I'm preparing an update for AFKMud that will clean up the compile for GCC 8 and ran into something really strange that I don't understand with the skills.cpp file. Here's the error:

Building AFKMud....
make -j2 -s afkmud
  Compiling o/skills.o....
In file included from /usr/include/c++/8/map:60,
                 from mud.h:37,
                 from skills.cpp:33:
/usr/include/c++/8/bits/stl_tree.h: In instantiation of ‘static const _Key& std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_S_key(std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_Const_Link_type) [with _Key = std::__cxx11::basic_string<char>; _Val = std::pair<const std::__cxx11::basic_string<char>, int>; _KeyOfValue = std::_Select1st<std::pair<const std::__cxx11::basic_string<char>, int> >; _Compare = string_sort; _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, int> >; std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_Const_Link_type = const std::_Rb_tree_node<std::pair<const std::__cxx11::basic_string<char>, int> >*]’:
/usr/include/c++/8/bits/stl_tree.h:2425:62:   required from ‘std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::iterator std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_emplace_hint_unique(std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::const_iterator, _Args&& ...) [with _Args = {const std::piecewise_construct_t&, std::tuple<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&>, std::tuple<>}; _Key = std::__cxx11::basic_string<char>; _Val = std::pair<const std::__cxx11::basic_string<char>, int>; _KeyOfValue = std::_Select1st<std::pair<const std::__cxx11::basic_string<char>, int> >; _Compare = string_sort; _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, int> >; std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::iterator = std::_Rb_tree_iterator<std::pair<const std::__cxx11::basic_string<char>, int> >; std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::const_iterator = std::_Rb_tree_const_iterator<std::pair<const std::__cxx11::basic_string<char>, int> >]’
/usr/include/c++/8/bits/stl_map.h:499:8:   required from ‘std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](const key_type& [with _Key = std::__cxx11::basic_string<char>; _Tp = int; _Compare = string_sort; _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, int> >; std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type = int; std::map<_Key, _Tp, _Compare, _Alloc>::key_type = std::__cxx11::basic_string<char>]’
skills.cpp:728:26:   required from here
/usr/include/c++/8/bits/stl_tree.h:777:8: error: static assertion failed: comparison object must be invocable as const
        is_invocable_v<const _Compare&, const _Key&, const _Key&>,
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Makefile:191: recipe for target 'o/skills.o' failed
make[1]: *** [o/skills.o] Error 1
Makefile:128: recipe for target 'all' failed
make: *** [all] Error 2

Yeah, a big fat mess, right? The Makefile currently includes the -std=c++1z directive, which it turns out is responsible for this. Backing off to -std=c++17 returns the same error, but -std=c++14 doesn't and the entire codebase will compile.

What I don't get is why this is happening, and why only on the one line. Here's the function:

void update_skill_index( skill_type * skill, int sn )
{
   string buf = strlower( skill->name );

   skill_table__index[buf] = sn; <---- This is line 728, the one throwing that huge error.

   switch ( skill->type )
   {
      default:
         bug( "%s: Invalid skill type %d", __func__, skill->type );
         break;

      case SKILL_SPELL:
         skill_table__spell[buf] = sn;
         break;

      case SKILL_SKILL:
         skill_table__skill[buf] = sn;
         break;

      case SKILL_RACIAL:
         skill_table__racial[buf] = sn;
         break;

      case SKILL_COMBAT:
         skill_table__combat[buf] = sn;
         break;

      case SKILL_TONGUE:
         skill_table__tongue[buf] = sn;
         break;

      case SKILL_LORE:
         skill_table__lore[buf] = sn;
         break;
   }
}

Here's the entire skill_index.h file for reference:

// This probably isn't necessary, but prefer to sort by the contents of the
// string rather than the binary value that std::less uses.
class string_sort
{
 public:
   string_sort(  );
   bool operator(  ) ( const string &, const string & );
};

#if defined(__APPLE__)
//Just deciding to go with default std:less for MacOSX
typedef map < string, int > SKILL_INDEX;
#else
typedef map < string, int, string_sort > SKILL_INDEX;
#endif

extern SKILL_INDEX skill_table__index;
extern SKILL_INDEX skill_table__spell;
extern SKILL_INDEX skill_table__skill;
extern SKILL_INDEX skill_table__racial;
extern SKILL_INDEX skill_table__combat;
extern SKILL_INDEX skill_table__tongue;
extern SKILL_INDEX skill_table__lore;

class find__skill_prefix
{
 public:
   string value;
   char_data *actor;

     find__skill_prefix( char_data *, const string & );
   bool operator(  ) ( pair < string, int >compare );
};

class find__skill_exact
{
 public:
   string value;
   char_data *actor;

     find__skill_exact( char_data *, const string & );
   bool operator(  ) ( pair < string, int >compare );
};

What I don't get is that it complains only about skill_table__index but none of the rest of the index tables in the function.

Note that I didn't write this indexing code so I have no idea what the error is even trying to tell me.

Ok, so, I don't know why this was necessary (cryptic errors suck) but the fix to get it compiling again was simple:

skill_index.h

class string_sort
{
 public:
   string_sort(  );
   bool operator(  ) ( const string &, const string & ) const;
};

skills.cpp

bool string_sort::operator(  ) ( const string & left, const string & right ) const
{
   return ( left.compare( right ) < 0 );
}

Sometimes going down the rabbit hole on Google leads to a good guess at a fix