[libc++] Simplify the implementation of LWG2762 (noexcept for unique_ptr)

[ldionne]

I had originally made some comments in https://reviews.llvm.org/D128214 that were followed when we implemented LWG2762. I don't understand why I made these comments anymore, but either way it seems like I was wrong since using unique_ptr<void>::operator* should be ill-formed. All other implementations also make that ill-formed.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Louis Dionne (ldionne)

Changes

I had originally made some comments in https://reviews.llvm.org/D128214 that were followed when we implemented LWG2762. I don't understand why I made these comments anymore, but either way it seems like I was wrong since using unique_ptr<void>::operator* should be ill-formed. All other implementations also make that ill-formed.

---

Full diff: https://github.com/llvm/llvm-project/pull/102032.diff

3 Files Affected:

• (modified) libcxx/include/__memory/unique_ptr.h (+1-13)
• (removed) libcxx/test/std/utilities/memory/unique.ptr/noexcept_operator_star.compile.pass.cpp (-25)
• (modified) libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.observers/dereference.single.pass.cpp (-4)

diff --git a/libcxx/include/__memory/unique_ptr.h b/libcxx/include/__memory/unique_ptr.h
index f75259473efb1..7f5e0ea243c95 100644
--- a/libcxx/include/__memory/unique_ptr.h
+++ b/libcxx/include/__memory/unique_ptr.h
@@ -36,7 +36,6 @@
 #include <__type_traits/is_trivially_relocatable.h>
 #include <__type_traits/is_void.h>
 #include <__type_traits/remove_extent.h>
-#include <__type_traits/remove_pointer.h>
 #include <__type_traits/type_identity.h>
 #include <__utility/declval.h>
 #include <__utility/forward.h>
@@ -52,17 +51,6 @@ _LIBCPP_PUSH_MACROS
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#ifndef _LIBCPP_CXX03_LANG
-
-template <class _Ptr>
-struct __is_noexcept_deref_or_void {
-  static constexpr bool value = noexcept(*std::declval<_Ptr>());
-};
-
-template <>
-struct __is_noexcept_deref_or_void<void*> : true_type {};
-#endif
-
 template <class _Tp>
 struct _LIBCPP_TEMPLATE_VIS default_delete {
   static_assert(!is_function<_Tp>::value, "default_delete cannot be instantiated for function types");
@@ -266,7 +254,7 @@ class _LIBCPP_UNIQUE_PTR_TRIVIAL_ABI _LIBCPP_TEMPLATE_VIS unique_ptr {
   }
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 __add_lvalue_reference_t<_Tp> operator*() const
-      _NOEXCEPT_(__is_noexcept_deref_or_void<pointer>::value) {
+      _NOEXCEPT_(_NOEXCEPT_(*std::declval<pointer>())) {
     return *__ptr_.first();
   }
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 pointer operator->() const _NOEXCEPT { return __ptr_.first(); }
diff --git a/libcxx/test/std/utilities/memory/unique.ptr/noexcept_operator_star.compile.pass.cpp b/libcxx/test/std/utilities/memory/unique.ptr/noexcept_operator_star.compile.pass.cpp
deleted file mode 100644
index a2d788005f0cd..0000000000000
--- a/libcxx/test/std/utilities/memory/unique.ptr/noexcept_operator_star.compile.pass.cpp
+++ /dev/null
@@ -1,25 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// 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
-
-// unique_ptr
-
-// add_lvalue_reference_t<T> operator*() const noexcept(noexcept(*declval<pointer>()));
-
-// Dereferencing pointer directly in noexcept fails for a void pointer.  This
-// is not SFINAE-ed away leading to a hard error. The issue was originally
-// triggered by
-// test/std/utilities/memory/unique.ptr/iterator_concept_conformance.compile.pass.cpp
-//
-// This test validates whether the code compiles.
-
-#include <memory>
-
-extern const std::unique_ptr<void> p;
-void f() { [[maybe_unused]] bool b = noexcept(p.operator*()); }
diff --git a/libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.observers/dereference.single.pass.cpp b/libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.observers/dereference.single.pass.cpp
index 17902a4ce27a4..ba7f8bbb76222 100644
--- a/libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.observers/dereference.single.pass.cpp
+++ b/libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.observers/dereference.single.pass.cpp
@@ -31,10 +31,6 @@ struct Deleter {
 #endif
 
 TEST_CONSTEXPR_CXX23 bool test() {
-  ASSERT_NOEXCEPT(*(std::unique_ptr<void>{}));
-#if TEST_STD_VER >= 11
-  static_assert(noexcept(*std::declval<std::unique_ptr<void>>()), "");
-#endif
   {
     std::unique_ptr<int> p(new int(3));
     assert(*p == 3);

[mordante]

IIRC there were issue to test static_assert(noexcept(unique_ptr<void>::operator*())); But let's see what the CI says.

[mordante]

This was the cause for adding this.

[ldionne]

Got it. FWIW this test passes after applying this patch locally. Perhaps it used to fail with an older compiler? I seem to recall older Clang instantiating noexcept too eagerly at some point in the past.

[ldionne]

IIRC there were issue to test static_assert(noexcept(unique_ptr<void>::operator*())); But let's see what the CI says.

My understanding is that static_assert(noexcept(unique_ptr<void>::operator*())); is ill-formed. All other implementations produce a hard-error for that case, and that seems to be what the Standard specifies.

[mordante]

LGTM with the CI passing.

[frederick-vs-ja]

It seems that the SFINAE-friendness of unique_ptr<void>::operator* was originally expected and tested in https://reviews.llvm.org/D100073 (57ebf3d). Should we just remove the test coverage?

[ldionne]

It seems that the SFINAE-friendness of unique_ptr<void>::operator* was originally expected and tested in https://reviews.llvm.org/D100073 (57ebf3d). Should we just remove the test coverage?

Yes, I think that makes the most sense.