Add a cmake check whether std::expected is supported or not

Note. Perhaps a wrong way. Please let me know if there is something better.

CMakePresets.json

@@ -1,5 +1,5 @@
 {
-  "version": 3,
+  "version": 6,
   "cmakeMinimumRequired": {
     "major": 3,
     "minor": 17,

cmake/helpers.cmake

@@ -0,0 +1,10 @@
+function(has_expected OUT_RESULT)
+    try_compile(COMPILE_RESULT
+        ${CMAKE_BINARY_DIR}/test_apps
+        ${CMAKE_SOURCE_DIR}/cmake/try-compile-apps/expected.cpp
+        CXX_STANDARD 23
+        CXX_STANDARD_REQUIRED TRUE
+        CXX_EXTENSIONS FALSE
+    )
+    set(${OUT_RESULT} ${COMPILE_RESULT} PARENT_SCOPE)
+endfunction()

cmake/try-compile-apps/expected.cpp

@@ -0,0 +1,12 @@
+#include <expected>
+#include <string>
+
+int main(int /*argc*/, char* /*argv*/[])
+{
+    using Expected = std::expected<int, std::string>;
+
+    Expected result{0};
+    Expected unxepectedResult = std::unexpected(std::string{"foo"});
+
+    return result != unxepectedResult ? *result : -1;
+}

test/CMakeLists.txt

@@ -1,3 +1,8 @@
+include(${CMAKE_SOURCE_DIR}/cmake/helpers.cmake)
+
+has_expected(HAS_EXPECTED_RESULT)
+message(STATUS "Has std::expected support: ${HAS_EXPECTED_RESULT}")
+
 find_package(Catch2 CONFIG REQUIRED)
 find_package(fmt CONFIG REQUIRED)
 find_package(spdlog CONFIG REQUIRED)