[libc++] Add an ABI flag to make __bit_iterator trivially copyable

libcxx/docs/ABIGuarantees.rst

@@ -157,6 +157,10 @@ This flag adds ``[[clang::trivial_abi]]`` to ``unique_ptr``, which makes it triv
 ---------------------------------------------
 This flag adds ``[[clang::trivial_abi]]`` to ``shared_ptr``, which makes it trivial for the purpose of calls.
 
+``_LIBCPP_ABI_TRIVIALLY_COPYABLE_BIT_ITERATOR``
+-----------------------------------------------
+This flag makes ``__bit_iterator`` (a.k.a. ``vector<bool>::iterator``) trivially copyable as well as trivial for the
+purpose of calls, since the copy constructor is made trivial.
 
 Types that public aliases reference
 ===================================

libcxx/include/__bit_reference

@@ -314,10 +314,15 @@ public:
   // When _IsConst=true, this is a converting constructor;
   // the copy and move constructors are implicitly generated
   // and trivial.
+#ifdef _LIBCPP_ABI_TRIVIALLY_COPYABLE_BIT_ITERATOR
+  template <bool _IsConstDep = _IsConst, __enable_if_t<_IsConstDep, int> = 0>
+#endif
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __bit_iterator(const __bit_iterator<_Cp, false>& __it) _NOEXCEPT
       : __seg_(__it.__seg_),
-        __ctz_(__it.__ctz_) {}
+        __ctz_(__it.__ctz_) {
+  }
 
+#ifndef _LIBCPP_ABI_TRIVIALLY_COPYABLE_BIT_ITERATOR
   // When _IsConst=false, we have a user-provided copy constructor,
   // so we must also provide a copy assignment operator because
   // the implicit generation of a defaulted one is deprecated.
@@ -329,6 +334,10 @@ public:
     __ctz_ = __it.__ctz_;
     return *this;
   }
+#else
+  _LIBCPP_HIDE_FROM_ABI __bit_iterator(const __bit_iterator&)            = default;
+  _LIBCPP_HIDE_FROM_ABI __bit_iterator& operator=(const __bit_iterator&) = default;
+#endif // _LIBCPP_ABI_TRIVIALLY_COPYABLE_BIT_ITERATOR
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference operator*() const _NOEXCEPT {
     _LIBCPP_ASSERT_INTERNAL(__ctz_ < __bits_per_word, "Dereferencing an invalid __bit_iterator.");

libcxx/include/__configuration/abi.h

@@ -84,6 +84,7 @@
 #  define _LIBCPP_ABI_USE_WRAP_ITER_IN_STD_ARRAY
 #  define _LIBCPP_ABI_USE_WRAP_ITER_IN_STD_STRING_VIEW
 #  define _LIBCPP_ABI_VARIANT_INDEX_TYPE_OPTIMIZATION
+#  define _LIBCPP_ABI_TRIVIALLY_COPYABLE_BIT_ITERATOR
 
 #elif _LIBCPP_ABI_VERSION == 1
 #  if !(defined(_LIBCPP_OBJECT_FORMAT_COFF) || defined(_LIBCPP_OBJECT_FORMAT_XCOFF))

libcxx/test/libcxx/containers/sequences/vector.bool/trivial_for_purposes_of_call.pass.cpp → libcxx/test/libcxx/containers/sequences/vector.bool/trivialty.compile.pass.cpp

@@ -38,19 +38,13 @@ using IsTrivialForCall = std::integral_constant<
     // Ignore the all-deleted case, it shouldn't occur here.
     >;
 
-void test_const_iterator() {
-  using It = std::vector<bool>::const_iterator;
-  static_assert(IsTrivialForCall<It>::value, "");
-}
-
-void test_non_const_iterator() {
-  using It = std::vector<bool>::iterator;
-  static_assert(!IsTrivialForCall<It>::value, "");
-}
-
-int main(int, char**) {
-  test_const_iterator();
-  test_non_const_iterator();
-
-  return 0;
-}
+static_assert(IsTrivialForCall<std::vector<bool>::const_iterator>::value, "");
+static_assert(std::is_trivially_copyable<std::vector<bool>::const_iterator>::value, "");
+
+#ifndef _LIBCPP_ABI_TRIVIALLY_COPYABLE_BIT_ITERATOR
+static_assert(!IsTrivialForCall<std::vector<bool>::iterator>::value, "");
+static_assert(!std::is_trivially_copyable<std::vector<bool>::iterator>::value, "");
+#else
+static_assert(IsTrivialForCall<std::vector<bool>::iterator>::value, "");
+static_assert(std::is_trivially_copyable<std::vector<bool>::iterator>::value, "");
+#endif