upgraded to C++20

Author: fbourgin

test/CMakeLists.txt

@@ -32,7 +32,7 @@ include(CheckCXXCompilerFlag)
 string(TOUPPER "${CMAKE_BUILD_TYPE}" U_CMAKE_BUILD_TYPE)
 
 if (CMAKE_CXX_COMPILER_ID MATCHES "Clang" OR CMAKE_CXX_COMPILER_ID MATCHES "GNU" OR CMAKE_CXX_COMPILER_ID MATCHES "Intel")
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -march=native -Wunused-parameter -Wextra -Wreorder -Wconversion")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -march=native -Wunused-parameter -Wextra -Wreorder -Wconversion -D_LIBCPP_DISABLE_AVAILABILITY")
     CHECK_CXX_COMPILER_FLAG("-std=c++20" HAS_CPP20_FLAG)
 
     if (HAS_CPP20_FLAG)