[libc++] Fix constant initialization of unique_ptr in C++17 and prior

libcxx/include/__memory/compressed_pair.h

@@ -56,7 +56,7 @@ template <class _ToPad>
 class __compressed_pair_padding {
   char __padding_[((is_empty<_ToPad>::value && !__libcpp_is_final<_ToPad>::value) || is_reference<_ToPad>::value)
                       ? 0
-                      : sizeof(_ToPad) - __datasizeof_v<_ToPad>];
+                      : sizeof(_ToPad) - __datasizeof_v<_ToPad>] = {};
 };
 
 #  define _LIBCPP_COMPRESSED_PAIR(T1, Initializer1, T2, Initializer2)                                                  \

libcxx/test/std/utilities/smartptr/unique.ptr/constinit.compile.pass.cpp

@@ -0,0 +1,19 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03
+
+// <memory>
+
+// Check that uniuqe_ptr<T> is constant initialized
+
+#include <memory>
+
+#include "test_macros.h"
+
+TEST_CONSTINIT std::unique_ptr<int> ptr;

libcxx/test/support/test_macros.h

@@ -221,9 +221,11 @@
 #endif
 
 #if TEST_STD_VER > 17
-#define TEST_CONSTINIT constinit
+#  define TEST_CONSTINIT constinit
+#elif __has_cpp_attribute(clang::require_constant_initialization)
+#  define TEST_CONSTINIT [[clang::require_constant_initialization]]
 #else
-#define TEST_CONSTINIT
+#  define TEST_CONSTINIT
 #endif
 
 #if TEST_STD_VER < 11