libstdc++: Fix std::reverse_iterator relational operators

My recent changes to reverse_iterator's comparisons was not the version
of the code (or tests) that I meant to commit, and broke the relational
operators. This fixes them to reverse the order of the comparisons on
the base() iterators.

This also replaces the SFINAE constraints in the return type of the
reverse_iterator and move_iterator comparisons with a requires-clause.
This ensures the constrained overloads are preferred to unconstrained
ones. This means the non-standard same-type overloads can be omitted for
C++20 because they're not needed to solve the problem with std::rel_ops
or the testsuite's greedy_ops::X type.

	* include/bits/stl_iterator.h (reverse_iterator): Use requires-clause
	to constrain C++20 versions of comparison operators. Fix backwards
	logic of relational operators.
	(move_iterator): Use requires-clause to constrain comparison operators
	in C++20. Do not declare non-standard same-type overloads for C++20.
	* testsuite/24_iterators/move_iterator/rel_ops_c++20.cc: Check result
	of comparisons and check using greedy_ops type.
	* testsuite/24_iterators/reverse_iterator/rel_ops_c++20.cc: Likewise.
	* testsuite/24_iterators/move_iterator/greedy_ops.cc: Remove redundant
	main function from compile-only test.
	* testsuite/24_iterators/reverse_iterator/greedy_ops.cc: Likewise.

6 files changed, 167 insertions, 84 deletions

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index 3b96b23..a016f64 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,3 +1,17 @@
+2020-03-28  Jonathan Wakely  <jwakely@redhat.com>
+
+	* include/bits/stl_iterator.h (reverse_iterator): Use requires-clause
+	to constrain C++20 versions of comparison operators. Fix backwards
+	logic of relational operators.
+	(move_iterator): Use requires-clause to constrain comparison operators
+	in C++20. Do not declare non-standard same-type overloads for C++20.
+	* testsuite/24_iterators/move_iterator/rel_ops_c++20.cc: Check result
+	of comparisons and check using greedy_ops type.
+	* testsuite/24_iterators/reverse_iterator/rel_ops_c++20.cc: Likewise.
+	* testsuite/24_iterators/move_iterator/greedy_ops.cc: Remove redundant
+	main function from compile-only test.
+	* testsuite/24_iterators/reverse_iterator/greedy_ops.cc: Likewise.
+
 2020-03-27  Jonathan Wakely  <jwakely@redhat.com>
 
 	* include/bits/range_cmp.h (__cpp_lib_ranges): Define.
diff --git a/libstdc++-v3/include/bits/stl_iterator.h b/libstdc++-v3/include/bits/stl_iterator.h
index e68f66a..d7972b7 100644
--- a/libstdc++-v3/include/bits/stl_iterator.h
+++ b/libstdc++-v3/include/bits/stl_iterator.h
@@ -341,9 +341,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
         { return __t.operator->(); }
     };
 
-  // Used in unevaluated expressions to test for implicit conversion to bool.
-  namespace __detail { bool __convbool(bool); }
-
   //@{
   /**
    *  @param  __x  A %reverse_iterator.
@@ -354,7 +351,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  iterators.
    *
   */
-#if __cplusplus <= 201703L
+#if __cplusplus <= 201703L || ! defined __cpp_lib_concepts
   template<typename _Iterator>
     inline _GLIBCXX17_CONSTEXPR bool
     operator==(const reverse_iterator<_Iterator>& __x,
@@ -430,46 +427,53 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { return !(__x < __y); }
 #else // C++20
   template<typename _IteratorL, typename _IteratorR>
-    constexpr auto
+    constexpr bool
     operator==(const reverse_iterator<_IteratorL>& __x,
 	       const reverse_iterator<_IteratorR>& __y)
-    -> decltype(__detail::__convbool(__x.base() == __y.base()))
+    requires requires { { __x.base() == __y.base() } -> convertible_to<bool>; }
     { return __x.base() == __y.base(); }
 
