[libc++] Makes `unique_ptr operator*() noexcept. (#98047)

This implements
 - LWG2762  unique_ptr operator*() should be noexcept.

Differential Revision: https://reviews.llvm.org/D128214

Author: mordante

libcxx/docs/Status/Cxx23Issues.csv

@@ -99,7 +99,7 @@
 "","","","","",""
 `2191 <https://wg21.link/LWG2191>`__,"Incorrect specification of ``match_results(match_results&&)``","October 2021","|Nothing To Do|",""
 `2381 <https://wg21.link/LWG2381>`__,"Inconsistency in parsing floating point numbers","October 2021","|Complete|","19.0"
-`2762 <https://wg21.link/LWG2762>`__,"``unique_ptr operator*()`` should be ``noexcept``","October 2021","",""
+`2762 <https://wg21.link/LWG2762>`__,"``unique_ptr operator*()`` should be ``noexcept``","October 2021","|Complete|","19.0"
 `3121 <https://wg21.link/LWG3121>`__,"``tuple`` constructor constraints for ``UTypes&&...`` overloads","October 2021","",""
 `3123 <https://wg21.link/LWG3123>`__,"``duration`` constructor from representation shouldn't be effectively non-throwing","October 2021","","","|chrono|"
 `3146 <https://wg21.link/LWG3146>`__,"Excessive unwrapping in ``std::ref/cref``","October 2021","|Complete|","14.0"

libcxx/include/__memory/unique_ptr.h

@@ -36,7 +36,9 @@
 #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>
 #include <__utility/move.h>
 #include <cstddef>
@@ -50,6 +52,17 @@ _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");
@@ -252,7 +265,8 @@ class _LIBCPP_UNIQUE_PTR_TRIVIAL_ABI _LIBCPP_TEMPLATE_VIS unique_ptr {
     return *this;
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 __add_lvalue_reference_t<_Tp> operator*() const {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 __add_lvalue_reference_t<_Tp> operator*() const
+      _NOEXCEPT_(__is_noexcept_deref_or_void<pointer>::value) {
     return *__ptr_.first();
   }
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 pointer operator->() const _NOEXCEPT { return __ptr_.first(); }

libcxx/include/memory

@@ -451,7 +451,8 @@ public:
     constexpr unique_ptr& operator=(nullptr_t) noexcept;                              // constexpr since C++23
 
     // observers
-    typename constexpr add_lvalue_reference<T>::type operator*() const;               // constexpr since C++23
+    constexpr
+    add_lvalue_reference<T>::type operator*() const noexcept(see below);              // constexpr since C++23
     constexpr pointer operator->() const noexcept;                                    // constexpr since C++23
     constexpr pointer get() const noexcept;                                           // constexpr since C++23
     constexpr deleter_type& get_deleter() noexcept;                                   // constexpr since C++23

libcxx/include/optional

@@ -136,12 +136,12 @@ namespace std {
     void swap(optional &) noexcept(see below ); // constexpr in C++20
 
     // [optional.observe], observers
-    constexpr T const *operator->() const;
-    constexpr T *operator->();
-    constexpr T const &operator*() const &;
-    constexpr T &operator*() &;
-    constexpr T &&operator*() &&;
-    constexpr const T &&operator*() const &&;
+    constexpr T const *operator->() const noexcept;
+    constexpr T *operator->() noexcept;
+    constexpr T const &operator*() const & noexcept;
+    constexpr T &operator*() & noexcept;
+    constexpr T &&operator*() && noexcept;
+    constexpr const T &&operator*() const && noexcept;
     constexpr explicit operator bool() const noexcept;
     constexpr bool has_value() const noexcept;
     constexpr T const &value() const &;
@@ -783,12 +783,12 @@ public:
     }
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr add_pointer_t<value_type const> operator->() const {
+  _LIBCPP_HIDE_FROM_ABI constexpr add_pointer_t<value_type const> operator->() const noexcept {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(this->has_value(), "optional operator-> called on a disengaged value");
     return std::addressof(this->__get());
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr add_pointer_t<value_type> operator->() {
+  _LIBCPP_HIDE_FROM_ABI constexpr add_pointer_t<value_type> operator->() noexcept {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(this->has_value(), "optional operator-> called on a disengaged value");
     return std::addressof(this->__get());
   }

libcxx/test/std/utilities/memory/unique.ptr/noexcept_operator_star.compile.pass.cpp

@@ -0,0 +1,25 @@
+//===----------------------------------------------------------------------===//
+//
+// 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*()); }

libcxx/test/std/utilities/optional/optional.object/optional.object.observe/dereference.pass.cpp

@@ -44,15 +44,7 @@ int main(int, char**)
     {
         optional<X> opt; ((void)opt);
         ASSERT_SAME_TYPE(decltype(*opt), X&);
-        LIBCPP_STATIC_ASSERT(noexcept(*opt));
-        // ASSERT_NOT_NOEXCEPT(*opt);
-        // FIXME: This assertion fails with GCC because it can see that
-        // (A) operator*() is constexpr, and
-        // (B) there is no path through the function that throws.
-        // It's arguable if this is the correct behavior for the noexcept
-        // operator.
-        // Regardless this function should still be noexcept(false) because
-        // it has a narrow contract.
+        ASSERT_NOEXCEPT(*opt);
     }
     {
         optional<X> opt(X{});

libcxx/test/std/utilities/optional/optional.object/optional.object.observe/dereference_const.pass.cpp

