placing boost::coroutine2 inside a class

The following code compiles without warnings or errors: (I am using clang 3.8 in -std=c++14 mode):

typedef boost::coroutines2::coroutine<int>   coro_t;

coro_t::push_type sink(
  [](coro_t::pull_type& source){
      while(source){
          std::cout << source.get() <<  " ";
          source();
      }
  });

However, if I place the same code inside a class:

class EnclosingClass {

coro_t::push_type sink(
  [](coro_t::pull_type& source){
      while(source){
          std::cout << source.get() <<  " ";
          source();
      }
  });

};

I get the following errors:

test.cpp:13:26: error: C++ requires a type specifier for all declarations
coro_t::push_type sink([](coro_t::pull_type &source) {
                       ^
test.cpp:13:26: error: 'type name' declared as array of functions of type 'int (coro_t::pull_type &)' (aka 'int (pull_coroutine<int> &)')
test.cpp:13:56: error: expected ')'
coro_t::push_type sink([](coro_t::pull_type &source) {
                                                     ^
test.cpp:13:25: note: to match this '('
coro_t::push_type sink([](coro_t::pull_type &source) {
                      ^

How can I have a boost::coroutine2 as a class member?

I was expecting code in the global context to also work OK inside a class, but it doesn't. Is there an interesting reason why?