[libc++][hardening] Constrain construction and use static_assert

... for `__{bounded.wrap}_iter`.

This PR restricts construction to cases where reference types of
source/destination iterators are (`T&`, `T&`) or (`T&`, `const T&`) (
where `T` can be const).

We can't assert `__libcpp_is_contiguous_iterator` for `__wrap_iter`
currently because `__wrap_iter` is currently used for wrapping
user-defined fancy pointers.

Author: frederick-vs-ja

libcxx/include/__iterator/bounded_iter.h

@@ -16,9 +16,14 @@
 #include <__config>
 #include <__iterator/iterator_traits.h>
 #include <__memory/pointer_traits.h>
+#include <__type_traits/conjunction.h>
+#include <__type_traits/disjunction.h>
 #include <__type_traits/enable_if.h>
 #include <__type_traits/integral_constant.h>
+#include <__type_traits/is_constructible.h>
 #include <__type_traits/is_convertible.h>
+#include <__type_traits/is_same.h>
+#include <__type_traits/make_const_lvalue_ref.h>
 #include <__utility/move.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -47,8 +52,11 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 //    pointer, it is undefined at the language level (see [expr.add]). If
 //    bounded iterators exhibited this undefined behavior, we risk compiler
 //    optimizations deleting non-redundant bounds checks.
-template <class _Iterator, class = __enable_if_t< __libcpp_is_contiguous_iterator<_Iterator>::value > >
+template <class _Iterator>
 struct __bounded_iter {
+  static_assert(__libcpp_is_contiguous_iterator<_Iterator>::value,
+                "Only contiguous iterators can be adapted by __bounded_iter.");
+
   using value_type        = typename iterator_traits<_Iterator>::value_type;
   using difference_type   = typename iterator_traits<_Iterator>::difference_type;
   using pointer           = typename iterator_traits<_Iterator>::pointer;
@@ -67,7 +75,13 @@ struct __bounded_iter {
   _LIBCPP_HIDE_FROM_ABI __bounded_iter(__bounded_iter const&) = default;
   _LIBCPP_HIDE_FROM_ABI __bounded_iter(__bounded_iter&&)      = default;
 
-  template <class _OtherIterator, __enable_if_t< is_convertible<_OtherIterator, _Iterator>::value, int> = 0>
+  template < class _OtherIterator,
+             __enable_if_t<
+                 _And<is_constructible<_Iterator, const _OtherIterator&>,
+                      is_convertible<const _OtherIterator&, _Iterator>,
+                      _Or<is_same<reference, __iter_reference<_OtherIterator> >,
+                          is_same<reference, __make_const_lvalue_ref<__iter_reference<_OtherIterator> > > > >::value,
+                 int> = 0>
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR __bounded_iter(__bounded_iter<_OtherIterator> const& __other) _NOEXCEPT
       : __current_(__other.__current_),
         __begin_(__other.__begin_),
@@ -247,7 +261,7 @@ struct __bounded_iter {
 private:
   template <class>
   friend struct pointer_traits;
-  template <class, class>
+  template <class>
   friend struct __bounded_iter;
   _Iterator __current_;       // current iterator
   _Iterator __begin_, __end_; // valid range represented as [begin, end]

libcxx/include/__iterator/wrap_iter.h

@@ -17,9 +17,14 @@
 #include <__iterator/iterator_traits.h>
 #include <__memory/addressof.h>
 #include <__memory/pointer_traits.h>
+#include <__type_traits/conjunction.h>
+#include <__type_traits/disjunction.h>
 #include <__type_traits/enable_if.h>
 #include <__type_traits/integral_constant.h>
+#include <__type_traits/is_constructible.h>
 #include <__type_traits/is_convertible.h>
+#include <__type_traits/is_same.h>
+#include <__type_traits/make_const_lvalue_ref.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
@@ -45,9 +50,15 @@ class __wrap_iter {
 
