CMake: leave C++ standard selection to the user

FindCoroutines.cmake tried to figure out the correct flags to use
coroutines. However, with MSVC 16.8 this is impossible. MSVC 16.8
supports both the coroutines-ts with its experimental headers
and experimental namespace as well as the C++20 coroutines.
The user can select the coroutines-ts with the /await flag and
the C++20 coroutines with /std:c++latest. Only the user can know
what she wants.
This patch changes the logic in FindCoroutines.cmake to basically:
If it does not compile without (/await|-fcoroutines-ts),
try adding (/await|-fcoroutines-ts) to the compiler flags.

README.md

@@ -2952,10 +2952,10 @@ target_link_libraries(app PRIVATE cppcoro::cppcoro)
 ```bash
 $ cmake . -Dcppcoro_ROOT=$HOME/.local
 # ...
--- Performing Test Coroutines_SUPPORTS_MS_FLAG
--- Performing Test Coroutines_SUPPORTS_MS_FLAG - Failed
--- Performing Test Coroutines_SUPPORTS_COROUTINES_FLAG
--- Performing Test Coroutines_SUPPORTS_COROUTINES_FLAG - Success
+-- Performing Test _CXX_COROUTINES_SUPPORTS_MS_FLAG
+-- Performing Test _CXX_COROUTINES_SUPPORTS_MS_FLAG - Failed
+-- Performing Test _CXX_COROUTINES_SUPPORTS_CORO_FLAG
+-- Performing Test _CXX_COROUTINES_SUPPORTS_CORO_FLAG - Success
 -- Looking for C++ include coroutine
 -- Looking for C++ include coroutine - not found
 -- Looking for C++ include experimental/coroutine

cmake/FindCoroutines.cmake

@@ -100,6 +100,7 @@ if(TARGET std::coroutines)
     return()
 endif()
 
+include(CheckCXXCompilerFlag)
 include(CMakePushCheckState)
 include(CheckIncludeFileCXX)
 include(CheckCXXSourceCompiles)
@@ -108,13 +109,16 @@ cmake_push_check_state()
 
 set(CMAKE_REQUIRED_QUIET ${Coroutines_FIND_QUIETLY})
 
-# All of our tests required C++17 or later
-if(("x${CMAKE_CXX_COMPILER_ID}" MATCHES "x.*Clang" AND "x${CMAKE_CXX_SIMULATE_ID}" STREQUAL "xMSVC") OR "x${CMAKE_CXX_COMPILER_ID}" STREQUAL "xMSVC")
-    set(_CXX_COROUTINES_STD17 "/std:c++17")
-    set(_CXX_COROUTINES_AWAIT "/await")
-else()
-    set(_CXX_COROUTINES_STD17 "-std=c++17")
-    set(_CXX_COROUTINES_AWAIT "-fcoroutines-ts")
+check_cxx_compiler_flag(/await _CXX_COROUTINES_SUPPORTS_MS_FLAG)
+check_cxx_compiler_flag(-fcoroutines-ts _CXX_COROUTINES_SUPPORTS_TS_FLAG)
+check_cxx_compiler_flag(-fcoroutines _CXX_COROUTINES_SUPPORTS_CORO_FLAG)
+
+if(_CXX_COROUTINES_SUPPORTS_MS_FLAG)
+    set(_CXX_COROUTINES_EXTRA_FLAGS "/await")
+elseif(_CXX_COROUTINES_SUPPORTS_TS_FLAG)
+    set(_CXX_COROUTINES_EXTRA_FLAGS "-fcoroutines-ts")
+elseif(_CXX_COROUTINES_SUPPORTS_CORO_FLAG)
+    set(_CXX_COROUTINES_EXTRA_FLAGS "-fcoroutines")
 endif()
 
 # Normalize and check the component list we were given
@@ -142,6 +146,13 @@ endif()
 
 if(find_final)
     check_include_file_cxx("coroutine" _CXX_COROUTINES_HAVE_HEADER)
+    if(NOT _CXX_COROUTINES_HAVE_HEADER)
+        cmake_push_check_state()
+        set(CMAKE_REQUIRED_FLAGS "${_CXX_COROUTINES_EXTRA_FLAGS}")
+        check_include_file_cxx("coroutine" _CXX_COROUTINES_HAVE_HEADER_WITH_FLAG)
+        set(_CXX_COROUTINES_HAVE_HEADER ${_CXX_COROUTINES_HAVE_HEADER_WITH_FLAG})
+        cmake_pop_check_state()
