Support for CMake 3.16 as well

CMakeLists.txt

 cmake_minimum_required(VERSION 3.16)
 project(ojl_bimap VERSION 0.0.1 LANGUAGES CXX)
-#TODO: does this project require C++ 20? Then that should be part of the compile features of the targets
 set(CMAKE_CXX_STANDARD 20)
 
 include(GNUInstallDirs)
@@ -14,11 +13,28 @@ target_include_directories(bimap
     $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/include>
     $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
 )
+
+target_compile_features(bimap
+    INTERFACE
+    cxx_std_20
+)
+
+# The above is not enough, because it let's some compilers through that don't support all of C++ 20...
+target_compile_options(bimap
+    INTERFACE
+    -std=c++20
+)
+
+#TODO: set the header files here as a list, so there's only one place where they are defined
+
+if(${CMAKE_VERSION} VERSION_LESS "3.23.0") 
+else()
 target_sources(bimap
     INTERFACE FILE_SET HEADERS
     BASE_DIRS ${PROJECT_SOURCE_DIR}/include/ojl
     FILES ${PROJECT_SOURCE_DIR}/include/ojl/bimap.hpp
 )
+endif()
 add_subdirectory(samples)
 
 option(OJL_BIMAP_BUILD_TESTS "Build tests" ON)
@@ -26,6 +42,16 @@ option(OJL_BIMAP_BUILD_TESTS "Build tests" ON)
 option(OJL_BIMAP_DOWNLOAD_DEPS "Download unsatisfied dependencies using FetchContent" ON)
 
 if (OJL_BIMAP_BUILD_TESTS)
+
+    #TODO: Only checks the initially set compiler? Have to run twice if you change the compiler (maybe this is just how it works, idk)
+    include(CheckCXXCompilerFlag)
+    check_cxx_compiler_flag(-std=c++20 CXX_20_SUPPORT)
+
+    if (NOT CXX_20_SUPPORT)
+        message(FATAL_ERROR "The C++ compiler does not support the -std=c++20 flag")
+    endif()
+
+
     if (OJL_BIMAP_DOWNLOAD_DEPS)
         include(cmake/fetch_catch2.cmake)
     endif()

cmake/install_bimap.cmake

 # Install config
 
-# Define export set bimapTargets, install headers
-install(TARGETS bimap
-    EXPORT ojl_bimapExports
-    FILE_SET HEADERS 
-    DESTINATION 
-      ${CMAKE_INSTALL_INCLUDEDIR}/ojl
-)
+if(${CMAKE_VERSION} VERSION_LESS "3.23.0") 
+    # Old way
+    # Define exports
+    install(TARGETS bimap
+        EXPORT ojl_bimapExports
+    )
+    # Install header directory
+    # Don't install bijection.hpp yet
+    install(DIRECTORY ${CMAKE_SOURCE_DIR}/include/ojl
+        DESTINATION 
+            ${CMAKE_INSTALL_INCLUDEDIR}
+        FILES_MATCHING PATTERN
+            *bimap.hpp
+            #*bijection.hpp
+    )
+
+        
+else()
+    # Define export set bimapTargets, install headers
+    install(TARGETS bimap
+        EXPORT ojl_bimapExports
+        FILE_SET HEADERS 
+        DESTINATION 
+          ${CMAKE_INSTALL_INCLUDEDIR}/ojl
+    )
+endif()
 
 # Generate the export set file
 install(EXPORT ojl_bimapExports

include/ojl/bijection.hpp

+//WARNING: WIP, not tested, use at your own risk
+
+
 #include <set>
 #include <vector>
 #include <utility>