libstdc++: Add comparisons to std::default_sentinel_t (LWG 3719)

This library defect was recently approved for C++23.

libstdc++-v3/ChangeLog:

	* include/bits/fs_dir.h (directory_iterator): Add comparison
	with std::default_sentinel_t. Remove redundant operator!= for
	C++20.
	* (recursive_directory_iterator): Likewise.
	* include/bits/iterator_concepts.h [!__cpp_lib_concepts]
	(default_sentinel_t, default_sentinel): Define even if concepts
	are not supported.
	* include/bits/regex.h (regex_iterator): Add comparison with
	std::default_sentinel_t. Remove redundant operator!= for C++20.
	(regex_token_iterator): Likewise.
	(regex_token_iterator::_M_end_of_seq()): Add noexcept.
	* testsuite/27_io/filesystem/iterators/lwg3719.cc: New test.
	* testsuite/28_regex/iterators/regex_iterator/lwg3719.cc:
	New test.
	* testsuite/28_regex/iterators/regex_token_iterator/lwg3719.cc:
	New test.

6 files changed, 169 insertions, 13 deletions

diff --git a/libstdc++-v3/include/bits/fs_dir.h b/libstdc++-v3/include/bits/fs_dir.h
index ca37952ec17..bec2b7674ef 100644
--- a/libstdc++-v3/include/bits/fs_dir.h
+++ b/libstdc++-v3/include/bits/fs_dir.h
@@ -36,8 +36,9 @@
 # include <bits/unique_ptr.h>
 # include <bits/shared_ptr.h>
 
-#if __cplusplus > 201703L
+#if __cplusplus >= 202002L
 # include <compare>	// std::strong_ordering
+# include <bits/iterator_concepts.h>	// std::default_sentinel_t
 #endif
 
 namespace std _GLIBCXX_VISIBILITY(default)
@@ -420,9 +421,6 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       return __pr;
     }
 
-  private:
-    directory_iterator(const path&, directory_options, error_code*);
-
     friend bool
     operator==(const directory_iterator& __lhs,
                const directory_iterator& __rhs) noexcept
@@ -431,10 +429,22 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 	&& !__lhs._M_dir.owner_before(__rhs._M_dir);
     }
 
+#if __cplusplus >= 202002L
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 3719. Directory iterators should be usable with default sentinel
+      bool operator==(default_sentinel_t) const noexcept
+      { return !_M_dir; }
+#endif
+
+#if __cpp_impl_three_way_comparison < 201907L
     friend bool
     operator!=(const directory_iterator& __lhs,
 	       const directory_iterator& __rhs) noexcept
     { return !(__lhs == __rhs); }
+#endif
+
+  private:
+    directory_iterator(const path&, directory_options, error_code*);
 
     friend class recursive_directory_iterator;
 
@@ -519,9 +529,6 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 
     void disable_recursion_pending() noexcept;
 
-  private:
-    recursive_directory_iterator(const path&, directory_options, error_code*);
-
     friend bool
     operator==(const recursive_directory_iterator& __lhs,
                const recursive_directory_iterator& __rhs) noexcept
@@ -530,10 +537,22 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 	&& !__lhs._M_dirs.owner_before(__rhs._M_dirs);
     }
 
+#if __cplusplus >= 202002L
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 3719. Directory iterators should be usable with default sentinel
+      bool operator==(default_sentinel_t) const noexcept
+      { return !_M_dirs; }
+#endif
+
+#if __cpp_impl_three_way_comparison < 201907L
     friend bool
     operator!=(const recursive_directory_iterator& __lhs,
                const recursive_directory_iterator& __rhs) noexcept
     { return !(__lhs == __rhs); }
+#endif
+
+  private:
+    recursive_directory_iterator(const path&, directory_options, error_code*);
 
     struct _Dir_stack;
     std::__shared_ptr<_Dir_stack> _M_dirs;
diff --git a/libstdc++-v3/include/bits/iterator_concepts.h b/libstdc++-v3/include/bits/iterator_concepts.h
index a04c970b03b..cf66c63f395 100644
--- a/libstdc++-v3/include/bits/iterator_concepts.h
+++ b/libstdc++-v3/include/bits/iterator_concepts.h
@@ -32,15 +32,35 @@
 
