[libc++][hardening] Add checks to forward_list element access.

In our implementation, failing these checks would result in a null
pointer access rather than an out-of-bounds access.

Author: var-const

libcxx/include/forward_list

@@ -766,8 +766,14 @@ public:
     return std::min<size_type>(__node_traits::max_size(this->__alloc_), numeric_limits<difference_type>::max());
   }
 
-  _LIBCPP_HIDE_FROM_ABI reference front() { return __base::__before_begin()->__next_->__get_value(); }
-  _LIBCPP_HIDE_FROM_ABI const_reference front() const { return __base::__before_begin()->__next_->__get_value(); }
+  _LIBCPP_HIDE_FROM_ABI reference front() {
+    _LIBCPP_ASSERT_NON_NULL(!empty(), "forward_list::front called on an empty list");
+    return __base::__before_begin()->__next_->__get_value();
+  }
+  _LIBCPP_HIDE_FROM_ABI const_reference front() const {
+    _LIBCPP_ASSERT_NON_NULL(!empty(), "forward_list::front called on an empty list");
+    return __base::__before_begin()->__next_->__get_value();
+  }
 
 #  ifndef _LIBCPP_CXX03_LANG
 #    if _LIBCPP_STD_VER >= 17
@@ -1085,6 +1091,7 @@ void forward_list<_Tp, _Alloc>::push_front(const value_type& __v) {
 
 template <class _Tp, class _Alloc>
 void forward_list<_Tp, _Alloc>::pop_front() {
+  _LIBCPP_ASSERT_NON_NULL(!empty(), "forward_list::pop_front called on an empty list");
   __node_pointer __p                = __base::__before_begin()->__next_;
   __base::__before_begin()->__next_ = __p->__next_;
   this->__delete_node(__p);

libcxx/test/libcxx/containers/sequences/forwardlist/assert.pass.cpp

@@ -0,0 +1,45 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <forward_list>
+
+// Test hardening assertions for std::forward_list.
+
+// REQUIRES: has-unix-headers
+// REQUIRES: libcpp-hardening-mode={{extensive|debug}}
+// UNSUPPORTED: c++03
+// XFAIL: libcpp-hardening-mode=debug && availability-verbose_abort-missing
+
+#include <forward_list>
+
+#include "check_assertion.h"
+
+int main(int, char**) {
+  { // Default-constructed list.
+    std::forward_list<int> c;
+    const auto& const_c = c;
+    TEST_LIBCPP_ASSERT_FAILURE(c.front(), "forward_list::front called on an empty list");
+    TEST_LIBCPP_ASSERT_FAILURE(const_c.front(), "forward_list::front called on an empty list");
+    TEST_LIBCPP_ASSERT_FAILURE(c.pop_front(), "forward_list::pop_front called on an empty list");
+  }
+
+  { // Non-empty list becomes empty.
+    std::forward_list<int> c;
+    const auto& const_c = c;
+    c.push_front(1);
+
+    (void)c.front(); // Check that there's no assertion on valid access.
+    (void)const_c.front(); // Check that there's no assertion on valid access.
+    c.pop_front();
+    TEST_LIBCPP_ASSERT_FAILURE(c.pop_front(), "forward_list::pop_front called on an empty list");
+    TEST_LIBCPP_ASSERT_FAILURE(c.front(), "forward_list::front called on an empty list");
+    TEST_LIBCPP_ASSERT_FAILURE(const_c.front(), "forward_list::front called on an empty list");
+  }
+
+  return 0;
+}