Require C23 and test __STDC_VERSION__ >= 202000L

Fix #2352.

CMakeLists.txt

@@ -33,13 +33,17 @@ include(FetchContent)
 include(FindPkgConfig)
 include(AvifExternalProjectUtils)
 
-option(AVIF_ENABLE_NODISCARD "Add [[nodiscard]] to some functions. CMake must be at least 3.21 to request C23." OFF)
+option(AVIF_ENABLE_NODISCARD "Add [[nodiscard]] to some functions. CMake must be at least 3.21 to force C23." OFF)
 
 # Set C99 as the default
-if(AVIF_ENABLE_NODISCARD AND CMAKE_VERSION VERSION_GREATER_EQUAL 3.21.0)
+if(AVIF_ENABLE_NODISCARD)
     # [[nodiscard]] requires C23. The supported value 23 for CMAKE_C_STANDARD
     # was added in CMake version 3.21.
+    if(CMAKE_VERSION VERSION_LESS 3.21.0)
+        message(FATAL_ERROR "CMake must be at least 3.21 to force C23, bailing out")
+    endif()
     set(CMAKE_C_STANDARD 23)
+    set(CMAKE_C_STANDARD_REQUIRED ON)
 else()
     set(CMAKE_C_STANDARD 99)
 endif()
@@ -585,6 +589,7 @@ if(AVIF_LIB_USE_CXX OR (AVIF_BUILD_APPS AND AVIF_ENABLE_EXPERIMENTAL_GAIN_MAP) O
     if(AVIF_ENABLE_NODISCARD)
         # [[nodiscard]] requires C++17.
         set(CMAKE_CXX_STANDARD 17)
+        set(CMAKE_CXX_STANDARD_REQUIRED ON)
     else()
         set(CMAKE_CXX_STANDARD 14)
     endif()

include/avif/avif.h

@@ -48,7 +48,14 @@ extern "C" {
 #define AVIF_API
 #endif // defined(AVIF_DLL)
 
-#if (defined(__cplusplus) && __cplusplus >= 201703L) || (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L)
+// [[nodiscard]] requires C++17 and C23.
+//
+// If the -std=c2x or -std=gnu2x option is specified, __STDC_VERSION__ is
+//   * 202000L in GCC 13.2.0, Clang 16.0.6, and Apple Clang 15.0.0, or
+//   * 202311L in Clang 19.0.0git.
+// If the /std:clatest option is specified, __STDC_VERSION__ is
+//   * 202312L in Microsoft Visual Studio 17.10.5.
+#if (defined(__cplusplus) && __cplusplus >= 201703L) || (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202000L)
 #define AVIF_NODISCARD [[nodiscard]]
 #else
 // Starting with 3.9, clang allows defining the warn_unused_result attribute for enums.