 public:
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 __wrap_iter() _NOEXCEPT : __i_() {}
-  template <class _Up, __enable_if_t<is_convertible<_Up, iterator_type>::value, int> = 0>
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 __wrap_iter(const __wrap_iter<_Up>& __u) _NOEXCEPT
-      : __i_(__u.base()) {}
+  template <
+      class _OtherIter,
+      __enable_if_t< _And<is_constructible<_Iter, const _OtherIter&>,
+                          is_convertible<const _OtherIter&, _Iter>,
+                          _Or<is_same<reference, __iter_reference<_OtherIter> >,
+                              is_same<reference, __make_const_lvalue_ref<__iter_reference<_OtherIter> > > > >::value,
+                     int> = 0>
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 __wrap_iter(const __wrap_iter<_OtherIter>& __u) _NOEXCEPT
+      : __i_(__u.__i_) {}
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 reference operator*() const _NOEXCEPT { return *__i_; }
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 pointer operator->() const _NOEXCEPT {
     return std::__to_address(__i_);

libcxx/test/libcxx/iterators/contiguous_iterators.verify.cpp

@@ -0,0 +1,22 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+//
+
+// <iterator>
+
+// __bounded_iter<_Iter>
+
+// Verify that __bounded_iter does not accept non-contiguous iterators as determined by __libcpp_is_contiguous_iterator.
+// static_assert should be used, see https://github.com/llvm/llvm-project/issues/115002.
+// __wrap_iter cannot be so handled because it may directly wrap user-defined fancy pointers in libc++'s vector.
+
+#include <deque>
+#include <vector>
+
+// expected-error-re@*:* {{static assertion failed due to requirement {{.*}}Only contiguous iterators can be adapted by __bounded_iter.}}
+std::__bounded_iter<std::deque<int>::iterator> bit;

libcxx/test/libcxx/iterators/wrap.bounded.iter.conv.compile.pass.cpp

@@ -0,0 +1,53 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+//
+
+// <iterator>
+
+// __bounded_iter<_Iter>
+// __wrap_iter<_Iter>
+
+// Verify that libc++-wrapped iterators do not permit slicing conversion or construction.
+
+#include <array>
+#include <vector>
+#include <span>
+#include <type_traits>
+
+#include "test_macros.h"
+
+struct Base {};
+struct Derived : Base {};
+
+#ifdef _LIBCPP_ABI_USE_WRAP_ITER_IN_STD_ARRAY
+static_assert(!std::is_convertible<std::array<Derived, 1>::iterator, std::array<Base, 1>::iterator>::value, "");
+static_assert(!std::is_convertible<std::array<Derived, 1>::iterator, std::array<Base, 1>::const_iterator>::value, "");
+static_assert(!std::is_convertible<std::array<Derived, 1>::const_iterator, std::array<Base, 1>::const_iterator>::value,
+              "");
+static_assert(!std::is_constructible<std::array<Base, 1>::iterator, std::array<Derived, 1>::iterator>::value, "");
+static_assert(!std::is_constructible<std::array<Base, 1>::iterator, std::array<Derived, 1>::const_iterator>::value, "");
+static_assert(
+    !std::is_constructible<std::array<Base, 1>::const_iterator, std::array<Derived, 1>::const_iterator>::value, "");
+#endif
+
+static_assert(!std::is_convertible<std::vector<Derived>::iterator, std::vector<Base>::iterator>::value, "");
+static_assert(!std::is_convertible<std::vector<Derived>::iterator, std::vector<Base>::const_iterator>::value, "");
+static_assert(!std::is_convertible<std::vector<Derived>::const_iterator, std::vector<Base>::const_iterator>::value, "");
+static_assert(!std::is_constructible<std::vector<Base>::iterator, std::vector<Derived>::iterator>::value, "");
+static_assert(!std::is_constructible<std::vector<Base>::iterator, std::vector<Derived>::const_iterator>::value, "");
+static_assert(!std::is_constructible<std::vector<Base>::const_iterator, std::vector<Derived>::const_iterator>::value,
+              "");
+
+#if TEST_STD_VER >= 20
+static_assert(!std::is_convertible_v<std::span<Derived>::iterator, std::span<Base>::iterator>);
+static_assert(!std::is_convertible_v<std::span<Derived>::iterator, std::span<Base>::const_iterator>);
+static_assert(!std::is_convertible_v<std::span<Derived>::const_iterator, std::span<Base>::const_iterator>);
+static_assert(!std::is_constructible_v<std::span<Base>::iterator, std::vector<Derived>::iterator>);
+static_assert(!std::is_constructible_v<std::span<Base>::iterator, std::vector<Derived>::const_iterator>);
+static_assert(!std::is_constructible_v<std::span<Base>::const_iterator, std::vector<Derived>::const_iterator>);
+#endif