What does these few lines do as a small test framework in C++?

Hi I am reading through the implementation of std::optional here and I found the following lines in its unit testing files.

struct caller {
    template <class T> caller(T fun) { fun(); }
};
# define CAT2(X, Y) X ## Y
# define CAT(X, Y) CAT2(X, Y)
# define TEST(NAME) caller CAT(__VAR, __LINE__) = []

I don't really understand what do these lines do. caller seems to be a template to invoke functions, but how can it be used as caller CAT ...? What does X ## Y mean here? Later in the file, the user define unit tests using TEST, but they do not appear in any executable code (I mean they are not called in main function at least), so I am not even sure if the compiled binary actually run the tests. Could you explain what is going on here? Thanks!