Unit conversion test error: conversion from 'nlohmann::json' to non-scalar type 'std::string_view' requested

[taylorhoward92]

• What is the issue you have?

When building v3.6.0 or v3.6.1 I get the following error from a unit-conversion test. The error is not present when building v3.5.0.

[ 46%] Building CXX object _deps/nlohmann_json-build/test/CMakeFiles/test-conversions.dir/src/unit-conversions.cpp.o
/tmp/tmp.KVk3RBpMpy/cmake-build-debug/_deps/nlohmann_json-src/test/src/unit-conversions.cpp: In function 'void ____C_A_T_C_H____T_E_S_T____0()':
/tmp/tmp.KVk3RBpMpy/cmake-build-debug/_deps/nlohmann_json-src/test/src/unit-conversions.cpp:483:34: error: conversion from 'nlohmann::json' {aka 'nlohmann::basic_json<>'} to non-scalar type 'std::string_view' {aka 'std::basic_string_view<char>'} requested
             std::string_view s = j;

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

Include nlohmann_json in a Cmake project using FetchContent:

FetchContent_Declare(
        nlohmann_json
        GIT_REPOSITORY https://github.com/nlohmann/json.git
        GIT_TAG v3.6.1
)

FetchContent_MakeAvailable(nlohmann_json)

include_directories(${nlohmann_json_SOURCE_DIR}/single_include)

• What is the expected behavior?

Tests pass without errors.

• And what is the actual behavior instead?

The above error occurs.

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

Ubuntu 18.04 with GCC 8.3.0.
Using C++17.

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

The test error occurs with v3.6.0 and v3.6.1. It does not occur with v3.5.0. I haven't tested any other versions.

• If you experience a compilation error: can you compile and run the unit tests?

See above.

[nickaein]

Thanks for reporting the issue. I have reproduced here (for 3.6.1 on gcc 8.3): https://godbolt.org/z/pl6XlF

This seems to be an oversight in fixing this unit-test on 3.6.1 which uses implicit conversion and is discouraged. It seems to be fixed on develop branch by replacing it with an explicit conversion.

Nevertheless, you should be able to use the library without building unit tests, if you want. Even more, I you can skip downloading the whole repository by replace the git URL with the following direct HTTP link to single header release 3.6.1:

https://github.com/nlohmann/json/releases/download/v3.6.1/json.hpp

The single library header is usually enough for using this library as a third-party in other applications.

[taylorhoward92]

Thanks for taking a look. It is fixed on the line you pointed out but not on this line.

[nlohmann]

Would you like to make a pull request with the fix?

[nlohmann]

Closed by merging #1639.