then() continuation not playing well with std::bind_front

[germandiagogomez]

Hello everyone,

It seems I cannot use std::bind_front with then() continuations when an equivalent (AFAIK) lambda will do.

Example that fails

auto wsock = response.acceptWebSocket(headers);
        return wsock->receive().then(std::bind_front(
            &WebsocketService::handleMessage, this, std::move(wsock)));

Example that works

auto wsock = response.acceptWebSocket(headers);
        return wsock->receive().then(
            [this,
             ws = std::move(wsock)](kj::WebSocket::Message &&message) mutable {
              return handleMessage(std::move(ws), std::move(message));
            });

Error

/Users/myuser/.conan/data/capnproto/0.10.3/_/_/package/13b953c004db54b2d9cf274425004a3d521cf731/include/kj/async-inl.h:504:22: error: no matching function for call to object of type 'std::__perfect_forward_impl<std::__bind_front_op, std::__tuple_types<kj::Promise<void> (WebsocketService::*)(kj::Own<kj::WebSocket> &&, kj::OneOf<kj::String, kj::Array<unsigned char>, kj::WebSocket::Close> &&), WebsocketService *, kj::Own<kj::WebSocket>>, std::__tuple_indices<0, 1, 2>>'
    typedef decltype(func(instance<ParamTypes>()...)) ReturnType;
                     ^~~~
/Users/myuser/.conan/data/capnproto/0.10.3/_/_/package/13b953c004db54b2d9cf274425004a3d521cf731/include/kj/async-inl.h:1085:76: note: in instantiation of function template specialization 'kj::_::GetFunctorStartAddress<kj::OneOf<kj::String, kj::Array<unsigned char>, kj::WebSocket::Close> &&>::apply<std::__perfect_forward_impl<std::__bind_front_op, std::__tuple_types<kj::Promise<void> (WebsocketService::*)(kj::Own<kj::WebSocket> &&, kj::OneOf<kj::String, kj::Array<unsigned char>, kj::WebSocket::Close> &&), WebsocketService *, kj::Own<kj::WebSocket>>, std::__tuple_indices<0, 1, 2>> &>' requested here
  void* continuationTracePtr = _::GetFunctorStartAddress<_::FixVoid<T>&&>::apply(func);
                                                                           ^
../programs/websocket-server/main.cpp:65:33: note: in instantiation of function template specialization 'kj::Promise<kj::OneOf<kj::String, kj::Array<unsigned char>, kj::WebSocket::Close>>::then<std::__perfect_forward_impl<std::__bind_front_op, std::__tuple_types<kj::Promise<void> (WebsocketService::*)(kj::Own<kj::WebSocket> &&, kj::OneOf<kj::String, kj::Array<unsigned char>, kj::WebSocket::Close> &&), WebsocketService *, kj::Own<kj::WebSocket>>, std::__tuple_indices<0, 1, 2>>, kj::_::PropagateException>' requested here
        return wsock->receive().then(std::bind_front(
                                ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__functional/perfect_forward.h:48:46: note: candidate function template not viable: expects an rvalue for object argument
    _LIBCPP_INLINE_VISIBILITY constexpr auto operator()(_Args&&... __args) &&
                                             ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__functional/perfect_forward.h:57:46: note: candidate function template not viable: expects an rvalue for object argument
    _LIBCPP_INLINE_VISIBILITY constexpr auto operator()(_Args&&... __args) const&&
                                             ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__functional/perfect_forward.h:36:46: note: candidate template ignored: substitution failure [with _Args = <kj::OneOf<kj::String, kj::Array<unsigned char>, kj::WebSocket::Close>>]: no matching function for call to '__call'
    _LIBCPP_INLINE_VISIBILITY constexpr auto operator()(_Args&&... __args) &
                                             ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__functional/perfect_forward.h:42:46: note: candidate template ignored: substitution failure [with _Args = <kj::OneOf<kj::String, kj::Array<unsigned char>, kj::WebSocket::Close>>]: no matching function for call to '__call'
    _LIBCPP_INLINE_VISIBILITY constexpr auto operator()(_Args&&... __args) const&
                                             ^
In file included from ../programs/websocket-server/main.cpp:2:
In file included from /Users/myuser/.conan/data/capnproto/0.10.3/_/_/package/13b953c004db54b2d9cf274425004a3d521cf731/include/kj/async.h:1188:
/Users/myuser/.conan/data/capnproto/0.10.3/_/_/package/13b953c004db54b2d9cf274425004a3d521cf731/include/kj/async-inl.h:505:12: error: no matching function for call to 'from'
    return PtmfHelper::from<ReturnType, Decay<Func>, ParamTypes...>(
