Xcode 26.2: Metal compile warning 'constexpr if is a C++17 extension' in generated steel_attention.h

[atlascodesai]

Environment

• Xcode 26.2 (Build 17C52)
• macOS 26.2 (Build 25C56)
• Building from an Xcode project that pulls mlx-swift via SwiftPM

mlx-swift revision

• main @ b2796b8 (from Package.resolved)

What happens
During a clean build, the Metal compilation for the generated Steel attention kernels emits warnings like this (seen on both macOS and iOS Simulator builds):

In file included from /tmp/LocalAIChat-DD-macOS/SourcePackages/checkouts/mlx-swift/Source/Cmlx/mlx-generated/metal/steel/attn/kernels/steel_attention.metal:7:
/tmp/LocalAIChat-DD-macOS/SourcePackages/checkouts/mlx-swift/Source/Cmlx/mlx-generated/metal/steel/attn/kernels/../../../steel/attn/kernels/steel_attention.h:349:16: warning: constexpr if is a C++17 extension [-Wc++17-extensions]
            if constexpr (is_bool) {
               ^
/tmp/LocalAIChat-DD-macOS/SourcePackages/checkouts/mlx-swift/Source/Cmlx/mlx-generated/metal/steel/attn/kernels/../../../steel/attn/kernels/steel_attention.h:419:14: warning: constexpr if is a C++17 extension [-Wc++17-extensions]
          if constexpr (BD == 128) {
             ^
/tmp/LocalAIChat-DD-macOS/SourcePackages/checkouts/mlx-swift/Source/Cmlx/mlx-generated/metal/steel/attn/kernels/../../../steel/attn/kernels/steel_attention.h:429:14: warning: constexpr if is a C++17 extension [-Wc++17-extensions]
          if constexpr (BD == 128) {
             ^
3 warnings generated.

Repro

• Build any Xcode project that depends on mlx-swift at the revision above.
• For example (from my app project):
  • xcodebuild -project "Local AI Chat.xcodeproj" -scheme "Seek Deep Local AI" -destination "generic/platform=macOS" -derivedDataPath /tmp/LocalAIChat-DD-macOS -quiet clean build
  • xcodebuild -project "Local AI Chat.xcodeproj" -scheme "Seek Deep Local AI" -destination "generic/platform=iOS Simulator" -derivedDataPath /tmp/LocalAIChat-DD-iOS -quiet clean build

Question
Is there an intended way to silence/avoid these warnings under Xcode 26.2 (e.g. Metal language standard flags), or should the generated MSL headers avoid using if constexpr?

[davidkoski]

This is from the mlx (core) side -- @awni do you know about this? In the CmakeLists I see:

• -Wno-c++17-extensions

here is one of the snippets:

template <int start, int stop, int step, typename F>
constexpr void const_for_loop(F f) {
  if constexpr (start < stop) {

This is coming from the metal compiler at build time.

[awni]

This is coming from the metal compiler at build time.

Are you sure it's at build time? I thought those kernels were in the JIT? Pre-built kernels should have -Wno-c++17-extensions which silences those warnings. I don't know how to silence them when compiling at run-time. You can't pass arbitrary options to the compiler.

[davidkoski]

Yes these are from the couple of kernels that are not JIT. I read that compiler flag backward -- that is in the mlx build but NOT in the mlx-swift build as we can't control the build arguments there as this is implicit in the SwiftPM handling of .metal files.

I think that explains it!

So @atlascodesai nothing that we can do today, though if you were to use the CMake or xcodeproj builds they would probably not emit that as they have more control. (xcodeproj might but we could suppress it)

[peterzhang0216]

Same issue here. Using mlx-swift-lm 2.30.6 (mlx-swift 0.30.6) with Xcode 26.0 on macOS 26.0. Getting the same constexpr if is a C++17 extension warnings in steel_attention.h and integral_constant.h during every build. Would appreciate a fix upstream.