 #pragma GCC system_header
 
+#if __cplusplus >= 202002L
 #include <concepts>
 #include <bits/ptr_traits.h>	// to_address
 #include <bits/ranges_cmp.h>	// identity, ranges::less
 
-#if __cpp_lib_concepts
 namespace std _GLIBCXX_VISIBILITY(default)
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
+  /** A sentinel type that can be used to check for the end of a range.
+   *
+   * For some iterator types the past-the-end sentinel value is independent
+   * of the underlying sequence, and a default sentinel can be used with them.
+   * For example, a `std::counted_iterator` keeps a count of how many elements
+   * remain, and so checking for the past-the-end value only requires checking
+   * if that count has reached zero. A past-the-end `std::istream_iterator` is
+   * equal to the default-constructed value, which can be easily checked.
+   *
+   * Comparing iterators of these types to `std::default_sentinel` is a
+   * convenient way to check if the end has been reached.
+   *
+   * @since C++20
+   */
+  struct default_sentinel_t { };
+
+  /// A default sentinel value.
+  inline constexpr default_sentinel_t default_sentinel{};
+
+#if __cpp_lib_concepts
   struct input_iterator_tag;
   struct output_iterator_tag;
   struct forward_iterator_tag;
@@ -924,9 +944,6 @@ namespace ranges
 
   inline constexpr unreachable_sentinel_t unreachable_sentinel{};
 
-  struct default_sentinel_t { };
-  inline constexpr default_sentinel_t default_sentinel{};
-
   // This is the namespace for [range.access] CPOs.
   namespace ranges::__cust_access
   {
@@ -983,7 +1000,8 @@ namespace ranges
 
   } // namespace __detail
 
+#endif // C++20 library concepts
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace std
-#endif // C++20 library concepts
+#endif // C++20
 #endif // _ITERATOR_CONCEPTS_H
diff --git a/libstdc++-v3/include/bits/regex.h b/libstdc++-v3/include/bits/regex.h
index 24298e35e2c..561c8fc564a 100644
--- a/libstdc++-v3/include/bits/regex.h
+++ b/libstdc++-v3/include/bits/regex.h
@@ -28,6 +28,10 @@
  *  Do not attempt to use it directly. @headername{regex}
  */
 
+#if __cplusplus >= 202002L
+# include <bits/iterator_concepts.h>	// std::default_sentinel_t
+#endif
+
 namespace std _GLIBCXX_VISIBILITY(default)
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
@@ -2760,12 +2764,21 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       bool
       operator==(const regex_iterator&) const noexcept;
 
+#if __cplusplus >= 202002L
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 3719. Directory iterators should be usable with default sentinel
+      bool operator==(default_sentinel_t) const noexcept
+      { return _M_pregex == nullptr; }
+#endif
+
+#if __cpp_impl_three_way_comparison < 201907L
       /**
        * @brief Tests the inequivalence of two regex iterators.
        */
       bool
       operator!=(const regex_iterator& __rhs) const noexcept
       { return !(*this == __rhs); }
+#endif
 
       /**
        * @brief Dereferences a %regex_iterator.
@@ -2968,12 +2981,21 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       bool
       operator==(const regex_token_iterator& __rhs) const;
 
+#if __cplusplus >= 202002L
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 3719. Directory iterators should be usable with default sentinel
+      bool operator==(default_sentinel_t) const noexcept
+      { return _M_end_of_seq(); }
+#endif
+
+#if __cpp_impl_three_way_comparison < 201907L
       /**
        * @brief Compares a %regex_token_iterator to another for inequality.
        */
       bool
       operator!=(const regex_token_iterator& __rhs) const
       { return !(*this == __rhs); }
+#endif
 
       /**
        * @brief Dereferences a %regex_token_iterator.
@@ -3022,7 +3044,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       }
 
       constexpr bool
-      _M_end_of_seq() const
+      _M_end_of_seq() const noexcept
       { return _M_result == nullptr; }
 