           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Users/myuser/.conan/data/capnproto/0.10.3/_/_/package/13b953c004db54b2d9cf274425004a3d521cf731/include/kj/async-inl.h:465:21: note: candidate function [with R = int, C = std::__perfect_forward_impl<std::__bind_front_op, std::__tuple_types<kj::Promise<void> (WebsocketService::*)(kj::Own<kj::WebSocket> &&, kj::OneOf<kj::String, kj::Array<unsigned char>, kj::WebSocket::Close> &&), WebsocketService *, kj::Own<kj::WebSocket>>, std::__tuple_indices<0, 1, 2>>, P = <kj::OneOf<kj::String, kj::Array<unsigned char>, kj::WebSocket::Close> &&>] not viable: no overload of 'operator()' matching 'int (std::__perfect_forward_impl<std::__bind_front_op, std::__tuple_types<kj::Promise<void> (WebsocketService::*)(kj::Own<kj::WebSocket> &&, kj::OneOf<kj::String, kj::Array<unsigned char>, kj::WebSocket::Close> &&), WebsocketService *, kj::Own<kj::WebSocket>>, std::__tuple_indices<0, 1, 2>>::*)(NoInfer<kj::OneOf<kj::String, kj::Array<unsigned char>, kj::WebSocket::Close> &&>)' for 1st argument
  static PtmfHelper from(R (C::*p)(NoInfer<P>...)) { BODY; }
                    ^
/Users/myuser/.conan/data/capnproto/0.10.3/_/_/package/13b953c004db54b2d9cf274425004a3d521cf731/include/kj/async-inl.h:467:21: note: candidate function [with R = int, C = std::__perfect_forward_impl<std::__bind_front_op, std::__tuple_types<kj::Promise<void> (WebsocketService::*)(kj::Own<kj::WebSocket> &&, kj::OneOf<kj::String, kj::Array<unsigned char>, kj::WebSocket::Close> &&), WebsocketService *, kj::Own<kj::WebSocket>>, std::__tuple_indices<0, 1, 2>>, P = <kj::OneOf<kj::String, kj::Array<unsigned char>, kj::WebSocket::Close> &&>] not viable: no overload of 'operator()' matching 'int (std::__perfect_forward_impl<std::__bind_front_op, std::__tuple_types<kj::Promise<void> (WebsocketService::*)(kj::Own<kj::WebSocket> &&, kj::OneOf<kj::String, kj::Array<unsigned char>, kj::WebSocket::Close> &&), WebsocketService *, kj::Own<kj::WebSocket>>, std::__tuple_indices<0, 1, 2>>::*)(NoInfer<kj::OneOf<kj::String, kj::Array<unsigned char>, kj::WebSocket::Close> &&>) const' for 1st argument
  static PtmfHelper from(R (C::*p)(NoInfer<P>...) const) { BODY; }
                    ^
/Users/myuser/.conan/data/capnproto/0.10.3/_/_/package/13b953c004db54b2d9cf274425004a3d521cf731/include/kj/async-inl.h:459:21: note: candidate template ignored: couldn't infer template argument 'F'
  static PtmfHelper from(F p) { BODY; }
                    ^
In file included from ../programs/websocket-server/main.cpp:2:
In file included from /Users/myuser/.conan/data/capnproto/0.10.3/_/_/package/13b953c004db54b2d9cf274425004a3d521cf731/include/kj/async.h:24:
/Users/myuser/.conan/data/capnproto/0.10.3/_/_/package/13b953c004db54b2d9cf274425004a3d521cf731/include/kj/async-prelude.h:156:12: error: no matching function for call to object of type 'std::__perfect_forward_impl<std::__bind_front_op, std::__tuple_types<kj::Promise<void> (WebsocketService::*)(kj::Own<kj::WebSocket> &&, kj::OneOf<kj::String, kj::Array<unsigned char>, kj::WebSocket::Close> &&), WebsocketService *, kj::Own<kj::WebSocket>>, std::__tuple_indices<0, 1, 2>>'
    return func(kj::mv(in));
           ^~~~
/Users/myuser/.conan/data/capnproto/0.10.3/_/_/package/13b953c004db54b2d9cf274425004a3d521cf731/include/kj/async-inl.h:566:57: note: in instantiation of function template specialization 'kj::_::MaybeVoidCaller<kj::OneOf<kj::String, kj::Array<unsigned char>, kj::WebSocket::Close>, kj::Promise<void>>::apply<std::__perfect_forward_impl<std::__bind_front_op, std::__tuple_types<kj::Promise<void> (WebsocketService::*)(kj::Own<kj::WebSocket> &&, kj::OneOf<kj::String, kj::Array<unsigned char>, kj::WebSocket::Close> &&), WebsocketService *, kj::Own<kj::WebSocket>>, std::__tuple_indices<0, 1, 2>>>' requested here
      output.as<T>() = handle(MaybeVoidCaller<DepT, T>::apply(func, kj::mv(*depValue)));
                                                        ^
/Users/myuser/.conan/data/capnproto/0.10.3/_/_/package/13b953c004db54b2d9cf274425004a3d521cf731/include/kj/async-inl.h:542:3: note: in instantiation of member function 'kj::_::TransformPromiseNode<kj::Promise<void>, kj::OneOf<kj::String, kj::Array<unsigned char>, kj::WebSocket::Close>, std::__perfect_forward_impl<std::__bind_front_op, std::__tuple_types<kj::Promise<void> (WebsocketService::*)(kj::Own<kj::WebSocket> &&, kj::OneOf<kj::String, kj::Array<unsigned char>, kj::WebSocket::Close> &&), WebsocketService *, kj::Own<kj::WebSocket>>, std::__tuple_indices<0, 1, 2>>, kj::_::PropagateException>::getImpl' requested here
