In file included from iostream:41:
In member function ‘std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(double) [with _CharT = char; _Traits = std::char_traits<char>]’,
    inlined from ‘int main()’ at foo.cc:16:32:
/nix/gcc-13.2.0/include/c++/13.2.0/ostream:223:25: warning: ‘r’ may be used uninitialized [-Wmaybe-uninitialized]
  223 |       { return _M_insert(__f); }
      |                ~~~~~~~~~^~~~~
foo.cc: In function ‘int main()’:
foo.cc:7:16: note: ‘r’ was declared here
    7 |         double r;
      |                ^