+    endif()
     mark_as_advanced(_CXX_COROUTINES_HAVE_HEADER)
     if(_CXX_COROUTINES_HAVE_HEADER)
         # We found the non-experimental header. Don't bother looking for the
@@ -154,6 +165,13 @@ endif()
 
 if(find_experimental)
     check_include_file_cxx("experimental/coroutine" _CXX_COROUTINES_HAVE_EXPERIMENTAL_HEADER)
+    if(NOT _CXX_COROUTINES_HAVE_EXPERIMENTAL_HEADER)
+        cmake_push_check_state()
+        set(CMAKE_REQUIRED_FLAGS "${_CXX_COROUTINES_EXTRA_FLAGS}")
+        check_include_file_cxx("experimental/coroutine" _CXX_COROUTINES_HAVE_EXPERIMENTAL_HEADER_WITH_FLAG)
+        set(_CXX_COROUTINES_HAVE_EXPERIMENTAL_HEADER ${_CXX_COROUTINES_HAVE_EXPERIMENTAL_HEADER_WITH_FLAG})
+        cmake_pop_check_state()
+    endif()
     mark_as_advanced(_CXX_COROUTINES_HAVE_EXPERIMENTAL_HEADER)
 else()
     set(_CXX_COROUTINES_HAVE_EXPERIMENTAL_HEADER FALSE)
@@ -188,7 +206,7 @@ if(CXX_COROUTINES_HAVE_COROUTINES)
             int result;
             present get_return_object() { return present{*this}; }
             @CXX_COROUTINES_NAMESPACE@::suspend_never initial_suspend() { return {}; }
-            @CXX_COROUTINES_NAMESPACE@::suspend_always final_suspend() { return {}; }
+            @CXX_COROUTINES_NAMESPACE@::suspend_always final_suspend() noexcept { return {}; }
             void return_value(int i) { result = i; }
             void unhandled_exception() {}
           };
@@ -223,7 +241,7 @@ if(CXX_COROUTINES_HAVE_COROUTINES)
 
     if(NOT CXX_COROUTINES_NO_AWAIT_NEEDED)
         # Add the -fcoroutines-ts (or /await) flag
-        set(CMAKE_REQUIRED_FLAGS "${_CXX_COROUTINES_STD17} ${_CXX_COROUTINES_AWAIT}")
+        set(CMAKE_REQUIRED_FLAGS "${_CXX_COROUTINES_EXTRA_FLAGS}")
         check_cxx_source_compiles("${code}" CXX_COROUTINES_AWAIT_NEEDED)
         set(can_link ${CXX_COROUTINES_AWAIT_NEEDED})
     endif()
@@ -235,7 +253,7 @@ if(CXX_COROUTINES_HAVE_COROUTINES)
         if(CXX_COROUTINES_NO_AWAIT_NEEDED)
             # Nothing to add...
         elseif(CXX_COROUTINES_AWAIT_NEEDED)
-            target_compile_options(std::coroutines INTERFACE ${_CXX_COROUTINES_AWAIT})
+            target_compile_options(std::coroutines INTERFACE ${_CXX_COROUTINES_EXTRA_FLAGS})
         endif()
     else()
         set(CXX_COROUTINES_HAVE_COROUTINES FALSE)
@@ -247,5 +265,5 @@ cmake_pop_check_state()
 set(Coroutines_FOUND ${_found} CACHE BOOL "TRUE if we can compile and link a program using std::coroutines" FORCE)
 
 if(Coroutines_FIND_REQUIRED AND NOT Coroutines_FOUND)
-    message(FATAL_ERROR "Cannot compile simple program using std::coroutines")
+    message(FATAL_ERROR "Cannot compile simple program using std::coroutines. Is C++17 or later activated?")
 endif()

lib/CMakeLists.txt

@@ -159,7 +159,7 @@ target_include_directories(cppcoro PUBLIC
 	$<INSTALL_INTERFACE:include>)
 target_compile_features(cppcoro PUBLIC cxx_std_20)
 
-find_package(Coroutines REQUIRED)
+find_package(Coroutines COMPONENTS Experimental Final REQUIRED)
 target_link_libraries(cppcoro PUBLIC std::coroutines)
 
 install(TARGETS cppcoro EXPORT cppcoroTargets