// Illustrates that the return type of th plus function depends of the type of the sume of 'a T-value' and an 'U-value'.
// Compiles with g++ -c dt2.cc -std=c++11 
// The example comes from The C++ Language (4ed) page 165.

template<typename T> class Matrix;

template<typename T, typename U>
auto plus(const Matrix<T>& a, const Matrix<U>& b) -> Matrix<decltype(T{}+U{})>;