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Question #228 Difficulty: 3

According to the C++23 standard, what is the output of this program? 

    template <typename ...Ts>
struct X {
  X(Ts ...args) : Var(0, args...) {}
  int Var;
};

int main() {
  X<> x;
}

Correct!

First, let's have a look at the initialization of X::Var. Var(0, args...) is valid if there are no template arguments, since then it's just Var(0). If there however are any template arguments, they expand to an invalid initializer for int, such as Var(0, arg1, arg2). So the only valid specialization of X is the one with no arguments.

X<> x in fact does just that, it instantiates the template with no arguments, which would be valid.

However, §[temp.res.general]¶6.3 says:

The program is ill-formed, no diagnostic required, if:

— (...)

— every valid specialization of a variadic template requires an empty template parameter pack, (...)

As we've seen, the only valid specialization requires an empty template parameter pack, so the program is ill-formed, no diagnostic required. When no diagnostic is required, the compiler is not required to diagnose the error, but execution of the program is undefined.

You can explore this question further on C++ Insights or Compiler Explorer!

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