C++20 issues

[davidcerny]

Issue from revbayes/revbayes#837 (comment) by @bredelings:

It looks like NCL has a few problems with C++20. I wonder if the upstream NCL fixes these.

(Hmm... it seems like it doesn't? So I guess this isn't really related to the current PR, so maybe I should move this comment to its own issue)

The issue seems to be that nxscharactersblock.h does

class NxsDiscreteDatatypeMapper;

class NxsCharactersBlock
{
...
		typedef std::pair<NxsDiscreteDatatypeMapper, NxsUnsignedSet> DatatypeMapperAndIndexSet;
		typedef std::vector<DatatypeMapperAndIndexSet> VecDatatypeMapperAndIndexSet;
...
};

class NxsDiscreteDatatypeMapper
{
...
		NxsDiscreteDatatypeMapper(NxsCharactersBlock::DataTypesEnum datatypeE, bool hasGaps);
...
};

The error is because the definition for NxsDiscreteDatatypeMapper is not known when std::pair<NxsDiscreteDatatypeMapper, NxsUnsignedSet> is first used. This was allowed in C++17 in some circumstances, but is not longer allowed in C++20.

We could simply move the definition of NxsDiscreteDatatypeMapper above the definition of NxsCharactersBlock, except that NxsDiscreteDatatypeMapper also references NxsCharactersBlock through NxsCharactersBlock::DataTypesEnum, creating a reference loop.

[davidcerny]

According to @bredelings, this no longer seems to be a problem.

[bredelings]

The problem occurs when compiling with clang, but not gcc.

In file included from ../example/ot-tree-inspect/ot_tree_inspect.cpp:46:
In file included from ../ncl/ncl.h:25:
In file included from /usr/lib/gcc/x86_64-linux-gnu/16/../../../../include/c++/16/cmath:49:
In file included from /usr/lib/gcc/x86_64-linux-gnu/16/../../../../include/c++/16/bits/cpp_type_traits.h:42:
/usr/lib/gcc/x86_64-linux-gnu/16/../../../../include/c++/16/type_traits:3647:64: error: incomplete type 'NxsDiscreteDatatypeMapper' used in type trait expression
 3647 |   inline constexpr bool is_assignable_v = __is_assignable(_Tp, _Up);
      |                                                                ^
/usr/lib/gcc/x86_64-linux-gnu/16/../../../../include/c++/16/bits/stl_pair.h:563:18: note: in instantiation of variable template specialization 'std::is_assignable_v<NxsDiscreteDatatypeMapper &, NxsDiscreteDatatypeMapper>' requested here
  563 |           if constexpr (is_assignable_v<_T1&, _U1>)
      |                         ^
/usr/lib/gcc/x86_64-linux-gnu/16/../../../../include/c++/16/bits/stl_pair.h:624:17: note: in instantiation of function template specialization 'std::pair<NxsDiscreteDatatypeMapper, std::set<unsigned int>>::_S_assignable<NxsDiscreteDatatypeMapper, std::set<unsigned int>>' requested here
  624 |       requires (_S_assignable<_T1, _T2>())
      |                 ^
/usr/lib/gcc/x86_64-linux-gnu/16/../../../../include/c++/16/bits/stl_pair.h:624:17: note: while substituting template arguments into constraint expression here
  624 |       requires (_S_assignable<_T1, _T2>())
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~
/usr/lib/gcc/x86_64-linux-gnu/16/../../../../include/c++/16/bits/stl_pair.h:622:7: note: while checking constraint satisfaction for function 'operator=' required here
  622 |       operator=(pair&& __p)
      |       ^~~~~~~~
/usr/lib/gcc/x86_64-linux-gnu/16/../../../../include/c++/16/type_traits:1505:30: note: in instantiation of template class 'std::pair<NxsDiscreteDatatypeMapper, std::set<unsigned int>>' requested here
 1505 |     : public __bool_constant<__is_trivially_destructible(_Tp)>
      |                              ^
/usr/lib/gcc/x86_64-linux-gnu/16/../../../../include/c++/16/bits/stl_construct.h:224:22: note: in instantiation of template class 'std::is_trivially_destructible<std::pair<NxsDiscreteDatatypeMapper, std::set<unsigned int>>>' requested here
  224 |       if constexpr (!is_trivially_destructible<_Value_type>::value)
      |                      ^
/usr/lib/gcc/x86_64-linux-gnu/16/../../../../include/c++/16/bits/alloc_traits.h:1045:12: note: in instantiation of function template specialization 'std::_Destroy<std::pair<NxsDiscreteDatatypeMapper, std::set<unsigned int>> *>' requested here
 1045 |       std::_Destroy(__first, __last);
      |            ^
/usr/lib/gcc/x86_64-linux-gnu/16/../../../../include/c++/16/bits/stl_vector.h:792:7: note: in instantiation of function template specialization 'std::_Destroy<std::pair<NxsDiscreteDatatypeMapper, std::set<unsigned int>> *, std::pair<NxsDiscreteDatatypeMapper, std::set<unsigned int>>>' requested here
  792 |         std::_Destroy(this->_M_impl._M_start, this->_M_impl._M_finish,
      |              ^
../ncl/nxscharactersblock.h:510:11: note: in instantiation of member function 'std::vector<std::pair<NxsDiscreteDatatypeMapper, std::set<unsigned int>>>::~vector' requested here
  510 |                 virtual ~NxsCharactersBlock() {}
      |                         ^
../ncl/nxscharactersblock.h:34:7: note: forward declaration of 'NxsDiscreteDatatypeMapper'
   34 | class NxsDiscreteDatatypeMapper;
      |       ^

[davidcerny]

Closed by #33. Thanks a lot, @bredelings!