Stream stream;
stream
    .map      ([] (int x)  { return 10*x; })      // error
    .forEach  ([] (int x)  { cout << x << " ";});

template argument deduction/substitution failed:
‘main(int, char**)::<lambda(int)>’ is not derived from ‘std::function<Re(int)>’
   .map       ([] (int x)  { return 10*x; })
requires a type parameter for `map`

.map<int>  ([] (int x)  { return 10*x; })

#include <iostream>
#include <functional>

using namespace std;

template <typename Tfrom, typename Tto> class MappedStream;

template <typename T>
class Stream
{
  public:
    void forEach(function< void(T) > action)
    {}

    template <typename Re>
    MappedStream<T,Re> map (function < Re(T)> mapping) {
        return MappedStream<T,Re>(*this, mapping);
    }
};

template <typename Tfrom, typename Tto>
class MappedStream
   : public Stream<Tto>
{
  public:
    MappedStream(Stream<Tfrom> & source,
                function <Tto(Tfrom)> mapping)
    {}
};

int main(int argc, char **argv)
{
  Stream<int> stream;
  stream
   .map<int> ([] (int x) { return 10*x; })
   //  XXXXX                          <- how to get rid of this?
   .forEach  ([] (int x) { cout << x << " ";});

   return 0;
}

template <typename F, typename Re = std::result_of_t<F&(T)>>
MappedStream<T,Re> map (F mapping) {
    return MappedStream<T,Re>(*this, mapping);
}