[Bug] macOS MLX build fails because warnings in MLX headers are treated as errors (-Werror)

[flamby]

Hi,

Building vllm.cpp on macOS fails while compiling metal_mlx_provider.mm because compiler warnings originating from MLX headers are promoted to errors by -Werror.

I reproduced the issue consistently across multiple macOS 26.6 releases and multiple MLX versions, including older MLX releases.

Reproduced configurations

macOS:

• 26.6.1
• 26.6.0

MLX:

• 0.32.0
• 0.31.2
• 0.30.6

The same compilation failure occurs with all of these combinations, on both master and latest release of vllm.cpp

Build error

In file included from /Users/angusyoung/dev/vllm.cpp/src/vt/metal/metal_mlx_provider.mm:59: /usr/local/include/mlx/allocator.h:39:36: error: unused parameter 'ptr' [-Werror,-Wunused-parameter] 39 | virtual Buffer make_buffer(void* ptr, size_t size) { | ^ /usr/local/include/mlx/allocator.h:39:48: error: unused parameter 'size' [-Werror,-Wunused-parameter] 39 | virtual Buffer make_buffer(void* ptr, size_t size) { | ^ /usr/local/include/mlx/allocator.h:42:31: error: unused parameter 'buffer' [-Werror,-Wunused-parameter] 42 | virtual void release(Buffer buffer) {} | ^ In file included from /Users/angusyoung/dev/vllm.cpp/src/vt/metal/metal_mlx_provider.mm:60: In file included from /usr/local/include/mlx/array.h:12: In file included from /usr/local/include/mlx/dtype.h:9: In file included from /usr/local/include/mlx/types/complex.h:5: In file included from /usr/local/include/mlx/types/half_types.h:32: /usr/local/include/mlx/types/bf16.h:29:3: error: definition of implicit copy assignment operator for '_MLX_BFloat16' is deprecated because it has a user-declared copy constructor [-Werror,-Wdeprecated-copy] 29 | _MLX_BFloat16(_MLX_BFloat16 const&) = default; | ^ /usr/local/include/mlx/types/bf16.h:38:19: note: in implicit copy assignment operator for 'mlx::core::_MLX_BFloat16' first required here 38 | return (*this = _MLX_BFloat16(x)); | ^ 4 errors generated. make[2]: *** [CMakeFiles/vllm.dir/src/vt/metal/metal_mlx_provider.mm.o] Error 1 make[2]: *** Waiting for unfinished jobs.... make[1]: *** [CMakeFiles/vllm.dir/all] Error 2 make: *** [all] Error 2

Analysis

The failure appears to be caused by MLX headers being compiled under the same warning policy as vllm.cpp itself.

In particular, -Werror turns warnings originating from third-party MLX headers into fatal compilation errors:

-Werror,-Wunused-parameter
-Werror,-Wdeprecated-copy

The warnings originate from:

/usr/local/include/mlx/allocator.h
/usr/local/include/mlx/types/bf16.h

while compiling:

src/vt/metal/metal_mlx_provider.mm

Since the issue is reproducible with MLX 0.30.6, 0.31.2 and 0.32.0, it does not appear to be a regression specific to the latest MLX release.

Likewise, it is reproducible on both macOS 26.6.0 and 26.6.1.

Environment

• macOS: 26.6.1, also reproduced on 26.6.0
• Architecture: Apple Silicon
• Compiler: Apple Clang
• Backend: Metal / MLX
• MLX versions tested: 0.32.0, 0.31.2, 0.30.6
• MLX headers: /usr/local/include/mlx
• Failing translation unit: src/vt/metal/metal_mlx_provider.mm

Thanks