[veljjanoski]

Fixes #2113.

What

On 32-bit x86, code built with Clang before 23 crashes when it resumes a __synthetic_coro_frame (added in #1974). That covers every co_await that goes through connect_awaitable, for example sync_wait of an exec::task.

Why

Before LLVM 23, Clang lowers coroutine_handle::resume() and destroy() to a fastcc call through the frame's function pointers (llvm/llvm-project#198943 changed that to the C convention). On i686, fastcc passes the frame address in ECX. The functions stored in __synthetic_coro_frame used the default C convention and read their argument from the stack, so __resume got a garbage pointer. On x86-64 and ARM both conventions pass the first argument in the same register, so only i686 is affected.

How

• A new macro in coroutine.hpp, STDEXEC_CORO_FRAME_CALLCONV, is __attribute__((__fastcall__)) for Clang < 23 on i686 and empty otherwise. fastcall also takes the first argument in ECX.
• It is applied to __synthetic_coro_frame::__callback_fn_t, __noop_fn and __connect_await::__promise::__resume.
• It can be predefined, for example as empty for a Clang that has the LLVM change backported.

The frames in the STDEXEC_MSVC_CORO_DESTROY_BUG_WORKAROUND block are only compiled by MSVC, where the macro is empty, so they are unchanged.

Verification

Linux x86-64 host (Docker, Debian 12), Clang 21.1 with libstdc++ 12, -m32, Debug, the same flags as the 32-bit CI job:

	main (9d292a4)	this PR
exec::task example from #2113, -O0 and -O2	segfault	runs
test.stdexec (641 test cases)	4 fail (connect_awaitable)	all pass
test.exec (454 test cases)	35 crash, 1 fails	all pass

The preprocessed #2113 example is the same as on main, apart from whitespace, for Clang 21 x86-64, GCC 12 x86-64, GCC 12 -m32, and Clang 21 -m32 with -DSTDEXEC_CORO_FRAME_CALLCONV=. clang-format 21 is clean.

There is no new test, because the existing tests already fail on i686 with Clang < 23. The 32-bit CI job uses GCC 14, so CI never runs them with Clang. I can add a Clang 32-bit job here or in a follow-up if you want one.

Not tested: clang-cl on 32-bit Windows, Release builds of the test suites, the other CI configurations.

AI-usage disclosure: Claude (Anthropic) was used as assistant.

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