Add C++17 copies of the test binaries (#3101)

* ⚗️ add C++17 copies of the test binaries
* ⚗️ use proper header for filesystem
* 🚨 fix warnings
* ⚗️ do not use too old compilers with C++17
* ✅ add test
* 🔨 add more constraints #3097
* ⚗️ use fix from https://github.com/nlohmann/json/pull/3101#issuecomment-998788786
* ⚗️ use fix from https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90050
* 👷 use published CI image

cmake/ci.cmake

@@ -814,11 +814,19 @@ add_custom_target(ci_cmake_flags
 foreach(COMPILER g++-4.8 g++-4.9 g++-5 g++-6 g++-7 g++-8 g++-9 g++-10 clang++-3.5 clang++-3.6 clang++-3.7 clang++-3.8 clang++-3.9 clang++-4.0 clang++-5.0 clang++-6.0 clang++-7 clang++-8 clang++-9 clang++-10 clang++-11 clang++-12 clang++-13)
     find_program(COMPILER_TOOL NAMES ${COMPILER})
     if (COMPILER_TOOL)
+        if ("${COMPILER}" STREQUAL "clang++-9")
+            # fix for https://github.com/nlohmann/json/pull/3101#issuecomment-998788786 / https://stackoverflow.com/a/64051725/266378
+            set(ADDITIONAL_FLAGS "-DCMAKE_CXX_FLAGS=--gcc-toolchain=/root/gcc/9")
+        else()
+            unset(ADDITIONAL_FLAGS)
+        endif()
+
         add_custom_target(ci_test_compiler_${COMPILER}
             COMMAND CXX=${COMPILER} ${CMAKE_COMMAND}
                 -DCMAKE_BUILD_TYPE=Debug -GNinja
                 -DJSON_BuildTests=ON -DJSON_FastTests=ON
                 -S${PROJECT_SOURCE_DIR} -B${PROJECT_BINARY_DIR}/build_compiler_${COMPILER}
+                ${ADDITIONAL_FLAGS}
             COMMAND ${CMAKE_COMMAND} --build ${PROJECT_BINARY_DIR}/build_compiler_${COMPILER}
             COMMAND cd ${PROJECT_BINARY_DIR}/build_compiler_${COMPILER} && ${CMAKE_CTEST_COMMAND} --parallel ${N} --exclude-regex "test-unicode" --output-on-failure
             COMMENT "Compile and test with ${COMPILER}"