Fix co_composed crash on MSVC

[annihilatorq]

When built with the Visual Studio generator in Release (/O2), MSVC may emit an aligned SSE store (movaps) for co_composed block storage that is only 8-byte aligned. At runtime this results with STATUS_ACCESS_VIOLATION; in the failing case the fault appears to come from a misaligned movaps store triggering a GP fault

I am reproducing this with

• blazeauth/aero at 0d125b94dbd3b35d4b8a4b67483e109a77794c3e
• Visual Studio 17 2022
• Visual Studio 18 2026
• Release config
• examples/chilkat.exe (the same issue also reproduces with other examples)
• any version of asio from 1.32 (not tested below) to 1.38

I don't see a problem with /O1, so far, it only appears on /O2
when using clang-cl with the same configuration as above, there are no errors

This looks like an MSVC backend/codegen bug rather than an asio logic bug, because generated code uses movaps even though the destination address is only 8-byte aligned in the failing case

The fix is to force the block storage used here to be at least 16-byte aligned on MSVC, which avoids the bad codegen assumption and makes the generated aligned SSE access safe:

#if defined(ASIO_MSVC) && !defined(__clang__)
  union alignas(16) block
#else
  union block
#endif

P.S.
screenshot of the failing movaps store:

[annihilatorq]

... or perhaps if this is considered a bug in the compiler's codegen, then asio will not take any action, and it would be better to report this to the MSVC as their bug?