[libc++] Fix PR35491 - std::array of zero-size doesn't work with non-default constructible types.

Summary:
This patch fixes llvm.org/PR35491 and LWG2157  (https://cplusplus.github.io/LWG/issue2157)

The fix attempts to maintain ABI compatibility by replacing the array with a instance of `aligned_storage`.

Reviewers: mclow.lists, EricWF

Reviewed By: EricWF

Subscribers: lichray, cfe-commits

Differential Revision: https://reviews.llvm.org/D41223

llvm-svn: 324182

libcxx/include/array

@@ -117,6 +117,55 @@ template <size_t I, class T, size_t N> const T&& get(const array<T, N>&&) noexce
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+template <class _Tp, size_t _Size>
+struct __array_traits {
+  typedef _Tp _StorageT[_Size];
+
+  _LIBCPP_INLINE_VISIBILITY
+  static _LIBCPP_CONSTEXPR_AFTER_CXX14 _Tp* __data(_StorageT& __store) {
+    return __store;
+  }
+
+  _LIBCPP_INLINE_VISIBILITY
+  static _LIBCPP_CONSTEXPR_AFTER_CXX14 _Tp const* __data(const _StorageT& __store) {
+    return __store;
+  }
+
+  _LIBCPP_INLINE_VISIBILITY
+  static void __swap(_StorageT& __lhs, _StorageT& __rhs) {
+    std::swap_ranges(__lhs, __lhs + _Size, __rhs);
+  }
+
+  _LIBCPP_INLINE_VISIBILITY
+  static void __fill(_StorageT& __arr, _Tp const& __val) {
+    _VSTD::fill_n(__arr, _Size, __val);
+  }
+};
+
+template <class _Tp>
+struct __array_traits<_Tp, 0> {
+  typedef typename aligned_storage<sizeof(_Tp), alignment_of<_Tp>::value>::type _StorageT;
+
+  _LIBCPP_INLINE_VISIBILITY
+  static _Tp* __data(_StorageT& __store) {
+    _StorageT *__ptr = std::addressof(__store);
+    return reinterpret_cast<_Tp*>(__ptr);
+  }
+
+  _LIBCPP_INLINE_VISIBILITY
+  static const _Tp* __data(const _StorageT& __store) {
+    const _StorageT *__ptr = std::addressof(__store);
+    return reinterpret_cast<const _Tp*>(__ptr);
+  }
+
+  _LIBCPP_INLINE_VISIBILITY
+  static void __swap(_StorageT&, _StorageT&) {}
+
+  _LIBCPP_INLINE_VISIBILITY
+  static void __fill(_StorageT&, _Tp const&) {
+  }
+};
+
 template <class _Tp, size_t _Size>
 struct _LIBCPP_TEMPLATE_VIS array
 {
@@ -134,31 +183,26 @@ struct _LIBCPP_TEMPLATE_VIS array
     typedef std::reverse_iterator<iterator>       reverse_iterator;
     typedef std::reverse_iterator<const_iterator> const_reverse_iterator;
 
-    value_type __elems_[_Size > 0 ? _Size : 1];
+    typedef __array_traits<_Tp, _Size> _Traits;
+    typename _Traits::_StorageT __elems_;
 
     // No explicit construct/copy/destroy for aggregate type
     _LIBCPP_INLINE_VISIBILITY void fill(const value_type& __u)
-        {_VSTD::fill_n(__elems_, _Size, __u);}
+        {_Traits::__fill(__elems_, __u);}
+
     _LIBCPP_INLINE_VISIBILITY
     void swap(array& __a) _NOEXCEPT_(_Size == 0 || __is_nothrow_swappable<_Tp>::value)
-        { __swap_dispatch((std::integral_constant<bool, _Size == 0>()), __a); }
+        { _Traits::__swap(__elems_, __a.__elems_); }
-
-    _LIBCPP_INLINE_VISIBILITY
-    void __swap_dispatch(std::true_type, array&) {}
-
-    _LIBCPP_INLINE_VISIBILITY
-    void __swap_dispatch(std::false_type, array& __a)
-        { _VSTD::swap_ranges(__elems_, __elems_ + _Size, __a.__elems_);}
 
