /opt/compiler-explorer/arm64/gcc-15.2.0/aarch64-unknown-linux-gnu/lib/gcc/aarch64-unknown-linux-gnu/15.2.0/../../../../aarch64-unknown-linux-gnu/include/c++/15.2.0/format:701:16: note: in call to '_S_parse_width_or_precision(&"{:<{}}"[4], &"{:<{}}"[6], __spec._M_width, __arg_id, __scanner._Scanner::_M_pc)'
     701 |         auto __next = _S_parse_width_or_precision(__first, __last, _M_width,
         |                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     702 |                                                   __arg_id, __pc);
         |                                                   ~~~~~~~~~~~~~~~
   /opt/compiler-explorer/arm64/gcc-15.2.0/aarch64-unknown-linux-gnu/lib/gcc/aarch64-unknown-linux-gnu/15.2.0/../../../../aarch64-unknown-linux-gnu/include/c++/15.2.0/format:1301:12: note: in call to '__spec._M_parse_width(&"{:<{}}"[3], &"{:<{}}"[6], __scanner._Scanner::_M_pc)'
    1301 |         __first = __spec._M_parse_width(__first, __last, __pc);
         |                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   /opt/compiler-explorer/arm64/gcc-15.2.0/aarch64-unknown-linux-gnu/lib/gcc/aarch64-unknown-linux-gnu/15.2.0/../../../../aarch64-unknown-linux-gnu/include/c++/15.2.0/format:2532:16: note: in call to 'this->_M_f.parse(__scanner._Scanner::_M_pc)'
    2532 |       { return _M_f.parse(__pc); }
         |                ^~~~~~~~~~~~~~~~
   /opt/compiler-explorer/arm64/gcc-15.2.0/aarch64-unknown-linux-gnu/lib/gcc/aarch64-unknown-linux-gnu/15.2.0/../../../../aarch64-unknown-linux-gnu/include/c++/15.2.0/format:4662:31: note: in call to '__f.parse(__scanner._Scanner::_M_pc)'
    4662 |               this->_M_pc.advance_to(__f.parse(this->_M_pc));
         |                                      ^~~~~~~~~~~~~~~~~~~~~~
   /opt/compiler-explorer/arm64/gcc-15.2.0/aarch64-unknown-linux-gnu/lib/gcc/aarch64-unknown-linux-gnu/15.2.0/../../../../aarch64-unknown-linux-gnu/include/c++/15.2.0/format:4648:3: note: in call to 'this->_M_parse_format_spec<char[2], int>(0)'
    4648 |                 _M_parse_format_spec<_Args...>(__id);
         |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   /opt/compiler-explorer/arm64/gcc-15.2.0/aarch64-unknown-linux-gnu/lib/gcc/aarch64-unknown-linux-gnu/15.2.0/../../../../aarch64-unknown-linux-gnu/include/c++/15.2.0/format:4556:2: note: in call to 'this->_M_format_arg(0)'
    4556 |         _M_format_arg(__id);
         |         ^~~~~~~~~~~~~~~~~~~
   /opt/compiler-explorer/arm64/gcc-15.2.0/aarch64-unknown-linux-gnu/lib/gcc/aarch64-unknown-linux-gnu/15.2.0/../../../../aarch64-unknown-linux-gnu/include/c++/15.2.0/format:4504:7: note: in call to 'this->_M_on_replacement_field()'
    4504 |                     _M_on_replacement_field();
         |                     ^~~~~~~~~~~~~~~~~~~~~~~~~
   /opt/compiler-explorer/arm64/gcc-15.2.0/aarch64-unknown-linux-gnu/lib/gcc/aarch64-unknown-linux-gnu/15.2.0/../../../../aarch64-unknown-linux-gnu/include/c++/15.2.0/format:4810:2: note: in call to '__scanner._M_scan()'
    4810 |         __scanner._M_scan();
         |         ^~~~~~~~~~~~~~~~~~~
   <source>:6:25: note: in call to 'basic_format_string<char[7]>("{:<{}}")'
       6 |   auto s3 = std::format("{:<{}}", "x", 1); // fails
         |                         ^~~~~~~~
   /opt/compiler-explorer/arm64/gcc-15.2.0/aarch64-unknown-linux-gnu/lib/gcc/aarch64-unknown-linux-gnu/15.2.0/../../../../aarch64-unknown-linux-gnu/include/c++/15.2.0/format:397:2: note: declared here
     397 |         __check_dynamic_spec(size_t __id) noexcept;
         |         ^
   1 error generated.
   Compiler returned: 1