clang-tidy reports uninitialized fields in format_string_checker

[kieranclancy]

clang-tidy reports uninitialized fields in format_string_checker when building with -std=c++20.

Minimal example on https://godbolt.org/z/o79qcj5oo :

#include <fmt/core.h>

std::string f()
{
    return fmt::format("hello world");
}

$ clang-tidy --extra-arg -Ifmt/include --extra-arg -std=c++20 test.cpp
...
fmt/include/fmt/core.h:2610:15: warning: 5 uninitialized fields at the end of the constructor call [clang-analyzer-optin.cplusplus.UninitializedObject]
        types_{mapped_type_constant<Args, buffer_context<Char>>::value...} {}
              ^
/tmp/example/fmt/include/fmt/core.h:725:7: note: uninitialized field 'this->context_.num_args_'
  int num_args_;
      ^~~~~~~~~
/tmp/example/fmt/include/fmt/core.h:726:15: note: uninitialized pointer 'this->context_.types_'
  const type* types_;
              ^~~~~~
/tmp/example/fmt/include/fmt/core.h:399:15: note: uninitialized pointer 'this->context_.basic_format_parse_context::format_str_.data_'
  const Char* data_;
              ^~~~~
/tmp/example/fmt/include/fmt/core.h:400:10: note: uninitialized field 'this->context_.basic_format_parse_context::format_str_.size_'
  size_t size_;
         ^~~~~
/tmp/example/fmt/include/fmt/core.h:655:7: note: uninitialized field 'this->context_.basic_format_parse_context::next_arg_id_'
  int next_arg_id_;
      ^~~~~~~~~~~~
/tmp/example/test.cpp:5:24: note: Calling constructor for 'basic_format_string<char, >'
    return fmt::format("hello world");
                       ^~~~~~~~~~~~~
/tmp/example/fmt/include/fmt/core.h:2733:5: note: Taking true branch
    if constexpr (detail::count_named_args<Args...>() ==
    ^
/tmp/example/fmt/include/fmt/core.h:2737:47: note: Calling constructor for 'format_string_checker<char, >'
      detail::parse_format_string<true>(str_, checker(s));
                                              ^~~~~~~~~~
/tmp/example/fmt/include/fmt/core.h:2610:15: note: 5 uninitialized fields at the end of the constructor call
        types_{mapped_type_constant<Args, buffer_context<Char>>::value...} {}
              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

fmtlib version: latest master at time of writing (661b23e), but as far as I can see this affects all versions after June 2022 (b135f1c).

I am getting this error with clang-tidy 15.0.7 on Linux but the same error also occurs on the clang-tidy in compiler explorer (linked above).

The fix seems to be relatively straightforward, I will submit a pull request.

[vitaut]

These are harmless because there is no uninitialized access but PRs to suppress are welcome provided that they are not very intrusive and don't cause regressions.

[galah92]

This seem to happen again for me with set(CMAKE_CXX_STANDARD 20). With C++17 clang-tidy does not complain.