Make array<const T, 0> non-CopyAssignable and make swap and fill ill-formed.

The standard isn't exactly clear how std::array should handle zero-sized arrays
with const element types. In particular W.R.T. copy assignment, swap, and fill.

This patch takes the position that those operations should be ill-formed,
and makes changes to libc++ to make it so.

This follows up on commit r324182.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@324185 91177308-0d34-0410-b5e6-96231b3b80d8

include/array

@@ -122,7 +122,8 @@ struct __array_traits {
   typedef _Tp _StorageT[_Size];
 
   _LIBCPP_INLINE_VISIBILITY
-  static _LIBCPP_CONSTEXPR_AFTER_CXX14 _Tp* __data(_StorageT& __store) {
+  static _LIBCPP_CONSTEXPR_AFTER_CXX14 typename remove_const<_Tp>::type*
+  __data(typename remove_const<_StorageT>::type& __store) {
     return __store;
   }
 
@@ -144,12 +145,16 @@ struct __array_traits {
 
 template <class _Tp>
 struct __array_traits<_Tp, 0> {
-  typedef typename aligned_storage<sizeof(_Tp), alignment_of<_Tp>::value>::type _StorageT;
+  typedef typename aligned_storage<sizeof(_Tp), alignment_of<_Tp>::value>::type
+      _NonConstStorageT;
+  typedef typename conditional<is_const<_Tp>::value, const _NonConstStorageT,
+                               _NonConstStorageT>::type _StorageT;
 
+  typedef typename remove_const<_Tp>::type _NonConstTp;
   _LIBCPP_INLINE_VISIBILITY
-  static _Tp* __data(_StorageT& __store) {
+  static _NonConstTp* __data(_NonConstStorageT& __store) {
     _StorageT *__ptr = std::addressof(__store);
-    return reinterpret_cast<_Tp*>(__ptr);
+    return reinterpret_cast<_NonConstTp*>(__ptr);
   }
 
   _LIBCPP_INLINE_VISIBILITY
@@ -162,8 +167,7 @@ struct __array_traits<_Tp, 0> {
   static void __swap(_StorageT&, _StorageT&) {}
 
   _LIBCPP_INLINE_VISIBILITY
-  static void __fill(_StorageT&, _Tp const&) {
-  }
+  static void __fill(_StorageT&, _Tp const&) {}
 };
 
 template <class _Tp, size_t _Size>
@@ -187,12 +191,19 @@ struct _LIBCPP_TEMPLATE_VIS array
     typename _Traits::_StorageT __elems_;
 
     // No explicit construct/copy/destroy for aggregate type
-    _LIBCPP_INLINE_VISIBILITY void fill(const value_type& __u)
-        {_Traits::__fill(__elems_, __u);}
+    _LIBCPP_INLINE_VISIBILITY void fill(const value_type& __u) {
+      static_assert(_Size != 0 || !is_const<_Tp>::value,
+                    "cannot fill zero-sized array of type 'const T'");
+      _Traits::__fill(__elems_, __u);
+    }
 
     _LIBCPP_INLINE_VISIBILITY
-    void swap(array& __a) _NOEXCEPT_(_Size == 0 || __is_nothrow_swappable<_Tp>::value)
-        { _Traits::__swap(__elems_, __a.__elems_); }
+    void swap(array& __a)
+        _NOEXCEPT_(_Size == 0 || __is_nothrow_swappable<_Tp>::value) {
+      static_assert(_Size != 0 || !is_const<_Tp>::value,
+                    "cannot swap zero-sized array of type 'const T'");
+      _Traits::__swap(__elems_, __a.__elems_);
+    }
 
