Improve error message for const fields

[AlexanderLanin]

The goal here is to propose an idea for a better error message, which says what is wrong instead of pointing to some internal implementation detail that does not compile.

As I'm a very novice user of the library and even less familiar with the code itself, I'll just create this draft for someone else to finish / abandon the idea. I'm not familiar enough with it to create a proper pull request. Sorry!

It should even be possible to test the static_assert, but that is another week of fiddling.
e.g. https://www.youtube.com/watch?v=zxDzMjfsgjg

The result is certainly not perfect, but it's significantly improved in both tested compilers.

Note: using enable_if or require doesn't really improve the situation.
It just says error: no matching member function for call to 'get_to'
And like 20 lines later ../single_include/nlohmann/json.hpp:20258:16: note: because 'const int' does not satisfy 'Writeable'

Scenario:

    struct person_with_const_age
    {
        std::string name{};
        const int age = 0;

        NLOHMANN_DEFINE_TYPE_INTRUSIVE(person_with_const_age, name, age)
    };

Output with g++ 9.3.0 without the patch:

/usr/bin/c++  -DDOCTEST_CONFIG_SUPER_FAST_ASSERTS -DJSON_DIAGNOSTICS=0 -DJSON_USE_IMPLICIT_CONVERSIONS=1 -Iinclude -I../test/thirdparty/doctest -I../test/thirdparty/fifo_map -I../single_include -Wno-deprecated -Wno-float-equal -Wno-deprecated-declarations -MD -MT test/CMakeFiles/test-udt_macro.dir/src/unit-udt_macro.cpp.o -MF test/CMakeFiles/test-udt_macro.dir/src/unit-udt_macro.cpp.o.d -o test/CMakeFiles/test-udt_macro.dir/src/unit-udt_macro.cpp.o -c ../test/src/unit-udt_macro.cpp
In file included from ../test/src/unit-udt_macro.cpp:34:
../single_include/nlohmann/json.hpp: In instantiation of ‘void nlohmann::detail::from_json(const BasicJsonType&, ArithmeticType&) [with BasicJsonType = nlohmann::basic_json<>; ArithmeticType = const int; typename std::enable_if<((((std::is_arithmetic<ArithmeticType>::value && (! std::is_same<ArithmeticType, typename BasicJsonType::number_unsigned_t>::value)) && (! std::is_same<ArithmeticType, typename BasicJsonType::number_integer_t>::value)) && (! std::is_same<ArithmeticType, typename BasicJsonType::number_float_t>::value)) && (! std::is_same<ArithmeticType, typename BasicJsonType::boolean_t>::value)), int>::type <anonymous> = 0]’:
../single_include/nlohmann/json.hpp:4255:25:   required from ‘decltype (nlohmann::detail::from_json(j, forward<T>(val))) nlohmann::detail::from_json_fn::operator()(const BasicJsonType&, T&&) const [with BasicJsonType = nlohmann::basic_json<>; T = const int&; decltype (nlohmann::detail::from_json(j, forward<T>(val))) = void]’
../single_include/nlohmann/json.hpp:4867:30:   required from ‘static decltype ((nlohmann::{anonymous}::from_json(forward<BasicJsonType>(j), val), void())) nlohmann::adl_serializer<T, SFINAE>::from_json(BasicJsonType&&, TargetType&) [with BasicJsonType = const nlohmann::basic_json<>&; TargetType = const int; ValueType = const int; <template-parameter-1-2> = void; decltype ((nlohmann::{anonymous}::from_json(forward<BasicJsonType>(j), val), void())) = void]’
../single_include/nlohmann/json.hpp:20260:45:   required from ‘ValueType& nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::get_to(ValueType&) const [with ValueType = const int; typename std::enable_if<((! nlohmann::detail::is_basic_json<BasicJsonType>::value) && nlohmann::detail::has_from_json<nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>, ValueType>::value), int>::type <anonymous> = 0; ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long int; NumberUnsignedType = long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer; BinaryType = std::vector<unsigned char>]’
../test/src/unit-udt_macro.cpp:238:9:   required from here
../single_include/nlohmann/json.hpp:4141:17: error: assignment of read-only reference ‘val’
 4141 |             val = static_cast<ArithmeticType>(*j.template get_ptr<const typename BasicJsonType::number_unsigned_t*>());
      |             ~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../single_include/nlohmann/json.hpp:4146:17: error: assignment of read-only reference ‘val’
 4146 |             val = static_cast<ArithmeticType>(*j.template get_ptr<const typename BasicJsonType::number_integer_t*>());
      |             ~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../single_include/nlohmann/json.hpp:4151:17: error: assignment of read-only reference ‘val’
 4151 |             val = static_cast<ArithmeticType>(*j.template get_ptr<const typename BasicJsonType::number_float_t*>());
      |             ~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../single_include/nlohmann/json.hpp:4156:17: error: assignment of read-only reference ‘val’
 4156 |             val = static_cast<ArithmeticType>(*j.template get_ptr<const typename BasicJsonType::boolean_t*>());
      |             ~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Output with g++ 9.3.0 with the patch:

