[gcc r10-6504] libstdc++: Enable three-way comparison for iota_view iterators

[Jonathan Wakely]

https://gcc.gnu.org/g:5713834e4b99e4c4c99eef15698a497f091b7dc4

commit r10-6504-g5713834e4b99e4c4c99eef15698a497f091b7dc4
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Feb 7 11:31:12 2020 +0000

    libstdc++: Enable three-way comparison for iota_view iterators
    
    The declaration of operator<=> was disabled due to a typo in the macro.
    The declaration was also ill-formed when three_way_comparable<_Winc> is
    not satisfied, which is a defect in the C++20 draft.
    
    	* include/std/ranges (iota_view::_Iterator): Fix typo in name of
    	__cpp_lib_three_way_comparison macro and use deduced return type for
    	operator<=>.
    	* testsuite/std/ranges/iota/iterator.cc: New test.

Diff:
---
 libstdc++-v3/ChangeLog                             |  7 +++++
 libstdc++-v3/include/std/ranges                    |  4 +--
 libstdc++-v3/testsuite/std/ranges/iota/iterator.cc | 35 ++++++++++++++++++++++
 3 files changed, 44 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index b9c7f43..18d02cb 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,3 +1,10 @@
+2020-02-07  Jonathan Wakely  <jwakely@redhat.com>
+
+	* include/std/ranges (iota_view::_Iterator): Fix typo in name of
+	__cpp_lib_three_way_comparison macro and use deduced return type for
+	operator<=>.
+	* testsuite/std/ranges/iota/iterator.cc: New test.
+
 2020-02-07  Patrick Palka  <ppalka@redhat.com>
 	    Jonathan Wakely  <jwakely@redhat.com>
 
diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 860f728..dc277a7 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -776,8 +776,8 @@ namespace ranges
 	  requires totally_ordered<_Winc>
 	{ return !(__x < __y); }
 
-#ifdef __cpp_lib_threeway_comparison
-	friend constexpr compare_three_way_result_t<_Winc>
+#ifdef __cpp_lib_three_way_comparison
+	friend constexpr auto
 	operator<=>(const _Iterator& __x, const _Iterator& __y)
 	  requires totally_ordered<_Winc> && three_way_comparable<_Winc>
 	{ return __x._M_value <=> __y._M_value; }
diff --git a/libstdc++-v3/testsuite/std/ranges/iota/iterator.cc b/libstdc++-v3/testsuite/std/ranges/iota/iterator.cc
new file mode 100644
index 0000000..4d47143
--- /dev/null
+++ b/libstdc++-v3/testsuite/std/ranges/iota/iterator.cc
@@ -0,0 +1,35 @@
+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+#include <ranges>
+
+auto i = std::ranges::iota_view<int>{}.begin();
+static_assert( std::three_way_comparable<decltype(i)> );
+
+struct Inc {
+  Inc& operator++();
+  Inc operator++(int);
+  friend long operator-(Inc, Inc);
+};
+static_assert( ! std::three_way_comparable<Inc> );
+
+// Instantiating iterator type must be valid despite !three_way_comparable<Inc>
+auto j = std::ranges::iota_view<Inc>{}.begin();
+static_assert( ! std::three_way_comparable<decltype(j)> );