Ambiguous equality/inequality operators under C++20

[tonyelewis]

Running clang++ -x c++ -std=c++2a -isystem include include/cereal/archives/json.hpp with a trunk version of Clang in the root of the cereal repo, gives:

In file included from include/cereal/archives/json.hpp:67:
include/cereal/external/rapidjson/document.h:813:58: error: use of overloaded operator '!=' is ambiguous (with operand types 'rapidjson::GenericValue<rapidjson::UTF8<char>, rapidjson::MemoryPoolAllocator<rapidjson::CrtAllocator> >::MemberIterator' (aka 'rapidjson::GenericMemberIterator<false, rapidjson::UTF8<char>, rapidjson::MemoryPoolAllocator<rapidjson::CrtAllocator> >') and 'rapidjson::GenericValue<rapidjson::UTF8<char>, rapidjson::MemoryPoolAllocator<rapidjson::CrtAllocator> >::MemberIterator')
                for (MemberIterator m = MemberBegin(); m != MemberEnd(); ++m)
                                                       ~ ^  ~~~~~~~~~~~
include/cereal/external/rapidjson/document.h:2150:5: note: in instantiation of member function 'rapidjson::GenericValue<rapidjson::UTF8<char>, rapidjson::MemoryPoolAllocator<rapidjson::CrtAllocator> >::~GenericValue' requested here
    GenericDocument(Allocator* allocator = 0, size_t stackCapacity = kDefaultStackCapacity, StackAllocator* stackAllocator = 0) :
    ^
include/cereal/archives/json.hpp:435:7: note: in instantiation of member function 'rapidjson::GenericDocument<rapidjson::UTF8<char>, rapidjson::MemoryPoolAllocator<rapidjson::CrtAllocator>, rapidjson::CrtAllocator>::GenericDocument' requested here
      JSONInputArchive(std::istream & stream) :
      ^
include/cereal/external/rapidjson/document.h:179:10: note: candidate function
    bool operator!=(ConstIterator that) const { return ptr_ != that.ptr_; }
         ^
include/cereal/external/rapidjson/document.h:178:10: note: candidate function
    bool operator==(ConstIterator that) const { return ptr_ == that.ptr_; }
         ^
include/cereal/external/rapidjson/document.h:178:10: note: candidate function (with reversed parameter order)
1 error generated.

AFAIU, the reworking of comparisons in C++20 means that the new language must now reject some previously acceptable code for causing ambiguities between comparison operators that the compiler could consider (eg see discussion here https://bugs.llvm.org/show_bug.cgi?id=43765#c4 ).

[erichkeane]

There's a handful of core-issues against C++20 about this, I'm hopeful they will fix this situation in some way.

The alternative I think is to add non-const versions of those comparisons to rapidjson (as this is an issue with that library).

[tonyelewis]

Do the above references to this issue suggest that grabbing an updated version of rapidjson would solve this problem?

[InBetweenNames]

Can confirm that the latest version of rapidjson has solved this problem for me.

[tonyelewis]

Thanks very much for this. Please may I add my voice to a request for a new release so we can get this updated version out via Conan? Thank you.

[dean0x7d]

I'd like to second the request for a new release with this fix.