C++ has a rule similar to the turbofish though:

    #include <Eigen/Dense> // a library where I know it's required

    using namespace Eigen;

    int main() {
        VectorXd zeros = VectorXd::zeros(5);
        VectorXf zeros_float = zeros.template cast<float>();
        return 0;
    }

I'm sure you know better, but at first glance this seems wrong. Reason: `main` is not itself a template, so `cast` is not a dependent name.

C++ compilers used to be liberal about accepting unnecessary instances of the `typename` keyword so lots of people would just sprinkle them everywhere. Modern versions of both Clang and GCC call this out.

The actual rule: https://en.cppreference.com/w/cpp/language/dependent_name#Th...

You're right, since I'm frequently using templated functions I encounter this issue quite often, but here when I wanted to give an example I forgot I needed to be in a dependent context. I guess I'd better test my example next time I want to give one...

And I have never internalized it! This is me, but for `typename` and `template` in expressions: https://xkcd.com/1168/