ambiguous conversion with nlohmann-json 3.10.4

[thkoch2001]

I'm working on an update for the nix Debian package to 2.5.1. In Debian we try to avoid vendorized code, thus I use the already packaged nlohmann-json3 in version 3.10.4.

The nlohmann-json version in nix seems to be 3.9.1.

I see this compilation error:

src/libstore/realisation.cc: In lambda function:                                                                                                                     [14/9368]
src/libstore/realisation.cc:81:16: error: conversion from ‘const nlohmann::basic_json<>’ to ‘std::optional<std::__cxx11::basic_string<char> >’ is ambiguous
   81 |         return *fieldIterator;
      |                ^~~~~~~~~~~~~~
In file included from src/libstore/realisation.cc:4:
/usr/include/nlohmann/json.hpp:3413:55: note: candidate: ‘nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFl
oatType, AllocatorType, JSONSerializer, BinaryType>::operator ValueType() const [with ValueType = std::optional<std::__cxx11::basic_string<char> >; typename std::enable_if<nl
ohmann::detail::conjunction<nlohmann::detail::negation<std::is_pointer<_Ptr> >, nlohmann::detail::negation<std::is_same<ValueType, nlohmann::detail::json_ref<nlohmann::basic_
json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType> > > >, nlohmann::detai
l::negation<std::is_same<ValueType, typename StringType::value_type> >, nlohmann::detail::negation<nlohmann::detail::is_basic_json<BasicJsonType> >, nlohmann::detail::negatio
n<std::is_same<ValueType, std::initializer_list<typename StringType::value_type> > >, nlohmann::detail::negation<std::is_same<ValueType, std::basic_string_view<char, std::cha
r_traits<char> > > >, nlohmann::detail::is_detected_lazy<nlohmann::detail::get_template_function, const nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, N
umberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>&, ValueType> >::value, int>::type <anonymous> = 0; ObjectType = std::map; Ar
rayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long int; NumberUnsignedType = long unsigned int; NumberFloatTyp
e = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer; BinaryType = std::vector<unsigned char>]’
 3413 |                                         JSON_EXPLICIT operator ValueType() const
      |                                                       ^~~~~~~~
In file included from src/libutil/error.hh:11,
                 from src/libutil/util.hh:4,
                 from src/libutil/serialise.hh:6,
                 from src/libutil/hash.hh:4,
                 from src/libstore/content-address.hh:4,
                 from src/libstore/path.hh:3,
                 from src/libstore/realisation.hh:3,
                 from src/libstore/realisation.cc:1:
/usr/include/c++/11/optional:704:9: note: candidate: ‘constexpr std::optional<_Tp>::optional(_Up&&) [with _Up = const nlohmann::basic_json<>&; typename std::enable_if<__and_v
<std::__not_<std::is_same<std::optional<_Tp>, typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type> >, std::__not_<std::is_same<std::in_place_t, typename
std::remove_cv<typename std::remove_reference<_Tp>::type>::type> >, std::is_constructible<_Tp, _Up>, std::is_convertible<_Up, _Tp> >, bool>::type <anonymous> = true; _Tp = st
d::__cxx11::basic_string<char>]’
  704 |         optional(_Up&& __t)
      |         ^~~~~~~~
make[1]: *** [mk/patterns.mk:3: src/libstore/realisation.o] Error 1

I'm not a C++ programmer, so please don't laugh. I'd guess that one would simply need to tell the compiler, which conversion to use?

This is the code in question from src/libstore/realisation.cc#L81:

    auto getOptionalField = [&](std::string fieldName) -> std::optional<std::string> {
        auto fieldIterator = json.find(fieldName);
        if (fieldIterator == json.end())
            return std::nullopt;
        return *fieldIterator;
    };

Is this related? nlohmann/json#3204

[thkoch2001]

I tried compilation now with nlohmann version 3.9.1 from Debian and this error did not appear anymore. Version 3.10.5 of nlohmann-json3 has been uploaded to Debian unstable today and I'll test this one and report back.

[thkoch2001]

If you came here because you care about the nix Debian package, then please also look at #5931

[thkoch2001]

Unfortunately the problem still persists with version 3.10.5 of nlohmann-json.