how to correctly find out the return type of a lambda

Question

basically how to make following code compile?

I know it failed because compiler was trying to evaluate something like ([](int &i){})(0) but how to s

Answer 1

Two ways. First:

using TResult = decltype(f(_e));

or second:

using TResult = typename std::result_of::type;

Your code implicity says that the TElement is a temprary/rvalue. The & above makes them lvalues.