JSON_BuildTests fail when JSON_DisableEnumSerialization is set to ON

[madamsrc]

Description

Setting JSON_DisableEnumSerialization to ON when building JSON for Modern C++ with testing enabled (JSON_BuildTests set also set to ON), the tests do not build and the build fails. Tests that rely on that setting should not be built for library clients who do not want EnumSerialization and the build should succeed.

Reproduction steps

From a fresh clone of nlohmann/json, I ran the following commands to configure and build the library using CMake:

/usr/bin/cmake -DJSON_DisableEnumSerialization=ON -DJSON_BuildTests=On -DJSON_Install=ON -DJSON_SystemInclude=ON -DCMAKE_INSTALL_PREFIX=/workspaces/json/install/gcc12-release -DCMAKE_EXPORT_COMPILE_COMMANDS=true -DCMAKE_BUILD_TYPE=Release -DCMAKE_CXX_COMPILER:FILEPATH=/usr/bin/g++-12 -S/workspaces/json -B/workspaces/json/build/gcc12-release -DCMAKE_BUILD_TYPE:STRING=Release -DCMAKE_C_COMPILER:FILEPATH=/usr/bin/gcc-12 -DCMAKE_CXX_COMPILER:FILEPATH=/usr/bin/g++-12 -DCMAKE_CXX_STANDARD:STRING=20 -DCMAKE_CXX_STANDARD_REQUIRED:BOOL=ON -DCMAKE_CXX_EXTENSIONS:BOOL=OFF -DCMAKE_CXX_FLAGS_INIT:STRING='-fPIC -O3 -mavx2 -mavx'
/usr/bin/cmake --build /workspaces/json/build/gcc12-release --parallel 17 --target all

Expected vs. actual results

I expect successful building and linking of all executable tests with no errors or warnings. In practice, the compiler produces errors and warnings as detailed below in the Error messages field in this form.

Minimal code example

N/A - just building the tests of the library

Error messages

/workspaces/json/tests/src/unit-conversions.cpp:1272:53: error: expected primary-expression before ‘)’ token
 1272 |         CHECK(json(cpp_enum::value_1).get<cpp_enum>() == cpp_enum::value_1);
      |                                                     ^
/workspaces/json/tests/thirdparty/doctest/doctest.h:2427:24: note: in definition of macro ‘DOCTEST_ASSERT_IMPLEMENT_1’
 2427 |                     << __VA_ARGS__) DOCTEST_CLANG_SUPPRESS_WARNING_POP
      |                        ^~~~~~~~~~~
/workspaces/json/tests/thirdparty/doctest/doctest.h:2969:20: note: in expansion of macro ‘DOCTEST_CHECK’
 2969 | #define CHECK(...) DOCTEST_CHECK(__VA_ARGS__)
      |                    ^~~~~~~~~~~~~
/workspaces/json/tests/src/unit-conversions.cpp:1272:9: note: in expansion of macro ‘CHECK’
 1272 |         CHECK(json(cpp_enum::value_1).get<cpp_enum>() == cpp_enum::value_1);
      |         ^~~~~
gmake[2]: *** [tests/CMakeFiles/test-conversions_cpp11.dir/build.make:76: tests/CMakeFiles/test-conversions_cpp11.dir/src/unit-conversions.cpp.o] Error 1
gmake[1]: *** [CMakeFiles/Makefile2:1763: tests/CMakeFiles/test-conversions_cpp11.dir/all] Error 2
gmake: *** [Makefile:146: all] Error

Compiler and operating system

gcc12, Ubuntu 22.04.1 x86_64

Library version

3.11.2

Validation

• The bug also occurs if the latest version from the develop branch is used.
• I can successfully compile and run the unit tests.