c++ - Partial class template argument deduction in C++17

Question

In the example below, we use the C++17 feature "Class template argument deduction" to deduce that val is of type Base<int, double, bool>:

template<class T, class U, class V>
struct Base {
    Base(T, U) { };
    Base(T, U, V) { };
    Base(V) { };
};

void func() {
    Base val(1, 4., false);
}

Now, is it possible to partially specify the template arguments, and let the remaining ones be deduced? Effectively something like this:

Base<V = bool> val1(1, 4.);        // U & V deduced --> Base<int, double, bool>
Base<T = bool, T = int> val2(5.);  // V deduced     --> Base<bool, int, double>

I've tried e.g.

template<class T, class U> using Base2 = Base<T, U, double>;

void func() {
    NewBase2 val(1, 2);
}

but it doesn't compile: 'Base2': use of alias template requires template argument list.

Is partial deduction possible somehow? If it is not possible directly, are there any good workarounds?

Answer 1

CTAD (Class Template Argument Deduction) is an all or nothing process currently. You either specify nothing and allow the compiler to deduce all of the arguments, or you specify all of the arguments taking the compiler out of the loop.

There is a paper (P1021R0) which asks for this and more, but it has not yet been accepted. There was a paper asking for partial specialization but after revisions it has been removed. The newest revision still includes a proposal to have CTAD function when using an alias.

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Per @Barry Support for Alias templates (P1814) and Aggregates (P1816) have been added to the working draft for C++20. No support for partial CTAD or CTAD with inherited constructors has been added.