Faster check of std::optional header using try_compile, which works w…

…ithout global variable CMAKE_CXX_STANDARD

cmake/CheckCXXOptional.cmake

@@ -1,57 +1,65 @@
 cmake_minimum_required(VERSION 3.14)
-function(check_optional_compiles)
-    include(CheckCXXSourceCompiles)
-    if(MSVC)
-        set(CMAKE_REQUIRED_FLAGS     "/std:c++17")
-    else()
-        set(CMAKE_REQUIRED_FLAGS     "-std=c++17")
-    endif()
-    check_cxx_source_compiles("
-        // Include optional or experimental/optional
-        namespace h5pp{}
-        #if __has_include(<optional>)
-        #include <optional>
-        #elif __has_include(<experimental/optional>)
-        #include <experimental/optional>
-        namespace h5pp{
-            namespace std{
-                constexpr const std::experimental::nullopt_t &nullopt = std::experimental::nullopt ;
-                template<typename T> using std::optional = std::experimental::optional<T>;
+function(try_compile_std_optional)
+    if(NOT OPTIONAL_COMPILES)
+        file(WRITE ${CMAKE_BINARY_DIR}/CMakeFiles/CMakeTmp/check-std-optional.cpp
+            "
+            // Include optional or experimental/optional
+            namespace h5pp{}
+            #if __has_include(<optional>)
+            #include <optional>
+            #elif __has_include(<experimental/optional>)
+            #include <experimental/optional>
+            namespace h5pp{
+                namespace std{
+                    constexpr const std::experimental::nullopt_t &nullopt = std::experimental::nullopt ;
+                    template<typename T> using std::optional = std::experimental::optional<T>;
+                }
             }
-        }
-        #else
-            #error Could not find <optional> or <experimental/optional>
-        #endif
+            #else
+                #error Could not find <optional> or <experimental/optional>
+            #endif
 
 
-        int main(){
-            using namespace h5pp;
-            std::optional<int> optVar = std::nullopt;
-            return 0;
-        }
-        " OPTIONAL_COMPILES)
-    set(OPTIONAL_COMPILES ${OPTIONAL_COMPILES} PARENT_SCOPE)
+            int main(){
+                using namespace h5pp;
+                [[maybe_unused]] std::optional<int> optVar = std::nullopt;
+                return 0;
+            }
+            ")
+        message(STATUS "Performing Test has_std_optional")
+        try_compile(OPTIONAL_COMPILES
+                ${CMAKE_BINARY_DIR}
+                ${CMAKE_BINARY_DIR}/CMakeFiles/CMakeTmp/check-std-optional.cpp
+                OUTPUT_VARIABLE OPTIONAL_COMPILES_OUT
+                CXX_STANDARD 17
+                CXX_EXTENSIONS OFF
+                )
+        if(OPTIONAL_COMPILES)
+            file(APPEND ${CMAKE_BINARY_DIR}/CMakeFiles/CMakeOutput.log "${OPTIONAL_COMPILES_OUT}")
+            message(STATUS "Performing Test has_std_optional - Success")
+        else()
+            file(APPEND ${CMAKE_BINARY_DIR}/CMakeFiles/CMakeError.log "${OPTIONAL_COMPILES_OUT}")
+            message(STATUS "Performing Test has_std_optional - Failed")
+        endif()
+        set(OPTIONAL_COMPILES ${OPTIONAL_COMPILES} PARENT_SCOPE)
+    endif()
 endfunction()
 
 function(CheckCXXOptional)
     cmake_policy(SET CMP0075 NEW)
     include(CheckIncludeFileCXX)
-    check_optional_compiles()
+    try_compile_std_optional()
     if(NOT OPTIONAL_COMPILES)
         check_include_file_cxx(optional    has_optional)
         check_include_file_cxx(experimental/optional has_experimental_optional )
         if(NOT has_optional AND NOT has_experimental_optional)
-            message(WARNING "
-                Missing <optional> and/or <experimental/optional> headers.\n\
+            message(FATAL_ERROR "
+                Missing both <optional> and <experimental/optional> headers.\n\
                 Consider using a newer compiler (GCC 6 or above, Clang 5 or above),\n\
-                or checking the compiler flags. If using Clang, pass the variable \n\
+                or checking the compiler flags. If using Clang, try passing the variable \n\
                 GCC_TOOLCHAIN=<path> \n\
-                where path is the install directory of a recent GCC.
+                where path is the install directory of a recent GCC to use its standard library.
         ")
         endif()
-        message(FATAL_ERROR "Unable to compile with <optional> or <experimental/optional> headers")
     endif()
-
-
-
 endfunction()