Fix compilation with MSVC 2019 in C++17 mode

Author: andreasbuhr

cmake/FindCoroutines.cmake

@@ -157,6 +157,10 @@ if(find_final)
         set(_CXX_COROUTINES_HAVE_HEADER ${_CXX_COROUTINES_HAVE_HEADER_WITH_FLAG})
         cmake_pop_check_state()
     endif()
+    check_cxx_source_compiles("#include <coroutine> \n typedef std::suspend_never blub; \nint main() {} " _CXX_COROUTINES_FINAL_HEADER_COMPILES)
+    if(NOT _CXX_COROUTINES_FINAL_HEADER_COMPILES)
+        set(_CXX_COROUTINES_HAVE_HEADER FALSE)
+    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

include/cppcoro/config.hpp

@@ -150,4 +150,17 @@
 # define CPPCORO_CPU_CACHE_LINE 64
 #endif
 
+#if CPPCORO_COMPILER_MSVC
+    #if __has_include(<coroutine>)
+    #include <yvals_core.h>
+    #ifdef __cpp_lib_coroutine
+    #define CPPCORO_COROHEADER_FOUND_AND_USABLE
+    #endif
+    #endif
+#else
+    #if __has_include(<coroutine>)
+    #define CPPCORO_COROHEADER_FOUND_AND_USABLE
+    #endif
+#endif
+
 #endif

include/cppcoro/coroutine.hpp

@@ -1,9 +1,11 @@
 #ifndef CPPCORO_COROUTINE_HPP_INCLUDED
 #define CPPCORO_COROUTINE_HPP_INCLUDED
 
-#if __has_include(<coroutine>)
+#include <cppcoro/config.hpp>
 
-#include <coroutine>
+#ifdef CPPCORO_COROHEADER_FOUND_AND_USABLE
+
+#includue <coroutine>
 
 namespace cppcoro {
   using std::coroutine_handle;