     // iterators:
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14

test/std/containers/sequences/array/array.cons/implicit_copy.pass.cpp

@@ -0,0 +1,93 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <array>
+
+// implicitly generated array constructors / assignment operators
+
+#include <array>
+#include <type_traits>
+#include <cassert>
+#include "test_macros.h"
+
+// 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"
+
+// FIXME: Clang generates copy assignment operators for types with const members
+// in C++03. The generated operator is ill-formed but still present.
+// I'm not sure if this is a Clang bug, but we need to work around it for now.
+#if TEST_STD_VER < 11 && defined(__clang__)
+#define TEST_NOT_COPY_ASSIGNABLE(T) ((void)0)
+#else
+#define TEST_NOT_COPY_ASSIGNABLE(T) static_assert(!std::is_copy_assignable<T>::value, "")
+#endif
+
+struct NoDefault {
+  NoDefault(int) {}
+};
+
+int main() {
+  {
+    typedef double T;
+    typedef std::array<T, 3> C;
+    C c = {1.1, 2.2, 3.3};
+    C c2 = c;
+    c2 = c;
+    static_assert(std::is_copy_constructible<C>::value, "");
+    static_assert(std::is_copy_assignable<C>::value, "");
+  }
+  {
+    typedef double T;
+    typedef std::array<const T, 3> C;
+    C c = {1.1, 2.2, 3.3};
+    C c2 = c;
+    ((void)c2);
+    static_assert(std::is_copy_constructible<C>::value, "");
+    TEST_NOT_COPY_ASSIGNABLE(C);
+  }
+  {
+    typedef double T;
+    typedef std::array<T, 0> C;
+    C c = {};
+    C c2 = c;
+    c2 = c;
+    static_assert(std::is_copy_constructible<C>::value, "");
+    static_assert(std::is_copy_assignable<C>::value, "");
+  }
+  {
+    // const arrays of size 0 should disable the implicit copy assignment operator.
+    typedef double T;
+    typedef std::array<const T, 0> C;
+    C c = {};
+    C c2 = c;
+    ((void)c2);
+    static_assert(std::is_copy_constructible<C>::value, "");
+    TEST_NOT_COPY_ASSIGNABLE(C);
+  }
+  {
+    typedef NoDefault T;
+    typedef std::array<T, 0> C;
+    C c = {};
+    C c2 = c;
+    c2 = c;
+    static_assert(std::is_copy_constructible<C>::value, "");
+    static_assert(std::is_copy_assignable<C>::value, "");
+  }
+  {
+    typedef NoDefault T;
+    typedef std::array<const T, 0> C;
+    C c = {};
+    C c2 = c;
+    ((void)c2);
+    static_assert(std::is_copy_constructible<C>::value, "");
+    TEST_NOT_COPY_ASSIGNABLE(C);
+  }
+
+}

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

@@ -36,6 +36,14 @@ int main()
         T* p = c.data();
         (void)p; // to placate scan-build
     }
+    {
+      typedef double T;
+      typedef std::array<const T, 0> C;
+      C c = {};
+      const T* p = c.data();
+      static_assert((std::is_same<decltype(c.data()), const T*>::value), "");
+      (void)p; // to placate scan-build
+    }
     {
       struct NoDefault {
         NoDefault(int) {}

test/std/containers/sequences/array/array.fill/fill.fail.cpp

@@ -0,0 +1,29 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <array>
+
+// void fill(const T& u);
+
+#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"
+
+int main() {
+  {
+    typedef double T;
+    typedef std::array<const T, 0> C;
+    C c = {};
+    // expected-error@array:* {{static_assert failed "cannot fill zero-sized array of type 'const T'"}}
+    c.fill(5.5); // expected-note {{requested here}}
+  }
+}

test/std/containers/sequences/array/array.swap/swap.fail.cpp

@@ -0,0 +1,30 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <array>
+
+// void swap(array& a);
+
+#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"
+
+int main() {
+  {
+    typedef double T;
+    typedef std::array<const T, 0> C;
+    C c = {};
+    C c2 = {};
+    // expected-error@array:* {{static_assert failed "cannot swap zero-sized array of type 'const T'"}}
+    c.swap(c2); // expected-note {{requested here}}
+  }
+}