Fix build error on Windows ARM64 with Clang due to missing "arm64_neon.h"

[komainu8]

Describe the problem

The changes introduced in #1317 cause the following build error:

C:/a/_temp/msys64/clangarm64/include/xsimd/memory/../config/../types/./xsimd_neon_register.hpp:22:10: fatal error: 'arm64_neon.h' file not found
   22 | #include <arm64_neon.h>
      |          ^~~~~~~~~~~~~~
1 error generated.

Error log URL: https://github.com/groonga/groonga/actions/runs/25546327568/job/74983334286#step:8:2549

Causes

The issue occurs because Clang does not currently provide arm64_neon.h.
When using Clang on Windows ARM64 (e.g., via MSYS2 with clang or clang-cl), the build fails because the compiler cannot find this header.

Currently, arm64_neon.h is included if #if defined(_WIN32) && XSIMD_WITH_NEON64 is true.
While this works for MSVC, it breaks the build for Clang-based environments on Windows ARM64, which still expect the standard arm_neon.h.

Environment

OS: Windows (ARM64)

Compiler: Clang (via MSYS2 with clang or clang-cl)

xsimd version: 14.0.2 or later

Suggestion

On Windows ARM64, Clang can be used via MSYS2 with clang or clang-cl in addition to MSVC. Since arm64_neon.h is a Microsoft-specific header, I believe it should be guarded by _MSC_VER instead of _WIN32.

How about changing the inclusion condition as follows?

--- a/include/xsimd/types/xsimd_neon_register.hpp
+++ b/include/xsimd/types/xsimd_neon_register.hpp
@@ -18,7 +18,7 @@
 #include "./xsimd_register.hpp"
 
 #if XSIMD_WITH_NEON
-#if defined(_WIN32) && XSIMD_WITH_NEON64
+#if defined(_MSC_VER) && XSIMD_WITH_NEON64
 #include <arm64_neon.h>
 #else
 #include <arm_neon.h>

If this looks good to you, I would be happy to submit a Pull Request with this patch.

Additional information

For reference, there is an ongoing effort to provide a wrapper for arm64_neon.h in LLVM (llvm/llvm-project#196014), but it has not been merged yet.

[mtl1979]

Clang defines _MSC_VER, so this workaround doesn't work.

[komainu8]

Thank you for pointing that out.
I see—so to distinguish them correctly, I should also use !defined(__clang__).

For example:

--- a/include/xsimd/types/xsimd_neon_register.hpp
+++ b/include/xsimd/types/xsimd_neon_register.hpp
@@ -18,7 +18,7 @@
 #include "./xsimd_register.hpp"
 
 #if XSIMD_WITH_NEON
-#if defined(_WIN32) && XSIMD_WITH_NEON64
+#if defined(_MSC_VER) && !defined(__clang__) && XSIMD_WITH_NEON64
 #include <arm64_neon.h>
 #else
 #include <arm_neon.h>

This should ensure that any Clang-based environment (including clang-cl) uses the standard arm_neon.h.

[mtl1979]

Thank you for pointing that out. I see—so to distinguish them correctly, I should also use !defined(__clang__).

Exactly... That way it will work on both current and future versions of Clang and Clang-cl.

[komainu8]

@serge-sans-paille Thank you for merging this.
@mtl1979 Thank you for the review and comment.