What does "typename =" mean in the template parameters?

Question

I have seen this expression in page 189 of the book "Effective Modern C++":

    template<typename T,              typename = typename std::enable_if<condition>::type>     explicit Person(T&& n); 

I am just wondering what does the part "typename =" mean. It certainly looks like a default argument for a template parameter. But don't you need something like "typename some_name = ..." in a default argument? There is no name for the second template argument, and I don't see the second template argument being used in this case.

P.S. When I search on google (or any other search engine) for an answer, the equal sign is always discarded, and this just makes finding an answer almost impossible...

Answer 1

That's an optional template parameter with no name and a default value.
It's used to apply the enable_if condition; it will create a compiler error if the condition is not met.

You can use exactly the same syntax for normal method arguments.