Template deduction for default template argument

Question

I have a class that looks similar to this:

template <class T = char>
struct C {
    T value;
};

Up to C++14, when I wanted to use it with the default template argument I always had to specify empty angle brackets:

void f() {
    C<> c;
    c.value = 'x';
}

Since C++17 supports class template argument deduction and explicit deduction guides, I wonder if there is a way now to make the above code work without specifying the empty angle brackets:

void f() {
    C c;
    c.value = 'x';
}

This code worked in GCC 8.0 if I compile it with -std=gnu++17. However, it still showed an error in Clang 6.0 and Visual Studio 15.7. Which compiler is correct in this situation?

I also tried specifying a deduction guide like this:

C() -> C<char>;

This also didn't help. Is that the correct syntax or is there even a way to specify deduction guides for a default constructor?

Answer 1

This program is correct:

template <class T = char>
struct C {
    T value;
};

int main() {
    C c;
    c.value = 'x';
}

clang just doesn't fully support class template argument deduction yet (note that it does compile on trunk).

---

Class template argument deduction will try to perform overload resolution on the candidate set:

template <class T=char> auto __f()     -> C<T>
template <class T=char> auto __f(C<T>) -> C<T>

with no initializer. That first one is a viable candidate (deducing T as char), the second one is not, so the first one is trivially the best viable candidate. So we end up with c having type C<char>. When we then do overload resolution on default construction, that works, so the program is fine.