compilation error when using with another library

[bl4ck5un]

• What is the issue you have?

To be honest, I'm not sure if this is a bug in this library, as it only happens when used together with another library, and only on macOS. But I'd appreciate any insight the authors might have. The error message appears agnostic to the other library in question.

/Users/fanz/dev/deco/notary/external/json/single_include/nlohmann/json.hpp:12450:106: error: no matching member function for call to 'to_char_type'
                                    string_buffer[bytes++] = detail::binary_writer<BasicJsonType, char>::to_char_type('\xEF');
                                                             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~
/Users/fanz/dev/deco/notary/external/json/single_include/nlohmann/json.hpp:12117:25: note: in instantiation of member function 'nlohmann::detail::serializer<nlohmann::basic_json<std::map, std::vector,
      std::__1::basic_string<char>, bool, long long, unsigned long long, double, std::allocator, adl_serializer> >::dump_escaped' requested here
                        dump_escaped(i->first, ensure_ascii);
                        ^
/Users/fanz/dev/deco/notary/external/json/single_include/nlohmann/json.hpp:14744:15: note: in instantiation of member function 'nlohmann::detail::serializer<nlohmann::basic_json<std::map, std::vector,
      std::__1::basic_string<char>, bool, long long, unsigned long long, double, std::allocator, adl_serializer> >::dump' requested here
            s.dump(*this, true, ensure_ascii, static_cast<unsigned int>(indent));
              ^
/Users/fanz/dev/deco/notary/external/json/single_include/nlohmann/json.hpp:20743:20: note: in instantiation of member function 'nlohmann::basic_json<std::map, std::vector, std::__1::basic_string<char>, bool, long
      long, unsigned long long, double, std::allocator, adl_serializer>::dump' requested here
        return h(j.dump());
                   ^
/Users/fanz/dev/deco/notary/external/json/single_include/nlohmann/json.hpp:10843:31: note: candidate template ignored: substitution failure [with C = char]: null non-type template argument must be cast to
      template parameter type 'enable_if_t<std::is_signed<char>::value && std::is_signed<char>::value> *' (aka 'void *')
    static constexpr CharType to_char_type(std::uint8_t x) noexcept
                              ^
/Users/fanz/dev/deco/notary/external/json/single_include/nlohmann/json.hpp:10850:21: note: candidate template ignored: requirement 'std::is_unsigned<char>::value' was not satisfied [with C = char]
    static CharType to_char_type(std::uint8_t x) noexcept
                    ^
/Users/fanz/dev/deco/notary/external/json/single_include/nlohmann/json.hpp:10861:31: note: candidate template ignored: requirement 'std::is_unsigned<char>::value' was not satisfied [with C = char]
    static constexpr CharType to_char_type(std::uint8_t x) noexcept
                              ^
/Users/fanz/dev/deco/notary/external/json/single_include/nlohmann/json.hpp:10872:31: note: candidate template ignored: substitution failure [with InputCharType = char, C = char]: null non-type template argument
      must be cast to template parameter type 'enable_if_t<std::is_signed<char>::value && std::is_signed<char>::value && std::is_same<char, typename std::remove_cv<char>::type>::value> *' (aka 'void *')
    static constexpr CharType to_char_type(InputCharType x) noexcept

• Please describe the steps to reproduce the issue. Can you provide a small but working code example?

In my case, as long as https://github.com/unbound-tech/blockchain-crypto-mpc/blob/master/src/utils/ub_common_def.h is included along with json.hpp, the compilation would fail.

• Which compiler and operating system are you using? Is it a supported compiler?

Apple LLVM version 10.0.1 (clang-1001.0.46.3)
Target: x86_64-apple-darwin18.5.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

• Did you use a released version of the library or the version from the develop branch?

v3.6.1 is used.

[Macr0Nerd]

@nlohmann or @theodelrieu correct me if I'm wrong, but it seems that there is some interference with to_char, presumably from the other library. It looks like one of the characters being input isn't being read as char.

[Macr0Nerd]

@bl4ck5un can you provide an example of what you input and where the supposed error is?

[bl4ck5un]

never mind guys, it's a messy project and hard to extract a small example. I can work it around by arranging the header carefully. Will reopen with a reproducible example when I have one. Thanks anyway.

[x-santiaga-x]

The compilation error is caused by nullptr has been defined as 0 in ub_common_def.h when compiling with clang.

#if defined(__GNUC__) && (GCC_VERSION < 0x0406) // 4.6
#define nullptr NULL
#endif

clang defines GNUC/MINOR/PATCHLEVEL as 4/2/1, i.e. compat with GCC 4.2.1.

Correct gcc version check could be
#if !defined(__clang__) && defined(__GNUC__) && (GCC_VERSION < 0x0406) // 4.6

So it might be helpful to insert some nullptr type checking with static_assert.

[gregmarr]

@x-santiaga-x Did you mean to submit this report to the library containing ub_common_def.h?

[x-santiaga-x]

I have discovered this error trying to compile my own project with the same #ifdef and it was hard to find.
Adding a static assertion can keep users from researching what no matching member function for call to 'to_char_type' might mean.