In file included from /usr/include/c++/9/bits/stl_algobase.h:71,
                 from /usr/include/c++/9/bits/char_traits.h:39,
                 from /usr/include/c++/9/ios:40,
                 from /usr/include/c++/9/istream:38,
                 from /usr/include/c++/9/sstream:38,
                 from /usr/include/c++/9/complex:45,
                 from /usr/include/c++/9/ccomplex:39,
                 from /usr/include/x86_64-linux-gnu/c++/9/bits/stdc++.h:54,
                 from foo.cc:1:
/usr/include/c++/9/bits/predefined_ops.h: In instantiation of 'bool __gnu_cxx::__ops::_Iter_comp_iter<_Compare>::operator()(_Iterator1, _Iterator2) [with _Iterator1 = int*; _Iterator2 = int*; _Compare = int*]':
/usr/include/c++/9/bits/stl_algo.h:1852:14:   required from 'void std::__insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = int*; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<int*>]'
/usr/include/c++/9/bits/stl_algo.h:1890:25:   required from 'void std::__final_insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = int*; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<int*>]'
/usr/include/c++/9/bits/stl_algo.h:1976:31:   required from 'void std::__sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = int*; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<int*>]'
/usr/include/c++/9/bits/stl_algo.h:4905:18:   required from 'void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = int*; _Compare = int*]'
foo.cc:24:18:   required from here
/usr/include/c++/9/bits/predefined_ops.h:143:18: error: expression cannot be used as a function
  143 |         { return bool(_M_comp(*__it1, *__it2)); }
      |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/9/bits/predefined_ops.h: In instantiation of 'bool __gnu_cxx::__ops::_Val_comp_iter<_Compare>::operator()(_Value&, _Iterator) [with _Value = int; _Iterator = int*; _Compare = int*]':
/usr/include/c++/9/bits/stl_algo.h:1833:20:   required from 'void std::__unguarded_linear_insert(_RandomAccessIterator, _Compare) [with _RandomAccessIterator = int*; _Compare = __gnu_cxx::__ops::_Val_comp_iter<int*>]'
/usr/include/c++/9/bits/stl_algo.h:1860:36:   required from 'void std::__insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = int*; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<int*>]'
/usr/include/c++/9/bits/stl_algo.h:1890:25:   required from 'void std::__final_insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = int*; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<int*>]'
/usr/include/c++/9/bits/stl_algo.h:1976:31:   required from 'void std::__sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = int*; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<int*>]'
/usr/include/c++/9/bits/stl_algo.h:4905:18:   required from 'void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = int*; _Compare = int*]'
foo.cc:24:18:   required from here
/usr/include/c++/9/bits/predefined_ops.h:215:11: error: expression cannot be used as a function
  215 |  { return bool(_M_comp(__val, *__it)); }
      |           ^~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/9/bits/predefined_ops.h: In instantiation of 'bool __gnu_cxx::__ops::_Iter_comp_val<_Compare>::operator()(_Iterator, _Value&) [with _Iterator = int*; _Value = int; _Compare = int*]':
/usr/include/c++/9/bits/stl_heap.h:133:48:   required from 'void std::__push_heap(_RandomAccessIterator, _Distance, _Distance, _Tp, _Compare&) [with _RandomAccessIterator = int*; _Distance = long int; _Tp = int; _Compare = __gnu_cxx::__ops::_Iter_comp_val<int*>]'
/usr/include/c++/9/bits/stl_heap.h:237:23:   required from 'void std::__adjust_heap(_RandomAccessIterator, _Distance, _Distance, _Tp, _Compare) [with _RandomAccessIterator = int*; _Distance = long int; _Tp = int; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<int*>]'
/usr/include/c++/9/bits/stl_heap.h:342:22:   required from 'void std::__make_heap(_RandomAccessIterator, _RandomAccessIterator, _Compare&) [with _RandomAccessIterator = int*; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<int*>]'
/usr/include/c++/9/bits/stl_algo.h:1677:23:   required from 'void std::__heap_select(_RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = int*; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<int*>]'
/usr/include/c++/9/bits/stl_algo.h:1938:25:   required from 'void std::__partial_sort(_RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = int*; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<int*>]'
/usr/include/c++/9/bits/stl_algo.h:1953:27:   required from 'void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = int*; _Size = long int; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<int*>]'
/usr/include/c++/9/bits/stl_algo.h:1973:25:   required from 'void std::__sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = int*; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<int*>]'
/usr/include/c++/9/bits/stl_algo.h:4905:18:   required from 'void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = int*; _Compare = int*]'
foo.cc:24:18:   required from here
/usr/include/c++/9/bits/predefined_ops.h:177:11: error: expression cannot be used as a function
  177 |  { return bool(_M_comp(*__it, __val)); }
      |           ^~~~~~~~~~~~~~~~~~~~~~~~~~~

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