@@ -37,15 +37,7 @@ int main(int, char**)
     {
         const optional<X> opt; ((void)opt);
         ASSERT_SAME_TYPE(decltype(*opt), X const&);
-        LIBCPP_STATIC_ASSERT(noexcept(*opt));
-        // ASSERT_NOT_NOEXCEPT(*opt);
-        // FIXME: This assertion fails with GCC because it can see that
-        // (A) operator*() is constexpr, and
-        // (B) there is no path through the function that throws.
-        // It's arguable if this is the correct behavior for the noexcept
-        // operator.
-        // Regardless this function should still be noexcept(false) because
-        // it has a narrow contract.
+        ASSERT_NOEXCEPT(*opt);
     }
     {
         constexpr optional<X> opt(X{});

libcxx/test/std/utilities/optional/optional.object/optional.object.observe/dereference_const_rvalue.pass.cpp

@@ -37,15 +37,7 @@ int main(int, char**)
     {
         const optional<X> opt; ((void)opt);
         ASSERT_SAME_TYPE(decltype(*std::move(opt)), X const &&);
-        LIBCPP_STATIC_ASSERT(noexcept(*opt));
-        // ASSERT_NOT_NOEXCEPT(*std::move(opt));
-        // FIXME: This assertion fails with GCC because it can see that
-        // (A) operator*() is constexpr, and
-        // (B) there is no path through the function that throws.
-        // It's arguable if this is the correct behavior for the noexcept
-        // operator.
-        // Regardless this function should still be noexcept(false) because
-        // it has a narrow contract.
+        ASSERT_NOEXCEPT(*std::move(opt));
     }
     {
         constexpr optional<X> opt(X{});

libcxx/test/std/utilities/optional/optional.object/optional.object.observe/dereference_rvalue.pass.cpp

@@ -44,15 +44,7 @@ int main(int, char**)
     {
         optional<X> opt; ((void)opt);
         ASSERT_SAME_TYPE(decltype(*std::move(opt)), X&&);
-        LIBCPP_STATIC_ASSERT(noexcept(*opt));
-        // ASSERT_NOT_NOEXCEPT(*std::move(opt));
-        // FIXME: This assertion fails with GCC because it can see that
-        // (A) operator*() is constexpr, and
-        // (B) there is no path through the function that throws.
-        // It's arguable if this is the correct behavior for the noexcept
-        // operator.
-        // Regardless this function should still be noexcept(false) because
-        // it has a narrow contract.
+        ASSERT_NOEXCEPT(*std::move(opt));
     }
     {
         optional<X> opt(X{});

libcxx/test/std/utilities/optional/optional.object/optional.object.observe/op_arrow.pass.cpp

@@ -41,14 +41,7 @@ int main(int, char**)
     {
         std::optional<X> opt; ((void)opt);
         ASSERT_SAME_TYPE(decltype(opt.operator->()), X*);
-        // ASSERT_NOT_NOEXCEPT(opt.operator->());
-        // FIXME: This assertion fails with GCC because it can see that
-        // (A) operator->() is constexpr, and
-        // (B) there is no path through the function that throws.
-        // It's arguable if this is the correct behavior for the noexcept
-        // operator.
-        // Regardless this function should still be noexcept(false) because
-        // it has a narrow contract.
+        ASSERT_NOEXCEPT(opt.operator->());
     }
     {
         optional<X> opt(X{});

libcxx/test/std/utilities/optional/optional.object/optional.object.observe/op_arrow_const.pass.cpp

@@ -40,14 +40,7 @@ int main(int, char**)
     {
         const std::optional<X> opt; ((void)opt);
         ASSERT_SAME_TYPE(decltype(opt.operator->()), X const*);
-        // ASSERT_NOT_NOEXCEPT(opt.operator->());
-        // FIXME: This assertion fails with GCC because it can see that
-        // (A) operator->() is constexpr, and
-        // (B) there is no path through the function that throws.
-        // It's arguable if this is the correct behavior for the noexcept
-        // operator.
-        // Regardless this function should still be noexcept(false) because
-        // it has a narrow contract.
+        ASSERT_NOEXCEPT(opt.operator->());
     }
     {
         constexpr optional<X> opt(X{});

libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.observers/dereference.single.pass.cpp

@@ -14,12 +14,50 @@
 
 #include <memory>
 #include <cassert>
+#include <vector>
 
 #include "test_macros.h"
 
+#if TEST_STD_VER >= 11
+struct ThrowDereference {
+  TEST_CONSTEXPR_CXX23 ThrowDereference& operator*() noexcept(false);
+  TEST_CONSTEXPR_CXX23 operator bool() const { return false; }
+};
+
+struct Deleter {
+  using pointer = ThrowDereference;
+  TEST_CONSTEXPR_CXX23 void operator()(ThrowDereference&) const {}
+};
+#endif
+
 TEST_CONSTEXPR_CXX23 bool test() {
-  std::unique_ptr<int> p(new int(3));
-  assert(*p == 3);
+  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);
+    ASSERT_NOEXCEPT(*p);
+  }
+#if TEST_STD_VER >= 11
+  {
+    std::unique_ptr<std::vector<int>> p(new std::vector<int>{3, 4, 5});
+    assert((*p)[0] == 3);
+    assert((*p)[1] == 4);
+    assert((*p)[2] == 5);
+    ASSERT_NOEXCEPT(*p);
+  }
+  {
+    std::unique_ptr<ThrowDereference> p;
+    ASSERT_NOEXCEPT(*p);
+  }
+  {
+    // The noexcept status of *unique_ptr<>::pointer should be propagated.
+    std::unique_ptr<ThrowDereference, Deleter> p;
+    ASSERT_NOT_NOEXCEPT(*p);
+  }
+#endif
 
   return true;
 }