template<typename T>
iDynTree::MatrixViewInternal::has_IsRowMajor<T, SpanUtils::void_t<decltype(T::IsRowMajor)>> struct

has_IsRowMajor is used to build a type-dependent expression that check if an element has IsRowMajor argument.

This specific implementation is used when the the object has not IsRowMajor, indeed void_t<> is used to detect ill-formed types in SFINAE context.