/usr/bin/c++  -DDOCTEST_CONFIG_SUPER_FAST_ASSERTS -DJSON_DIAGNOSTICS=0 -DJSON_USE_IMPLICIT_CONVERSIONS=1 -Iinclude -I../test/thirdparty/doctest -I../test/thirdparty/fifo_map -I../single_include -Wno-deprecated -Wno-float-equal -Wno-deprecated-declarations -MD -MT test/CMakeFiles/test-udt_macro.dir/src/unit-udt_macro.cpp.o -MF test/CMakeFiles/test-udt_macro.dir/src/unit-udt_macro.cpp.o.d -o test/CMakeFiles/test-udt_macro.dir/src/unit-udt_macro.cpp.o -c ../test/src/unit-udt_macro.cpp
In file included from ../test/src/unit-udt_macro.cpp:34:
../single_include/nlohmann/json.hpp: In instantiation of ‘ValueType& nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::get_to(ValueType&) const [with ValueType = const int; typename std::enable_if<((! nlohmann::detail::is_basic_json<BasicJsonType>::value) && nlohmann::detail::has_from_json<nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>, ValueType>::value), int>::type <anonymous> = 0; ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long int; NumberUnsignedType = long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer; BinaryType = std::vector<unsigned char>]’:
../test/src/unit-udt_macro.cpp:238:9:   required from here
../single_include/nlohmann/json.hpp:20259:23: error: static assertion failed: Cannot deserialize into constant fields
20259 |         static_assert(!std::is_const<ValueType>::value, "Cannot deserialize into constant fields");
      |                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Output with clang++ 10.0.0 without the patch:

/usr/bin/clang++  -DDOCTEST_CONFIG_SUPER_FAST_ASSERTS -DJSON_DIAGNOSTICS=0 -DJSON_USE_IMPLICIT_CONVERSIONS=1 -Iinclude -I../test/thirdparty/doctest -I../test/thirdparty/fifo_map -I../single_include -Wno-deprecated -Wno-float-equal -MD -MT test/CMakeFiles/test-udt_macro.dir/src/unit-udt_macro.cpp.o -MF test/CMakeFiles/test-udt_macro.dir/src/unit-udt_macro.cpp.o.d -o test/CMakeFiles/test-udt_macro.dir/src/unit-udt_macro.cpp.o -c ../test/src/unit-udt_macro.cpp
In file included from ../test/src/unit-udt_macro.cpp:34:
../single_include/nlohmann/json.hpp:4141:17: error: cannot assign to variable 'val' with const-qualified type 'const int &'
            val = static_cast<ArithmeticType>(*j.template get_ptr<const typename BasicJsonType::number_unsigned_t*>());
            ~~~ ^
../single_include/nlohmann/json.hpp:4255:16: note: in instantiation of function template specialization 'nlohmann::detail::from_json<nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char>, bool, long, unsigned long, double, std::allocator, adl_serializer, std::vector<unsigned char, std::allocator<unsigned char> > >, const int, 0>' requested here
        return from_json(j, std::forward<T>(val));
               ^
../single_include/nlohmann/json.hpp:4867:9: note: in instantiation of function template specialization 'nlohmann::detail::from_json_fn::operator()<nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char>, bool, long, unsigned long, double, std::allocator, adl_serializer, std::vector<unsigned char, std::allocator<unsigned char> > >, const int &>' requested here
        ::nlohmann::from_json(std::forward<BasicJsonType>(j), val);
        ^
../single_include/nlohmann/json.hpp:20260:36: note: in instantiation of function template specialization 'nlohmann::adl_serializer<const int, void>::from_json<const nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char>, bool, long, unsigned long, double, std::allocator, adl_serializer, std::vector<unsigned char, std::allocator<unsigned char> > > &, const int>' requested here
        JSONSerializer<ValueType>::from_json(*this, v);
                                   ^
../test/src/unit-udt_macro.cpp:238:9: note: in instantiation of function template specialization 'nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char>, bool, long, unsigned long, double, std::allocator, adl_serializer, std::vector<unsigned char, std::allocator<unsigned char> > >::get_to<const int, 0>' requested here
        NLOHMANN_DEFINE_TYPE_INTRUSIVE(person_with_const_age, name, age)
        ^
        [~30 more lines]

Output with clang++ 10.0.0 with the patch:

/usr/bin/clang++  -DDOCTEST_CONFIG_SUPER_FAST_ASSERTS -DJSON_DIAGNOSTICS=0 -DJSON_USE_IMPLICIT_CONVERSIONS=1 -Iinclude -I../test/thirdparty/doctest -I../test/thirdparty/fifo_map -I../single_include -Wno-deprecated -Wno-float-equal -MD -MT test/CMakeFiles/test-udt_macro.dir/src/unit-udt_macro.cpp.o -MF test/CMakeFiles/test-udt_macro.dir/src/unit-udt_macro.cpp.o.d -o test/CMakeFiles/test-udt_macro.dir/src/unit-udt_macro.cpp.o -c ../test/src/unit-udt_macro.cpp
In file included from ../test/src/unit-udt_macro.cpp:34:
../single_include/nlohmann/json.hpp:20261:9: error: static_assert failed due to requirement '!std::is_const<const int>::value' "Cannot deserialize into constant fields"
        static_assert(!std::is_const<ValueType>::value, "Cannot deserialize into constant fields");
        ^             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../test/src/unit-udt_macro.cpp:238:9: note: in instantiation of function template specialization 'nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char>, bool, long, unsigned long, double, std::allocator, adl_serializer, std::vector<unsigned char, std::allocator<unsigned char> > >::get_to<const int, 0>' requested here
        NLOHMANN_DEFINE_TYPE_INTRUSIVE(person_with_const_age, name, age)
        ^
        [~50 more lines]

---

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[nlohmann]

We once had static asserts, but there was a reason to remove them, see #960. I know this situation different, but I am hesitant to merge right now.

[coveralls]

Coverage remained the same at 99.982% when pulling 2494932 on AlexanderLanin:const into 18a5f4c on nlohmann:develop.

[AlexanderLanin]

Indeed "interesting". Especially since adding !std::is_const<ValueType> to enable_if / concepts is way less helpful. As mentioned above instead of "this is the correct function, but you can't call it", you get "by the way there was this one function, but nah... doesn't fit".

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.