CMakeLists.txt: Guard for static_assert

[Dave-Allured]

• Add CMake guard for C11 static_assert, which was added in libavif 1.4.2.
• This will not fix missing support, but will catch insufficient compiler at configure time.
• 1.4.2 fails on static_assert with older versions of clang #3319

I am not experienced with CMake. Please adjust or reposition this patch, as appropriate. Thanks.

[wantehchang]

cc: @vrabaud

Dave: Thank you for the pull request. We can try the following simple change:

diff --git a/CMakeLists.txt b/CMakeLists.txt
index c9a7f82d..f29d4b39 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -51,10 +51,10 @@ option(AVIF_ENABLE_NODISCARD "Add [[nodiscard]] to some functions." OFF)
 if(AVIF_ENABLE_NODISCARD)
     # [[nodiscard]] requires C23.
     set(CMAKE_C_STANDARD 23)
-    set(CMAKE_C_STANDARD_REQUIRED ON)
 else()
     set(CMAKE_C_STANDARD 11)
 endif()
+set(CMAKE_C_STANDARD_REQUIRED ON)
 
 # SOVERSION scheme: MAJOR.MINOR.PATCH
 #   If there was an incompatible interface change:

If we want to avoid setting CMAKE_C_STANDARD globally, I would prefer to use c_std_11 instead of c_static_assert because we may use other C11 features such as alignas in the future. Also, we would need to call target_compile_features(... PRIVATE c_std_11) on all the C targets, not just avif.

[Dave-Allured]

@wantehchang I understand your concerns. I think you should avoid setting CMAKE_C_STANDARD globally, only for this purpose. I am concerned about possible wider disruption, which seems unnecessary right now. My intent was a very minor improvement in diagnostic messaging for legacy compilers. I was looking for a way to set c_static_assert globally, but I did not find this in CMake documentation.

Let's cancel this PR and not disturb the code base. I have a viable solution for the problem that I raised in #3319.

[wantehchang]

Dave: I will look into avoiding setting CMAKE_C_STANDARD globally. I believe target_compile_features() is the recommended method:

https://cmake.org/cmake/help/latest/manual/cmake-compile-features.7.html#requiring-language-standards

I need to figure out a way to use either c_std_11 or c_std_23 depending on whether AVIF_ENABLE_NODISCARD is enabled.

[Dave-Allured]

I have reconsidered this and checked (briefly) the CMake documentation. c_static_assert is a fine-grained approach which targets only the code base in its current state at 1.4.2. Meta-options such as c_std_11 carry more requirements, with a slight risk of disturbing current builds in some contexts. I see this as unnecessary disturbance for new language features which are not yet used or needed.

https://cmake.org/cmake/help/latest/prop_gbl/CMAKE_C_KNOWN_FEATURES.html

Therefore I recommend only two conservative alternatives at this time.

• Either apply c_static_assert as I originally suggested,
• Or make no change at this time, wait for new code requirements before adding new compiler requirements.

This addition of CMake c_static_assert is a simple one-line update. You can easily discard or change this at any time in the future, as code requirements change.