       // [28.12.2.2.4]
diff --git a/libstdc++-v3/testsuite/27_io/filesystem/iterators/lwg3719.cc b/libstdc++-v3/testsuite/27_io/filesystem/iterators/lwg3719.cc
new file mode 100644
index 00000000000..c19cddc74f9
--- /dev/null
+++ b/libstdc++-v3/testsuite/27_io/filesystem/iterators/lwg3719.cc
@@ -0,0 +1,39 @@
+// { dg-options "-std=gnu++20" }
+// { dg-do run { target c++20 } }
+// { dg-require-filesystem-ts "" }
+
+#include <filesystem>
+#include <iterator>
+#include <testsuite_hooks.h>
+
+// LWG 3719. Directory iterators should be usable with default sentinel
+
+void
+test_dir_iter()
+{
+  std::filesystem::directory_iterator d0;
+  VERIFY( d0 == std::default_sentinel );
+  std::filesystem::directory_iterator d1(".");
+  VERIFY( d1 != std::default_sentinel );
+
+  static_assert( noexcept(d0 == std::default_sentinel) );
+  static_assert( noexcept(d0 != std::default_sentinel) );
+}
+
+void
+test_recursive_dir_iter()
+{
+  std::filesystem::recursive_directory_iterator d0;
+  VERIFY( d0 == std::default_sentinel );
+  std::filesystem::recursive_directory_iterator d1(".");
+  VERIFY( d1 != std::default_sentinel );
+
+  static_assert( noexcept(d0 == std::default_sentinel) );
+  static_assert( noexcept(d0 != std::default_sentinel) );
+}
+
+int main()
+{
+  test_dir_iter();
+  test_recursive_dir_iter();
+}
diff --git a/libstdc++-v3/testsuite/28_regex/iterators/regex_iterator/lwg3719.cc b/libstdc++-v3/testsuite/28_regex/iterators/regex_iterator/lwg3719.cc
new file mode 100644
index 00000000000..e8c8f79364a
--- /dev/null
+++ b/libstdc++-v3/testsuite/28_regex/iterators/regex_iterator/lwg3719.cc
@@ -0,0 +1,29 @@
+// { dg-options "-std=gnu++20" }
+// { dg-do run { target c++20 } }
+
+#include <regex>
+#include <iterator>
+#include <testsuite_hooks.h>
+
+// LWG 3719. Directory iterators should be usable with default sentinel
+
+void
+test_iter()
+{
+  std::sregex_token_iterator r0;
+  VERIFY( r0 == std::default_sentinel );
+  std::string haystack = "a needle in a haystack";
+  std::regex needle("needle");
+  std::sregex_iterator r1(haystack.begin(), haystack.end(), needle);
+  VERIFY( r1 != std::default_sentinel );
+  ++r1;
+  VERIFY( r1 == std::default_sentinel );
+
+  static_assert( noexcept(r0 == std::default_sentinel) ); // GCC extension
+  static_assert( noexcept(r0 != std::default_sentinel) ); // GCC extension
+}
+
+int main()
+{
+  test_iter();
+}
diff --git a/libstdc++-v3/testsuite/28_regex/iterators/regex_token_iterator/lwg3719.cc b/libstdc++-v3/testsuite/28_regex/iterators/regex_token_iterator/lwg3719.cc
new file mode 100644
index 00000000000..5c36acead74
--- /dev/null
+++ b/libstdc++-v3/testsuite/28_regex/iterators/regex_token_iterator/lwg3719.cc
@@ -0,0 +1,29 @@
+// { dg-options "-std=gnu++20" }
+// { dg-do run { target c++20 } }
+
+#include <regex>
+#include <iterator>
+#include <testsuite_hooks.h>
+
+// LWG 3719. Directory iterators should be usable with default sentinel
+
+void
+test_iter()
+{
+  std::sregex_iterator r0;
+  VERIFY( r0 == std::default_sentinel );
+  std::string haystack = "a needle in a haystack";
+  std::regex needle("needle");
+  std::sregex_iterator r1(haystack.begin(), haystack.end(), needle);
+  VERIFY( r1 != std::default_sentinel );
+  ++r1;
+  VERIFY( r1 == std::default_sentinel );
+
+  static_assert( noexcept(r0 == std::default_sentinel) ); // GCC extension
+  static_assert( noexcept(r0 != std::default_sentinel) ); // GCC extension
+}
+
+int main()
+{
+  test_iter();
+}