   template<typename _IteratorL, typename _IteratorR>
-    constexpr auto
+    constexpr bool
     operator!=(const reverse_iterator<_IteratorL>& __x,
 	       const reverse_iterator<_IteratorR>& __y)
-    -> decltype(__detail::__convbool(__x.base() != __y.base()))
+    requires requires { { __x.base() != __y.base() } -> convertible_to<bool>; }
     { return __x.base() != __y.base(); }
 
   template<typename _IteratorL, typename _IteratorR>
-    constexpr auto
+    constexpr bool
     operator<(const reverse_iterator<_IteratorL>& __x,
 	      const reverse_iterator<_IteratorR>& __y)
-    -> decltype(__detail::__convbool(__x.base() < __y.base()))
-    { return __x.base() < __y.base(); }
+    requires requires { { __x.base() > __y.base() } -> convertible_to<bool>; }
+    { return __x.base() > __y.base(); }
 
   template<typename _IteratorL, typename _IteratorR>
-    constexpr auto
+    constexpr bool
     operator>(const reverse_iterator<_IteratorL>& __x,
 	      const reverse_iterator<_IteratorR>& __y)
-    -> decltype(__detail::__convbool(__x.base() > __y.base()))
-    { return __x.base() > __y.base(); }
+    requires requires { { __x.base() < __y.base() } -> convertible_to<bool>; }
+    { return __x.base() < __y.base(); }
 
   template<typename _IteratorL, typename _IteratorR>
-    constexpr auto
+    constexpr bool
     operator<=(const reverse_iterator<_IteratorL>& __x,
 	       const reverse_iterator<_IteratorR>& __y)
-    -> decltype(__detail::__convbool(__x.base() <= __y.base()))
-    { return __x.base() <= __y.base(); }
+    requires requires { { __x.base() >= __y.base() } -> convertible_to<bool>; }
+    { return __x.base() >= __y.base(); }
 
   template<typename _IteratorL, typename _IteratorR>
-    constexpr auto
+    constexpr bool
     operator>=(const reverse_iterator<_IteratorL>& __x,
 	       const reverse_iterator<_IteratorR>& __y)
-    -> decltype(__detail::__convbool(__x.base() >= __y.base()))
-    { return __x.base() >= __y.base(); }
+    requires requires { { __x.base() <= __y.base() } -> convertible_to<bool>; }
+    { return __x.base() <= __y.base(); }
+
+  template<typename _IteratorL,
+	   three_way_comparable_with<_IteratorL> _IteratorR>
+    constexpr compare_three_way_result_t<_IteratorL, _IteratorR>
+    operator<=>(const reverse_iterator<_IteratorL>& __x,
+		const reverse_iterator<_IteratorR>& __y)
+    { return __y.base() <=> __x.base(); }
 #endif // C++20
   //@}
 
@@ -1413,24 +1417,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #endif // C++20
     };
 
-  // Note: See __normal_iterator operators note from Gaby to understand
-  // why there are always 2 versions for most of the move_iterator
-  // operators.
   template<typename _IteratorL, typename _IteratorR>
     inline _GLIBCXX17_CONSTEXPR bool
     operator==(const move_iterator<_IteratorL>& __x,
 	       const move_iterator<_IteratorR>& __y)
 #if __cplusplus > 201703L && __cpp_lib_concepts
-    requires requires { __detail::__convbool(__x.base() == __y.base()); }
+    requires requires { { __x.base() == __y.base() } -> convertible_to<bool>; }
 #endif
     { return __x.base() == __y.base(); }
 
-  template<typename _Iterator>
-    inline _GLIBCXX17_CONSTEXPR bool
-    operator==(const move_iterator<_Iterator>& __x,
-	       const move_iterator<_Iterator>& __y)
-    { return __x.base() == __y.base(); }
-
 #if __cpp_lib_three_way_comparison
   template<typename _IteratorL,
 	   three_way_comparable_with<_IteratorL> _IteratorR>
@@ -1444,12 +1439,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     operator!=(const move_iterator<_IteratorL>& __x,
 	       const move_iterator<_IteratorR>& __y)
     { return !(__x == __y); }
-
-  template<typename _Iterator>
-    inline _GLIBCXX17_CONSTEXPR bool
-    operator!=(const move_iterator<_Iterator>& __x,
-	       const move_iterator<_Iterator>& __y)
-    { return !(__x == __y); }
 #endif
 
