QtHttpServer::route()/afterRequest(): Compile errors when assigning the lambda to variable and passing that

In file included from include/QtHttpServer/qhttpserverrouterviewtraits.h:34,
                 from include/QtHttpServer/qhttpserverrouter.h:34,
                 from include/QtHttpServer/qhttpserver.h:35,
                 from include/QtHttpServer/QtHttpServer:6,
                 from main.cpp:52:
include/QtHttpServer/qhttpserverviewtraits_impl.h: In instantiation of ‘struct QtPrivate::FunctionTraits<main(int, char**)::<lambda(const QHttpServerRequest&)>&>’:
include/QtHttpServer/qhttpserverviewtraits_impl.h:117:11:   required from ‘struct QtPrivate::ViewTraits<main(int, char**)::<lambda(const QHttpServerRequest&)>&, false>’
include/QtHttpServer/qhttpserverrouterviewtraits.h:44:8:   required from ‘struct QtPrivate::RouterViewTraitsHelper<main(int, char**)::<lambda(const QHttpServerRequest&)>&, false>’
include/QtHttpServer/qhttpserverrouterviewtraits.h:152:11:   required from ‘struct QHttpServerRouterViewTraits<main(int, char**)::<lambda(const QHttpServerRequest&)>&, false>’
include/QtHttpServer/qhttpserver.h:82:46:   required from ‘bool QHttpServer::route(Args&& ...) [with Rule = QHttpServerRouterRule; Args = {const char (&)[7], main(int, char**)::<lambda(const QHttpServerRequest&)>&}]’
main.cpp:72:36:   required from here
include/QtHttpServer/qhttpserverviewtraits_impl.h:92:56: error: ‘operator()’ is not a member of ‘main(int, char**)::<lambda(const QHttpServerRequest&)>&’
   92 | struct FunctionTraits : public FunctionTraits<decltype(&T::operator())>{};
      |                                                        ^~~~~~~~~~~~~~
include/QtHttpServer/qhttpserverviewtraits_impl.h: In instantiation of ‘struct QtPrivate::ViewTraits<main(int, char**)::<lambda(const QHttpServerRequest&)>&, false>’:
include/QtHttpServer/qhttpserverrouterviewtraits.h:44:8:   required from ‘struct QtPrivate::RouterViewTraitsHelper<main(int, char**)::<lambda(const QHttpServerRequest&)>&, false>’
include/QtHttpServer/qhttpserverrouterviewtraits.h:152:11:   required from ‘struct QHttpServerRouterViewTraits<main(int, char**)::<lambda(const QHttpServerRequest&)>&, false>’
include/QtHttpServer/qhttpserver.h:82:46:   required from ‘bool QHttpServer::route(Args&& ...) [with Rule = QHttpServerRouterRule; Args = {const char (&)[7], main(int, char**)::<lambda(const QHttpServerRequest&)>&}]’
main.cpp:72:36:   required from here
include/QtHttpServer/qhttpserverviewtraits_impl.h:117:11: error: ‘ArgumentCount’ is not a member of ‘QtPrivate::FunctionTraits<main(int, char**)::<lambda(const QHttpServerRequest&)>&>’
  117 |     using ArgumentIndexes = typename Indexes<FTraits::ArgumentCount>::Value;
      |           ^~~~~~~~~~~~~~~
In file included from include/QtHttpServer/qhttpserverrouter.h:34,
                 from include/QtHttpServer/qhttpserver.h:35,
                 from include/QtHttpServer/QtHttpServer:6,
                 from main.cpp:52:
include/QtHttpServer/qhttpserverrouterviewtraits.h: In instantiation of ‘struct QHttpServerRouterViewTraits<main(int, char**)::<lambda(const QHttpServerRequest&)>&, false>’:
include/QtHttpServer/qhttpserver.h:82:46:   required from ‘bool QHttpServer::route(Args&& ...) [with Rule = QHttpServerRouterRule; Args = {const char (&)[7], main(int, char**)::<lambda(const QHttpServerRequest&)>&}]’
main.cpp:72:36:   required from here
include/QtHttpServer/qhttpserverrouterviewtraits.h:152:11: error: no type named ‘ReturnType’ in ‘using FunctionTraits = using FTraits = struct QtPrivate::FunctionTraits<main(int, char**)::<lambda(const QHttpServerRequest&)>&>’ {aka ‘struct QtPrivate::FunctionTraits<main(int, char**)::<lambda(const QHttpServerRequest&)>&>’}
  152 |     using ReturnType = typename Helpers::FunctionTraits::ReturnType;
      |           ^~~~~~~~~~
In file included from include/QtHttpServer/QtHttpServer:6,
                 from main.cpp:52:
include/QtHttpServer/qhttpserver.h:142:10: error: ‘bool QHttpServer::routeHelper(QtPrivate::IndexesList<Px ...>, Args&& ...) [with Rule = QHttpServerRouterRule; ViewHandler = main(int, char**)::<lambda(const QHttpServerRequest&)>&; ViewTraits = QHttpServerRouterViewTraits<main(int, char**)::<lambda(const QHttpServerRequest&)>&, false>; int ...I = {0}; Args = {const char (&)[7], main(int, char**)::<lambda(const QHttpServerRequest&)>&}]’, declared using local type ‘main(int, char**)::<lambda(const QHttpServerRequest&)>’, is used but never defined [-fpermissive]
  142 |     bool routeHelper(QtPrivate::IndexesList<I...>, Args &&... args)
      |          ^~~~~~~~~~~
include/QtHttpServer/qhttpserver.h:142:10: warning: ‘bool QHttpServer::routeHelper(QtPrivate::IndexesList<Px ...>, Args&& ...) [with Rule = QHttpServerRouterRule; ViewHandler = main(int, char**)::<lambda(const QHttpServerRequest&)>&; ViewTraits = QHttpServerRouterViewTraits<main(int, char**)::<lambda(const QHttpServerRequest&)>&, false>; int ...I = {0}; Args = {const char (&)[7], main(int, char**)::<lambda(const QHttpServerRequest&)>&}]’ used but never defined
make: *** [Makefile:891: main.o] Error 1