Copy templated function argument in Eigen

The declaration of the Eigen::Matrix variable M should be something like:

Eigen::Matrix<typename Derived::Scalar, Eigen::Dynamic, Eigen::Dynamic, Eigen::RowMajor> M; 

Then your code will compile. See Demo.

The detailed description of each of the template parameters can be found at this link.

A generic way of declaring your type would be to use the declaration generating it as a source, this way you don't have to look at specific ways to declare complicated template types, this is an example based on your code:

decltype(static_cast<Eigen::DenseBase<Derived> *>(nullptr)->eval()) M;

don't worry there is no nullptr dereference here because the code inside decltype is not executed.

As pointed in the comments there's a cleaner way of writing this:

decltype(declval<Eigen::DenseBase<Derived>>().eval()) M;

and if you're worried that the type might be a reference and don't want that:

remove_reference_t<decltype(declval<Eigen::DenseBase<Derived>>().eval())> M;

also don't forget to #include <type_traits> and either prefix everything with std:: or add using namespace std; to your code.

To make the syntax simpler for future use add this to the beginning of your code:

template<typename T, typename T::f>
using member_function_return_t = remove_reference_t<decltype(declval<T>().f())>;

and than declare the variable as:

member_function_return_t<Eigen::DenseBase<Derived>, Eigen::DenseBase<Derived>::eval> M;

Your container needs the actual "plain type" as template argument:

template <typename PlainType>
class A
{
    PlainType M; 
public:
    template<class Derived>
    A(const Eigen::MatrixBase<Derived> & V) : M(V) {}
};

And you need an additional template deduction rule:

template<class Derived>
A(const Eigen::MatrixBase<Derived> & V) -> A<typename Derived::PlainObject>;

Usage example (on godbolt):

template<class X>
void bar(X&); // just to read full type of A

void foo(Eigen::Matrix2d const& M)
{
    A a = M*M;
    bar(a);  // calls bar<A<Matrix2d>>();
}