   template<typename _IteratorL, typename _IteratorR>
@@ -1457,60 +1446,81 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     operator<(const move_iterator<_IteratorL>& __x,
 	      const move_iterator<_IteratorR>& __y)
 #if __cplusplus > 201703L && __cpp_lib_concepts
-    requires requires { __detail::__convbool(__x.base() < __y.base()); }
+    requires requires { { __x.base() < __y.base() } -> convertible_to<bool>; }
 #endif
     { return __x.base() < __y.base(); }
 
-  template<typename _Iterator>
-    inline _GLIBCXX17_CONSTEXPR bool
-    operator<(const move_iterator<_Iterator>& __x,
-	      const move_iterator<_Iterator>& __y)
-    { return __x.base() < __y.base(); }
-
   template<typename _IteratorL, typename _IteratorR>
     inline _GLIBCXX17_CONSTEXPR bool
     operator<=(const move_iterator<_IteratorL>& __x,
 	       const move_iterator<_IteratorR>& __y)
 #if __cplusplus > 201703L && __cpp_lib_concepts
-    requires requires { __detail::__convbool(__y.base() < __x.base()); }
+    requires requires { { __y.base() < __x.base() } -> convertible_to<bool>; }
 #endif
     { return !(__y < __x); }
 
-  template<typename _Iterator>
-    inline _GLIBCXX17_CONSTEXPR bool
-    operator<=(const move_iterator<_Iterator>& __x,
-	       const move_iterator<_Iterator>& __y)
-    { return !(__y < __x); }
-
   template<typename _IteratorL, typename _IteratorR>
     inline _GLIBCXX17_CONSTEXPR bool
     operator>(const move_iterator<_IteratorL>& __x,
 	      const move_iterator<_IteratorR>& __y)
 #if __cplusplus > 201703L && __cpp_lib_concepts
-    requires requires { __detail::__convbool(__y.base() < __x.base()); }
+    requires requires { { __y.base() < __x.base() } -> convertible_to<bool>; }
 #endif
     { return __y < __x; }
 
-  template<typename _Iterator>
-    inline _GLIBCXX17_CONSTEXPR bool
-    operator>(const move_iterator<_Iterator>& __x,
-	      const move_iterator<_Iterator>& __y)
-    { return __y < __x; }
-
   template<typename _IteratorL, typename _IteratorR>
     inline _GLIBCXX17_CONSTEXPR bool
     operator>=(const move_iterator<_IteratorL>& __x,
 	       const move_iterator<_IteratorR>& __y)
 #if __cplusplus > 201703L && __cpp_lib_concepts
-    requires requires { __detail::__convbool(__x.base() < __y.base()); }
+    requires requires { { __x.base() < __y.base() } -> convertible_to<bool>; }
 #endif
     { return !(__x < __y); }
 
+#if ! (__cplusplus > 201703L && __cpp_lib_concepts)
+  // Note: See __normal_iterator operators note from Gaby to understand
+  // why we have these extra overloads for some move_iterator operators.
+
+  // These extra overloads are not needed in C++20, because the ones above
+  // are constrained with a requires-clause and so overload resolution will
+  // prefer them to greedy unconstrained function templates.
+
+  template<typename _Iterator>
+    inline _GLIBCXX17_CONSTEXPR bool
+    operator==(const move_iterator<_Iterator>& __x,
+	       const move_iterator<_Iterator>& __y)
+    { return __x.base() == __y.base(); }
+
+  template<typename _Iterator>
+    inline _GLIBCXX17_CONSTEXPR bool
+    operator!=(const move_iterator<_Iterator>& __x,
+	       const move_iterator<_Iterator>& __y)
+    { return !(__x == __y); }
+
+  template<typename _Iterator>
+    inline _GLIBCXX17_CONSTEXPR bool
+    operator<(const move_iterator<_Iterator>& __x,
+	      const move_iterator<_Iterator>& __y)
+    { return __x.base() < __y.base(); }
+
+  template<typename _Iterator>
+    inline _GLIBCXX17_CONSTEXPR bool
+    operator<=(const move_iterator<_Iterator>& __x,
+	       const move_iterator<_Iterator>& __y)
+    { return !(__y < __x); }
+
+  template<typename _Iterator>
+    inline _GLIBCXX17_CONSTEXPR bool
+    operator>(const move_iterator<_Iterator>& __x,
+	      const move_iterator<_Iterator>& __y)
+    { return __y < __x; }
+
   template<typename _Iterator>
     inline _GLIBCXX17_CONSTEXPR bool
     operator>=(const move_iterator<_Iterator>& __x,
 	       const move_iterator<_Iterator>& __y)
     { return !(__x < __y); }
+#endif // ! C++20
 
