fix: make Task promise_type a non-aggregate to prevent Clang 21 SIGSEGV - #2580
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Clang 21 aggregate-initializes Task<T>::promise_type from the coroutine arguments because it declares no user-declared constructor. The first argument initializes the first member, std::optional<T> value, so any implicit conversion to T runs during promise construction, before the coroutine body. If that conversion throws (e.g. nlohmann::json's implicit operator ValueType() on a non-string value), the exception escapes the partially constructed coroutine frame and the process crashes inside _Unwind_Resume during unwinding. Declare a default constructor for Task<T>::promise_type and Task<void>::promise_type so they are no longer aggregates: the promise is then default-constructed, which is the behavior prescribed when no promise constructor takes the coroutine parameters, and matches pre-Clang-21 compilers. AsyncTask::promise_type has no data members and is unaffected. Add a regression test to CoroutineTest.cc: a Task<std::string> whose argument is implicitly convertible to std::string and throws used to segfault at the call, before the test body could run; it now completes normally. Fixes drogonframework#2579
marty1885
approved these changes
Sep 3, 2026
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Fixes #2579
What
drogon::Task<T>::promise_typedeclares no constructor, which makes it an aggregate. Clang 21 aggregate-initializes the promise from the coroutine arguments, positionally: the first argument initializes the first member,std::optional<T> value. If the argument is implicitly convertible toTand theconversion throws (most commonly
nlohmann::json's implicitoperator ValueType()on a mismatched value, throwingtype_error.302), the exception is thrown during promise construction — before the coroutine body runs — and unwinding from the partially constructed coroutine frame crashes the process inside_Unwind_Resume(SIGSEGV; the exception is never catchable).How
Add a user-declared default constructor to
Task<T>::promise_type(andTask<void>::promise_type, same latent hazard), making it a non-aggregate. Compilers then fall back to default-constructing the promise — exactly the behavior prescribed when no promise constructor takes the coroutine parameters, and thebehavior of pre-Clang-21 compilers.
AsyncTask::promise_typehas no data members and is unaffected.Test
PromiseNotAggregateInitializedFromArgsinCoroutineTest.cc: aTask<std::string>whose argument is implicitly convertible tostd::stringand throws. Before this change the process segfaulted at the call, before the test body could run; after it, the coroutine completes and returns normally.Verified on Apple clang 21.0.0 (clang-2100.1.1.101), macOS 26.5.2 (arm64), C++20. A standalone minimal repro and the full crash analysis are in #2579.