Why do I get an error trying to call a template member function with an explicit type parameter?

I don't get it, it seems to me that the call to f is completely unambiguous, but it fails to compile with expected primary-expression before ‘int’. If I comment out the line with the call to f, it compiles fine.

template<typename T>
struct A {
    template<typename S>
    void f() { }
};

template<typename T>
struct B : A<T> {
    void g() {
        this->f<int>();
    }
};

This is due to a really obscure provision of the standard in which if you have a template that tries to access a template function in an object whose type depends on a template argument, you have to use the template keyword in a weird way:

this->template f<int>();

This is similar to the weirdness with typename that comes up with dependent types, except as applied to functions. In particular, if you leave out the template keyword, there's a parsing ambiguity between

this->f<int>()

(what you intended), and

((this->f) < int) > ()

which makes no sense (hence your error). The use of the keyword template here disambiguates and forces the compiler to recognize that it's looking at a perfectly valid call to a templated member function rather than a garbled mass of symbols.

Hope this helps!