   // DR 685.
   template<typename _IteratorL, typename _IteratorR>
diff --git a/libstdc++-v3/testsuite/24_iterators/move_iterator/greedy_ops.cc b/libstdc++-v3/testsuite/24_iterators/move_iterator/greedy_ops.cc
index 66da0a4..0e6d598 100644
--- a/libstdc++-v3/testsuite/24_iterators/move_iterator/greedy_ops.cc
+++ b/libstdc++-v3/testsuite/24_iterators/move_iterator/greedy_ops.cc
@@ -24,7 +24,7 @@ void test01()
   typedef std::move_iterator<greedy_ops::X*> iterator_type;
 
   iterator_type it(nullptr);
-  
+
   it == it;
   it != it;
   it < it;
@@ -35,9 +35,3 @@ void test01()
   1 + it;
   it + 1;
 }
-
-int main() 
-{ 
-  test01();
-  return 0;
-}
diff --git a/libstdc++-v3/testsuite/24_iterators/move_iterator/rel_ops_c++20.cc b/libstdc++-v3/testsuite/24_iterators/move_iterator/rel_ops_c++20.cc
index 4e7b9d0..a530923 100644
--- a/libstdc++-v3/testsuite/24_iterators/move_iterator/rel_ops_c++20.cc
+++ b/libstdc++-v3/testsuite/24_iterators/move_iterator/rel_ops_c++20.cc
@@ -127,3 +127,37 @@ static_assert( has_le<2> );
 static_assert( ! has_ge<0> );
 static_assert( has_ge<1> );
 static_assert( ! has_ge<2> );
+
+int arr[3] = { 1, 2, 3 };
+constexpr std::move_iterator<int*> beg{std::begin(arr)};
+constexpr std::move_iterator<const int*> cbeg{std::cbegin(arr)};
+static_assert( beg == cbeg );
+static_assert( beg <= cbeg );
+static_assert( beg >= cbeg );
+static_assert( std::is_eq(beg <=> cbeg) );
+constexpr std::move_iterator<const int*> cend{std::cend(arr)};
+static_assert( beg != cend );
+static_assert( beg < cend );
+static_assert( cend > beg );
+static_assert( beg <= cend );
+static_assert( cend >= beg );
+static_assert( std::is_lt(beg <=> cend) );
+
+#include <testsuite_greedy_ops.h>
+
+void test01()
+{
+  typedef std::move_iterator<greedy_ops::X*> iterator_type;
+
+  iterator_type it(nullptr);
+
+  it == it;
+  it != it;
+  it < it;
+  it <= it;
+  it > it;
+  it >= it;
+  // it - it;  // See PR libstdc++/71771
+  1 + it;
+  it + 1;
+}
diff --git a/libstdc++-v3/testsuite/24_iterators/reverse_iterator/greedy_ops.cc b/libstdc++-v3/testsuite/24_iterators/reverse_iterator/greedy_ops.cc
index 60d4611..a5ea452 100644
--- a/libstdc++-v3/testsuite/24_iterators/reverse_iterator/greedy_ops.cc
+++ b/libstdc++-v3/testsuite/24_iterators/reverse_iterator/greedy_ops.cc
@@ -24,7 +24,7 @@ void test01()
   typedef std::reverse_iterator<greedy_ops::X*> iterator_type;
 
