Error(s):

source_file.cpp: In instantiation of ‘struct awaitable<int>’:
source_file.cpp:17:20:   required from here
source_file.cpp:8:71: error: redefinition of ‘template<class U> std::enable_if_t<(! std::is_void<_Tp>::value), awaitable<void> > operator&&(awaitable<U>, awaitable<void>)’
     friend std::enable_if_t<!std::is_void<U>::value, awaitable<void>> operator&&(awaitable<U> a1, awaitable<void> a2)
                                                                       ^
source_file.cpp:8:71: note: ‘template<class U> std::enable_if_t<(! std::is_void<_Tp>::value), awaitable<void> > operator&&(awaitable<U>, awaitable<void>)’ previously defined here