  TransformPromiseNode(Own<PromiseNode>&& dependency, Func&& func, ErrorFunc&& errorHandler,
  ^
/Users/myuser/.conan/data/capnproto/0.10.3/_/_/package/13b953c004db54b2d9cf274425004a3d521cf731/include/kj/memory.h:469:21: note: in instantiation of member function 'kj::_::TransformPromiseNode<kj::Promise<void>, kj::OneOf<kj::String, kj::Array<unsigned char>, kj::WebSocket::Close>, std::__perfect_forward_impl<std::__bind_front_op, std::__tuple_types<kj::Promise<void> (WebsocketService::*)(kj::Own<kj::WebSocket> &&, kj::OneOf<kj::String, kj::Array<unsigned char>, kj::WebSocket::Close> &&), WebsocketService *, kj::Own<kj::WebSocket>>, std::__tuple_indices<0, 1, 2>>, kj::_::PropagateException>::TransformPromiseNode' requested here
  return Own<T>(new T(kj::fwd<Params>(params)...), _::HeapDisposer<T>::instance);
                    ^
/Users/myuser/.conan/data/capnproto/0.10.3/_/_/package/13b953c004db54b2d9cf274425004a3d521cf731/include/kj/async-inl.h:1087:7: note: in instantiation of function template specialization 'kj::heap<kj::_::TransformPromiseNode<kj::Promise<void>, kj::OneOf<kj::String, kj::Array<unsigned char>, kj::WebSocket::Close>, std::__perfect_forward_impl<std::__bind_front_op, std::__tuple_types<kj::Promise<void> (WebsocketService::*)(kj::Own<kj::WebSocket> &&, kj::OneOf<kj::String, kj::Array<unsigned char>, kj::WebSocket::Close> &&), WebsocketService *, kj::Own<kj::WebSocket>>, std::__tuple_indices<0, 1, 2>>, kj::_::PropagateException>, kj::Own<kj::_::PromiseNode>, std::__perfect_forward_impl<std::__bind_front_op, std::__tuple_types<kj::Promise<void> (WebsocketService::*)(kj::Own<kj::WebSocket> &&, kj::OneOf<kj::String, kj::Array<unsigned char>, kj::WebSocket::Close> &&), WebsocketService *, kj::Own<kj::WebSocket>>, std::__tuple_indices<0, 1, 2>>, kj::_::PropagateException, void *&>' requested here
      heap<_::TransformPromiseNode<ResultT, _::FixVoid<T>, Func, ErrorFunc>>(
      ^
../programs/websocket-server/main.cpp:65:33: note: in instantiation of function template specialization 'kj::Promise<kj::OneOf<kj::String, kj::Array<unsigned char>, kj::WebSocket::Close>>::then<std::__perfect_forward_impl<std::__bind_front_op, std::__tuple_types<kj::Promise<void> (WebsocketService::*)(kj::Own<kj::WebSocket> &&, kj::OneOf<kj::String, kj::Array<unsigned char>, kj::WebSocket::Close> &&), WebsocketService *, kj::Own<kj::WebSocket>>, std::__tuple_indices<0, 1, 2>>, kj::_::PropagateException>' requested here
        return wsock->receive().then(std::bind_front(
                                ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__functional/perfect_forward.h:48:46: note: candidate function template not viable: expects an rvalue for object argument
    _LIBCPP_INLINE_VISIBILITY constexpr auto operator()(_Args&&... __args) &&
                                             ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__functional/perfect_forward.h:57:46: note: candidate function template not viable: expects an rvalue for object argument
    _LIBCPP_INLINE_VISIBILITY constexpr auto operator()(_Args&&... __args) const&&
                                             ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__functional/perfect_forward.h:36:46: note: candidate template ignored: substitution failure [with _Args = <kj::OneOf<kj::String, kj::Array<unsigned char>, kj::WebSocket::Close>>]: no matching function for call to '__call'
    _LIBCPP_INLINE_VISIBILITY constexpr auto operator()(_Args&&... __args) &
                                             ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__functional/perfect_forward.h:42:46: note: candidate template ignored: substitution failure [with _Args = <kj::OneOf<kj::String, kj::Array<unsigned char>, kj::WebSocket::Close>>]: no matching function for call to '__call'
    _LIBCPP_INLINE_VISIBILITY constexpr auto operator()(_Args&&... __args) const&

[kentonv]

What is the signature of your WebsocketService::handleMessage?

It looks to me like bind_front doesn't believe the arguments you have provided fit that function. Maybe try saving the result of bind_front to a local and verify it is actually callable with a WebSocket::Message -- I suspect it isn't.

[germandiagogomez]

@kentonv

kj::Promise<void> MyWebSocketService::handleMessage(kj::Own<kj::WebSocket> &&ws,
                                  kj::WebSocket::Message &&message);

[kentonv]

Sorry but I don't have bandwidth to debug this. If there's a simple change that we could make to fix this I'm happy to accept a PR.