Build problem with upcoming gcc-16

[hhoffstaette]

Originally reported in Gentoo: https://bugs.gentoo.org/964308

Probably since commit https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=876d1a22dfaf873d167bd2ffad190a1d07a81b22 there seems to be something wrong with the comparator function.

test_tree.cc:292:11:   
  292 |                 tr3.sort(tr3.begin(bdy), tr3.end(bdy), comp, true); // false: no sorting of subtrees
      |                 ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/lib/gcc/x86_64-pc-linux-gnu/16/include/g++-v16/bits/stl_tree.h:1402:72: error: static assertion failed: comparison object must be invocable with two arguments of key type
 1402 |             __is_invocable<const _Compare&, const _Key&, const _Key&>::value,
      |                                                                        ^~~~~
  • ‘std::integral_constant<bool, false>::value’ evaluates to false
/usr/lib/gcc/x86_64-pc-linux-gnu/16/include/g++-v16/bits/stl_tree.h:1406:40: error: no match for call to ‘(const tree<std::__cxx11::basic_string<char> >::compare_nodes<std::less<std::__cxx11::basic_string<char> > >) (tree_node_<std::__cxx11::basic_string<char> >* const&, tree_node_<std::__cxx11::basic_string<char> >* const&)’
 1406 |           return _M_impl._M_key_compare(__k1, __k2);
      |                  ~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~
  • there is 1 candidate
    tree.hh:504:23:
      504 |                 class compare_nodes {
          |                       ^~~~~~~~~~~~~
    • candidate 1: ‘bool tree<T, tree_node_allocator>::compare_nodes<StrictWeakOrdering>::operator()(const tree<T, tree_node_allocator>::tree_node*, const tree<T, tree_node_allocator>::tree_node*) [with StrictWeakOrdering = std::less<std::__cxx11::basic_string<char> >; T = std::__cxx11::basic_string<char>; tree_node_allocator = std::allocator<tree_node_<std::__cxx11::basic_string<char> > >; tree<T, tree_node_allocator>::tree_node = tree_node_<std::__cxx11::basic_string<char> >]’ (near match)
      tree.hh:508:38:
        508 |                                 bool operator()(const tree_node *a, const tree_node *b)
            |                                      ^~~~~~~~
      • passing ‘const tree<std::__cxx11::basic_string<char> >::compare_nodes<std::less<std::__cxx11::basic_string<char> > >*’ as ‘this’ argument discards qualifiers