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

[philnik777]

This is already tested in std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.ctor/default.pass.cpp except that TEST_CONSTINIT doesn't do anything before C++20 without this patch.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Nikolas Klauser (philnik777)

Changes

---

Full diff: https://github.com/llvm/llvm-project/pull/108956.diff

3 Files Affected:

• (modified) libcxx/include/__memory/compressed_pair.h (+1-1)
• (added) libcxx/test/std/utilities/smartptr/unique.ptr/constinit.compile.pass.cpp (+19)
• (modified) libcxx/test/support/test_macros.h (+4-2)

diff --git a/libcxx/include/__memory/compressed_pair.h b/libcxx/include/__memory/compressed_pair.h
index 629e3ad8848ffa..afa78f99612693 100644
--- a/libcxx/include/__memory/compressed_pair.h
+++ b/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)                                                  \
diff --git a/libcxx/test/std/utilities/smartptr/unique.ptr/constinit.compile.pass.cpp b/libcxx/test/std/utilities/smartptr/unique.ptr/constinit.compile.pass.cpp
new file mode 100644
index 00000000000000..a52089385211fb
--- /dev/null
+++ b/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;
diff --git a/libcxx/test/support/test_macros.h b/libcxx/test/support/test_macros.h
index 3aa818af1d2695..ae171b3f223ab3 100644
--- a/libcxx/test/support/test_macros.h
+++ b/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

[mstorsjo]

LGTM, thanks! This does indeed seem to fix my build issues.

[ldionne]

Would these tests fail if we refactored to run them in a constexpr context, or is this required to use constinit? I'm worried that this is a roundabout way of testing this and that we could lose the coverage in the future via some refactoring.

[philnik777]

Would these tests fail if we refactored to run them in a constexpr context, or is this required to use constinit? I'm worried that this is a roundabout way of testing this and that we could lose the coverage in the future via some refactoring.

It requires constant initialization, since unique_ptr isn't fully constexpr before C++23(or 26?).