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

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);
+  }
+
+}