Apple Clang miscompiles coroutine exception handling at -O2 with default visibility

[mvandeberg]

Summary

Two when_any tests fail on Apple Clang Release builds (-O2 and above) with default symbol visibility. The compiler-generated coroutine state machine silently drops exceptions thrown in the coroutine body, causing the run_async error handler to never fire.

Failing tests:

• testEmptyVectorThrows — when_any.cpp:1348
• testEmptyRangeThrows — when_any.cpp:2157

Both verify that when_any(empty_range) delivers std::invalid_argument through the run_async error handler.

Environment

• Apple Clang 17.0.0 (clang-1700.0.13.3), arm64-apple-darwin25.3.0
• Failure boundary: passes at -O1, fails at -O2 and above
• -DNDEBUG and debug symbols have no effect

Root cause

Assembly diff between -O2 (failing) and -O2 -fvisibility=hidden (passing) shows the difference: with default visibility, template instantiation symbols (weak external linkage) are accessed through the GOT (@GOTPAGE/@GOTPAGEOFF). With hidden visibility, they use direct references (@PAGE/@PAGEOFF). The coroutine optimizer generates incorrect exception tables when these function references are GOT-indirect.

The bug is in compiler-generated code (the coroutine resume function's implicit try/catch that routes to unhandled_exception()), not in user-written code. This was confirmed by applying __attribute__((optnone)) to invoke_impl — the handler dispatch function — which had no effect, proving the exception never reaches it.

Approaches tried (all failed)

Approach	Result
__attribute__((optnone)) on invoke_impl	FAIL
__attribute__((noinline)) on exception() / unhandled_exception()	FAIL
volatile bool has_ep_	FAIL
Cache exception_ptr locally	FAIL
asm volatile memory barrier on function pointer	FAIL
co_await noop IoAwaitable before throw	FAIL
[[noreturn]] __attribute__((noinline)) throw wrapper	FAIL

Current workaround

Build with -fvisibility=hidden on Apple Clang. This eliminates GOT indirection for weak template symbols and produces correct exception routing at all optimization levels.

Related upstream issues

• LLVM #61900 — incorrect code for coroutines when unhandled_exception() exits via exception
• LLVM #105595 — runtime crash at -O2 in coroutines with co_await
• CWG2935 / PR #177628 — coroutine startup exception safety

TODO

• When a new Apple Clang version ships, test with default visibility at -O2 to check if the upstream fix has landed. Reproduce with: cmake -DCMAKE_CXX_FLAGS="-O2 -DNDEBUG" .. && cmake --build . && ctest
• If fixed upstream, remove the -fvisibility=hidden workaround from CI and document the minimum Apple Clang version