Confused by default constructor description of std::tuple in the ISO C++ Standard

Question

The Standard says that std::tuple has the following member functions

constexpr tuple();
explicit tuple(const Types&...);

C

Answer 1

I guess the definition given in the standard is supposed to be pseudocode. That is the case with many of the definitions in the standard; it contains several requirements that are given verbally, but are satisfiable only with tricks like enable_if. This seems to be an example where the C++-like pseudocode notation can actually lead to illegal C++ when trying to instantiate such an empty tuple (or it might just be an omission).

Both stdlibc++ and libc++ have an explicit specialization for the zero-element tuple. For example, in stdlibc++:

  // Explicit specialization, zero-element tuple.
  template<>  
    class tuple<>
    {
    public:
      void swap(tuple&) noexcept { /* no-op */ }
    };

with an implicitly-defined unambiguous default constructor.

Libc++ does not explicitly declare the parameterless default constructor. Presumably the templated constructor is then chosen as default constructor for non-empty tuples.

Interestingly, the two libraries disagree on what members the empty tuple has. For example, the following compiles with libc++, but not with libstdc++:

#include 
#include 

int main() {
  std::tuple<> t(std::allocator_arg, std::allocator());
}