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.

libstdc++-v3/ChangeLog

+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>
 

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; }

libstdc++-v3/testsuite/std/ranges/iota/iterator.cc

+// 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)> );