Ensure 16-byte block alignment in co_composed's implementation on MSVC.

Author: annihilatorq

include/asio/co_composed.hpp

@@ -994,7 +994,13 @@ class co_composed_promise
 
   union block
   {
+#if defined(ASIO_MSVC) && !defined(__clang__)
+    // Force 16-byte alignment as std::max_align_t is only 8-byte aligned on
+    // MSVC, but the compiler may emit aligned SSE stores into the storage.
+    alignas(16) std::max_align_t max_align;
+#else // defined(ASIO_MSVC) && !defined(__clang__)
     std::max_align_t max_align;
+#endif // defined(ASIO_MSVC) && !defined(__clang__)
     alignas(allocator_type) char pad[alignof(allocator_type)];
   };