Error(s):

source_file.cpp: In function ‘auto f(int&)’:
source_file.cpp:5:25: error: missing template arguments before ‘(’ token
     return std::function([&] { return x; });
                         ^
source_file.cpp: In function ‘int main()’:
source_file.cpp:10:17: error: ‘void y’ has incomplete type
     auto y = f(x);
                 ^