c++ - Deduction guide and variadic templates

Question

Consider the following code:

#include <tuple>
#include <iostream>

template <class T>
struct custom_wrapper
{
    template <class Arg>
    custom_wrapper(Arg arg): data(arg) {}
    T data;
};

template <class Arg>
custom_wrapper(Arg arg) -> custom_wrapper<Arg>;

template <class... T>
struct custom_tuple
{
    template <class... Args>
    custom_tuple(Args... args): data(args...) {}
    std::tuple<T...> data;
};

template <class... Args>
custom_tuple(Args... args) -> custom_tuple<Args...>;

int main(int argc, char* argv[])
{
    custom_wrapper<int> w1(42);  // OK
    custom_wrapper w2(42);       // OK
    custom_tuple<int> t1(42);    // OK
    custom_tuple t2(42);         // Fails
    return 0;
}

The line that fails return the following error under g++7:

variadic_deduction_guide.cpp: In instantiation of 'custom_tuple<T>::custom_tuple(Args ...) [with Args = {int}; T = {}]':
variadic_deduction_guide.cpp:31:23:   required from here
variadic_deduction_guide.cpp:19:45: error: no matching function for call to 'std::tuple<>::tuple(int&)'
     custom_tuple(Args... args): data(args...) {}

Is that normal or is that a compiler bug?

Answer 1

This is gcc bug 80871. What follows is an explanation of why the code is well-formed (and clang is correct in deciding that t2 is a custom_tuple<int>).

---

The process for figuring out what to do with

custom_tuple t2(42);

basically involves synthesizing a bunch of functions and performing overload resolution on them. The relevant candidates are the synthesized functions from the one constructor and the deduction guide:

template <class... T, class... Args>
custom_tuple<T...> foo(Args... );     // the constructor

template <class... Args>
custom_tuple<Args...> foo(Args... );  // the deduction guide

From this point it's a choose your own adventure based on your interpretation of what a "trailing parameter pack" is according to [temp.arg.explicit]/3:

A trailing template parameter pack not otherwise deduced will be deduced to an empty sequence of template arguments. If all of the template arguments can be deduced, they may all be omitted; in this case, the empty template argument list <> itself may also be omitted.

T... isn't trailing

This case is easy. We only have one viable candidate (because T... isn't deducible) - the deduction-guide candidate. We deduce Args... as {int}, so we end up with custom_tuple<int>.

T... is trailing

Both gcc and clang actually do consider deduction to succeed for the constructor. So we go to the tiebreakers in [over.match.best]:

Given these definitions, a viable function F1 is defined to be a better function than another viable function F2 if [...]

• F1 and F2 are function template specializations, and the function template for F1 is more specialized than the template for F2 according to the partial ordering rules described in [temp.func.order], or, if not that,
• F1 is generated from a deduction-guide ([over.match.class.deduct]) and F2 is not, or, if not that, [...]

For purposes of partial ordering, the relevant types are just those which correspond to function parameters, and we're allowed to ignore unused template parameters, so neither function template is considered more specialized than the other.

This leaves us to simply preferring the deduction-guide, which has been the simplest step of this whole process. We deduce Args... as {int}, so we end up with custom_tuple<int>.

---

Either way, custom_tuple<int> is the correct decision.