Structured bindings with pair/tuple not producing references

[vittorioromeo]

The following code:

#include <utility>

int main()
{
    std::pair<int, char> p;
  	auto [a, b] = p;
}

Produces this after being run in cppinsights:

#include <utility>

int main()
{
  std::pair<int, char> p = std::pair<int, char>();
  std::pair<int, char> __p6 = std::pair<int, char>(p);
  int a = std::get<0UL>(static_cast<std::pair<int, char> &&>(__p6));
  char b = std::get<1UL>(static_cast<std::pair<int, char> &&>(__p6));
}

I think that is incorrect. It should be:

Produces this after being run in cppinsights:

#include <utility>

int main()
{
  std::pair<int, char> p = std::pair<int, char>();
  std::pair<int, char> __p6 = std::pair<int, char>(p);
  int& a = std::get<0UL>(static_cast<std::pair<int, char> &&>(__p6));
  char& b = std::get<1UL>(static_cast<std::pair<int, char> &&>(__p6));
}

According to cppreference:

For each identifier, a variable whose type is "reference to std::tuple_element<i, E>::type" is introduced: [...]

[andreasfertig]

Hello @vittorioromeo,

thanks for reporting this. I took me some time but I think I figured the correct behavior out. The patch will contain additional information, but I will explain it here as well for documentation purposes and discussions.

I agree that what C++ Insights showed was wrong. However, the resulting code should be

  std::pair<int, char> p = std::pair<int, char>();
  std::pair<int, char> __p6 = std::pair<int, char>(p);
  int&& a = std::get<0UL>(static_cast<std::pair<int, char> &&>(__p6));
  char&& b = std::get<1UL>(static_cast<std::pair<int, char> &&>(__p6));

Both a and b are rvalue reference because get is called with an rvalue of __p6. That is also what the quote from cppreference describes. The text there seems to agree what the standard says in eel.is/c++draft/dcl.struct.bind#4.

Andreas