Detect toolchains that don't support co_await in Boost.Asio

Author: wravery

cmake/test_boost_beast.cpp

@@ -0,0 +1,15 @@
+// This is a dummy program that just needs to compile to tell us if Boost.Asio
+// supports co_await and Boost.Beast is installed.
+
+#include <boost/asio/use_awaitable.hpp>
+
+#include <boost/beast.hpp>
+
+int main()
+{
+#ifdef BOOST_ASIO_HAS_CO_AWAIT
+	return 0;
+#else
+	#error BOOST_ASIO_HAS_CO_AWAIT is undefined
+#endif
+}

samples/CMakeLists.txt

@@ -12,11 +12,17 @@ if(GRAPHQL_BUILD_HTTP_SAMPLE)
   find_package(Boost QUIET)
   if(Boost_FOUND)
     if(Boost_VERSION VERSION_GREATER_EQUAL "1.81.0")
-      if(EXISTS "${Boost_INCLUDE_DIR}/boost/beast.hpp")
+      try_compile(TEST_RESULT
+        ${CMAKE_CURRENT_BINARY_DIR}
+        ${CMAKE_CURRENT_SOURCE_DIR}/../cmake/test_boost_beast.cpp
+        CMAKE_FLAGS -DINCLUDE_DIRECTORIES:STRING=${Boost_INCLUDE_DIR}
+        CXX_STANDARD 20)
+
+      if(TEST_RESULT)
         message(STATUS "Using Boost.Beast ${Boost_VERSION}")
         add_subdirectory(proxy)
       else()
-        message(WARNING "GRAPHQL_BUILD_HTTP_SAMPLE requires the Boost.Beast header-only library, but it was not found in ${Boost_INCLUDE_DIR}.")
+        message(WARNING "GRAPHQL_BUILD_HTTP_SAMPLE requires the Boost.Beast header-only library and a toolchain that supports co_await in Boost.Asio.")
       endif()
     else()
       message(WARNING "GRAPHQL_BUILD_HTTP_SAMPLE requires the Boost.Beast header-only library >= 1.81.0, but only ${Boost_VERSION} was found in ${Boost_INCLUDE_DIR}.")