[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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@324526 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/containers/sequences/array/array.zero/db_back.pass.cpp

@@ -0,0 +1,49 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: libcpp-no-exceptions
+// MODULES_DEFINES: _LIBCPP_DEBUG=1
+// MODULES_DEFINES: _LIBCPP_DEBUG_USE_EXCEPTIONS
+
+// Can't test the system lib because this test enables debug mode
+// UNSUPPORTED: with_system_cxx_lib
+
+// test array<T, 0>::front() throws a debug exception.
+
+#define _LIBCPP_DEBUG 1
+#define _LIBCPP_DEBUG_USE_EXCEPTIONS
+#include <array>
+
+template <class Array>
+inline bool CheckDebugThrows(Array& Arr) {
+  try {
+    Arr.back();
+  } catch (std::__libcpp_debug_exception const&) {
+    return true;
+  }
+  return false;
+}
+
+int main()
+{
+  {
+    typedef std::array<int, 0> C;
+    C c = {};
+    C const& cc = c;
+    assert(CheckDebugThrows(c));
+    assert(CheckDebugThrows(cc));
+  }
+  {
+    typedef std::array<const int, 0> C;
+    C c = {{}};
+    C const& cc = c;
+    assert(CheckDebugThrows(c));
+    assert(CheckDebugThrows(cc));
+  }
+}

test/libcxx/containers/sequences/array/array.zero/db_front.pass.cpp

@@ -0,0 +1,49 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: libcpp-no-exceptions
+// MODULES_DEFINES: _LIBCPP_DEBUG=1
+// MODULES_DEFINES: _LIBCPP_DEBUG_USE_EXCEPTIONS
+
+// Can't test the system lib because this test enables debug mode
+// UNSUPPORTED: with_system_cxx_lib
+
+// test array<T, 0>::front() throws a debug exception.
+
+#define _LIBCPP_DEBUG 1
+#define _LIBCPP_DEBUG_USE_EXCEPTIONS
+#include <array>
+
+template <class Array>
+inline bool CheckDebugThrows(Array& Arr) {
+  try {
+    Arr.front();
+  } catch (std::__libcpp_debug_exception const&) {
+    return true;
+  }
+  return false;
+}
+
+int main()
+{
+  {
+    typedef std::array<int, 0> C;
+    C c = {};
+    C const& cc = c;
+    assert(CheckDebugThrows(c));
+    assert(CheckDebugThrows(cc));
+  }
+  {
+    typedef std::array<const int, 0> C;
+    C c = {{}};
+    C const& cc = c;
+    assert(CheckDebugThrows(c));
+    assert(CheckDebugThrows(cc));
+  }
+}

test/libcxx/containers/sequences/array/array.zero/db_indexing.pass.cpp

@@ -0,0 +1,53 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: libcpp-no-exceptions
+// MODULES_DEFINES: _LIBCPP_DEBUG=1
+// MODULES_DEFINES: _LIBCPP_DEBUG_USE_EXCEPTIONS
+
+// Can't test the system lib because this test enables debug mode
+// UNSUPPORTED: with_system_cxx_lib
+
+// test array<T, 0>::operator[] throws a debug exception.
+
+#define _LIBCPP_DEBUG 1
+#define _LIBCPP_DEBUG_USE_EXCEPTIONS
+#include <array>
+
+template <class Array>
+inline bool CheckDebugThrows(Array& Arr, size_t Index) {
+  try {
+    Arr[Index];
+  } catch (std::__libcpp_debug_exception const&) {
+    return true;
+  }
+  return false;
+}
+
+int main()
+{
+  {
+    typedef std::array<int, 0> C;
+    C c = {};
+    C const& cc = c;
+    assert(CheckDebugThrows(c, 0));
+    assert(CheckDebugThrows(c, 1));
+    assert(CheckDebugThrows(cc, 0));
+    assert(CheckDebugThrows(cc, 1));
+  }
+  {
+    typedef std::array<const int, 0> C;
+    C c = {{}};
+    C const& cc = c;
+    assert(CheckDebugThrows(c, 0));
+    assert(CheckDebugThrows(c, 1));
+    assert(CheckDebugThrows(cc, 0));
+    assert(CheckDebugThrows(cc, 1));
+  }
+}