     // iterators:
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
-    iterator begin() _NOEXCEPT {return iterator(__elems_);}
+    iterator begin() _NOEXCEPT {return iterator(data());}
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
-    const_iterator begin() const _NOEXCEPT {return const_iterator(__elems_);}
+    const_iterator begin() const _NOEXCEPT {return const_iterator(data());}
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
-    iterator end() _NOEXCEPT {return iterator(__elems_ + _Size);}
+    iterator end() _NOEXCEPT {return iterator(data() + _Size);}
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
-    const_iterator end() const _NOEXCEPT {return const_iterator(__elems_ + _Size);}
+    const_iterator end() const _NOEXCEPT {return const_iterator(data() + _Size);}
 
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
     reverse_iterator rbegin() _NOEXCEPT {return reverse_iterator(end());}
@@ -201,9 +245,9 @@ struct _LIBCPP_TEMPLATE_VIS array
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11 const_reference back() const  {return __elems_[_Size > 0 ? _Size-1 : 0];}
 
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
-    value_type* data() _NOEXCEPT {return __elems_;}
+    value_type* data() _NOEXCEPT {return _Traits::__data(__elems_);}
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
-    const value_type* data() const _NOEXCEPT {return __elems_;}
+    const value_type* data() const _NOEXCEPT {return _Traits::__data(__elems_);}
 };
 
 template <class _Tp, size_t _Size>

libcxx/test/std/containers/sequences/array/array.cons/default.pass.cpp

@@ -14,6 +14,14 @@
 #include <array>
 #include <cassert>
 
+// std::array is explicitly allowed to be initialized with A a = { init-list };.
+// Disable the missing braces warning for this reason.
+#include "disable_missing_braces_warning.h"
+
+struct NoDefault {
+  NoDefault(int) {}
+};
+
 int main()
 {
     {
@@ -28,4 +36,13 @@ int main()
         C c;
         assert(c.size() == 0);
     }
+    {
+      typedef std::array<NoDefault, 0> C;
+      C c;
+      assert(c.size() == 0);
+      C c1 = {};
+      assert(c1.size() == 0);
+      C c2 = {{}};
+      assert(c2.size() == 0);
+    }
 }

libcxx/test/std/containers/sequences/array/array.data/data.pass.cpp

@@ -36,4 +36,14 @@ int main()
         T* p = c.data();
         (void)p; // to placate scan-build
     }
+    {
+      struct NoDefault {
+        NoDefault(int) {}
+      };
+      typedef NoDefault T;
+      typedef std::array<T, 0> C;
+      C c = {};
+      T* p = c.data();
+      assert(p != nullptr);
+    }
 }

libcxx/test/std/containers/sequences/array/array.data/data_const.pass.cpp

@@ -38,6 +38,16 @@ int main()
         const T* p = c.data();
         (void)p; // to placate scan-build
     }
+    {
+      struct NoDefault {
+        NoDefault(int) {}
+      };
+      typedef NoDefault T;
+      typedef std::array<T, 0> C;
+      const C c = {};
+      const T* p = c.data();
+      assert(p != nullptr);
+    }
 #if TEST_STD_VER > 14
     {
         typedef std::array<int, 5> C;

libcxx/test/std/containers/sequences/array/begin.pass.cpp

@@ -31,4 +31,13 @@ int main()
         *i = 5.5;
         assert(c[0] == 5.5);
     }
+    {
+      struct NoDefault {
+        NoDefault(int) {}
+      };
+      typedef NoDefault T;
+      typedef std::array<T, 0> C;
+      C c = {};
+      assert(c.begin() == c.end());
+    }
 }