   iterator_type it;
-  
+
   it == it;
   it != it;
   it < it;
@@ -37,9 +37,3 @@ void test01()
   1 + it;
   it + 1;
 }
-
-int main() 
-{ 
-  test01();
-  return 0;
-}
diff --git a/libstdc++-v3/testsuite/24_iterators/reverse_iterator/rel_ops_c++20.cc b/libstdc++-v3/testsuite/24_iterators/reverse_iterator/rel_ops_c++20.cc
index 3e91a03..a382ae5 100644
--- a/libstdc++-v3/testsuite/24_iterators/reverse_iterator/rel_ops_c++20.cc
+++ b/libstdc++-v3/testsuite/24_iterators/reverse_iterator/rel_ops_c++20.cc
@@ -78,10 +78,10 @@ reverse_iterator<long*> r{nullptr};
 
 bool b0 = l0 == r;
 bool b1 = l1 != r;
-bool b2 = l2 < r;
-bool b3 = l3 > r;
-bool b4 = l4 <= r;
-bool b5 = l5 >= r;
+bool b2 = l2 > r;
+bool b3 = l3 < r;
+bool b4 = l4 >= r;
+bool b5 = l5 <= r;
 
 template<int N>
   concept has_eq
@@ -129,15 +129,15 @@ static_assert( ! has_ne<5> );
 
 static_assert( ! has_lt<0> );
 static_assert( ! has_lt<1> );
-static_assert( has_lt<2> );
-static_assert( ! has_lt<3> );
+static_assert( ! has_lt<2> );
+static_assert( has_lt<3> );
 static_assert( ! has_lt<4> );
 static_assert( ! has_lt<5> );
 
 static_assert( ! has_gt<0> );
 static_assert( ! has_gt<1> );
-static_assert( ! has_gt<2> );
-static_assert( has_gt<3> );
+static_assert( has_gt<2> );
+static_assert( ! has_gt<3> );
 static_assert( ! has_gt<4> );
 static_assert( ! has_gt<5> );
 
@@ -145,12 +145,49 @@ static_assert( ! has_le<0> );
 static_assert( ! has_le<1> );
 static_assert( ! has_le<2> );
 static_assert( ! has_le<3> );
-static_assert( has_le<4> );
-static_assert( ! has_le<5> );
+static_assert( ! has_le<4> );
+static_assert( has_le<5> );
 
 static_assert( ! has_ge<0> );
 static_assert( ! has_ge<1> );
 static_assert( ! has_ge<2> );
 static_assert( ! has_ge<3> );
-static_assert( ! has_ge<4> );
-static_assert( has_ge<5> );
+static_assert( has_ge<4> );
+static_assert( ! has_ge<5> );
+
+int arr[3] = { 1, 2, 3 };
+constexpr std::reverse_iterator<int*> rbeg = std::rbegin(arr);
+constexpr std::reverse_iterator<const int*> crbeg = std::crbegin(arr);
+static_assert( rbeg == crbeg );
+static_assert( rbeg <= crbeg );
+static_assert( rbeg >= crbeg );
+static_assert( std::is_eq(rbeg <=> crbeg) );
+constexpr std::reverse_iterator<const int*> crend = std::crend(arr);
+static_assert( rbeg != crend );
+static_assert( rbeg < crend );
+static_assert( crend > rbeg );
+static_assert( rbeg <= crend );
+static_assert( crend >= rbeg );
+static_assert( std::is_lt(rbeg <=> crend) );
+
+#include <testsuite_greedy_ops.h>
+
+// copied from 24_iterators/reverse_iterator/greedy_ops.cc
+void test01()
+{
+  typedef std::reverse_iterator<greedy_ops::X*> iterator_type;
+
+  iterator_type it;
+
+  it == it;
+  it != it;
+  it < it;
+  it <= it;
+  it > it;
+  it >= it;
+#if __cplusplus < 201103L
+  it - it; // See PR libstdc++/71771
+#endif
+  1 + it;
+  it + 1;
+}