[PATCH] Proposed change to the constructor of std::pair

[Eric Fiselier]

The code as it currently stands now will fail to compile the following code (that currently works).

  #include <type_traits>
  
  template <class T>
  struct IllFormedDefaultImp {
      IllFormedDefaultImp(T x) : value(x) {}
      constexpr IllFormedDefaultImp() {}
      T value;
  };
  
  typedef IllFormedDefaultImp<int &> IllFormedDefault;
  
  template <class T, class U>
  struct pair
  {
  #if defined(USE_SFINAE)
    template <bool Dummy = true,
      class = typename std::enable_if<
           std::is_default_constructible<T>::value
        && std::is_default_constructible<U>::value
        && Dummy>::type
      >
  #endif
    constexpr pair() : first(), second() {}
  
    pair(T const & t, U const & u) : first(t), second(u) {}
  
    T first;
    U second;
  };
  
  int main()
  {
    int x = 1;
    IllFormedDefault v(x);
    pair<IllFormedDefault, IllFormedDefault> p(v, v);
  }

`clang++ -std=c++11 test.cpp` compiles just fine.
`clang++ -std=c++11 -DUSE_SFINAE test.cpp` fails to compile.

You can work around this by using something like this

  template <class T, bool>
  struct dependent_is_default_constructible : std::is_default_constructible<T> {};


http://reviews.llvm.org/D7384

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