libstdc++: LWG 3286 ranges::size is not required to be valid after ...

... a call to ranges::begin on an input range.

This implements LWG 3286.  The new wording for the single-argument constructor
for subrange is implemented by splitting the constructor into two delegating
constructors, one constrained by _S_store_size and the other by !_S_store_size.

Tested on x86_64-pc-linux-gnu, both added tests fail before the patch and pass
with the patch.

libstdc++-v3/ChangeLog:

	LWG 3286 ranges::size is not required to be valid after a call to
	ranges::begin on an input range
	* include/std/ranges (subrange::subrange): Split single-argument
	constructor into two, one constrained by _S_store_size and another by
	!_S_store_size.
	(take_view::begin): Call size() before calling ranges::begin(_M_base).
	* testsuite/std/ranges/adaptors/lwg3286.cc: New test.
	* testsuite/std/ranges/subrange/lwg3286.cc: New test.

libstdc++-v3/ChangeLog

+2020-03-11  Patrick Palka  <ppalka@redhat.com>
+
+	LWG 3286 ranges::size is not required to be valid after a call to
+	ranges::begin on an input range
+	* include/std/ranges (subrange::subrange): Split single-argument
+	constructor into two, one constrained by _S_store_size and another by
+	!_S_store_size.
+	(take_view::begin): Call size() before calling ranges::begin(_M_base).
+	* testsuite/std/ranges/adaptors/lwg3286.cc: New test.
+	* testsuite/std/ranges/subrange/lwg3286.cc: New test.
+
 2020-03-10  Jonathan Wakely  <jwakely@redhat.com>
 
 	* include/std/ranges (split_view::_OuterIter::__at_end): Use __current

libstdc++-v3/include/std/ranges

@@ -281,12 +281,18 @@ namespace ranges
 	  && __detail::__convertible_to_non_slicing<iterator_t<_Rng>, _It>
 	  && convertible_to<sentinel_t<_Rng>, _Sent>
 	constexpr
-	subrange(_Rng&& __r) requires (!_S_store_size || sized_range<_Rng>)
+	subrange(_Rng&& __r) requires _S_store_size && sized_range<_Rng>
+	: subrange{__r, ranges::size(__r)}
+	{ }
+
+      template<__detail::__not_same_as<subrange> _Rng>
+	requires borrowed_range<_Rng>
+	  && __detail::__convertible_to_non_slicing<iterator_t<_Rng>, _It>
+	  && convertible_to<sentinel_t<_Rng>, _Sent>
+	constexpr
+	subrange(_Rng&& __r) requires (!_S_store_size)
 	: subrange{ranges::begin(__r), ranges::end(__r)}
-	{
-	  if constexpr (_S_store_size)
-	    _M_size._M_size = ranges::size(__r);
-	}
+	{ }
 
       template<borrowed_range _Rng>
 	requires __detail::__convertible_to_non_slicing<iterator_t<_Rng>, _It>
@@ -2032,7 +2038,10 @@ namespace views
 	    if constexpr (random_access_range<_Vp>)
 	      return ranges::begin(_M_base);
 	    else
-	      return counted_iterator{ranges::begin(_M_base), size()};
+	      {
+		auto __sz = size();
+		return counted_iterator{ranges::begin(_M_base), __sz};
+	      }
 	  }
 	else
 	  return counted_iterator{ranges::begin(_M_base), _M_count};
@@ -2046,7 +2055,10 @@ namespace views
 	    if constexpr (random_access_range<const _Vp>)
 	      return ranges::begin(_M_base);
 	    else
-	      return counted_iterator{ranges::begin(_M_base), size()};
+	      {
+		auto __sz = size();
+		return counted_iterator{ranges::begin(_M_base), __sz};
+	      }
 	  }
 	else
 	  return counted_iterator{ranges::begin(_M_base), _M_count};

libstdc++-v3/testsuite/std/ranges/adaptors/lwg3286.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 run { target c++2a } }
+
+#include <ranges>
+#include <testsuite_iterators.h>
+#include <testsuite_hooks.h>
+
+using __gnu_test::test_input_range;
+
+namespace ranges = std::ranges;
+namespace views = std::views;
+
+struct my_range
+{
+  static inline int x[] = {1,2,3};
+  static inline test_input_range<int> r{x};
+
+  bool called_begin = false;
+
+  auto
+  begin()
+  {
+    called_begin = true;
+    return r.begin();
+  }
+
+  auto
+  end()
+  {
+    return r.end();
+  }
+
+  ranges::range_difference_t<decltype(r)>
+  size()
+  {
+    VERIFY( !called_begin );
+    return 3;
+  }
+};
+
+void
+test01()
+{
+  my_range r;
+  static_assert(!ranges::forward_range<my_range>);
+  static_assert(ranges::sized_range<my_range>);
+  auto v = r | views::take(3);
+  auto i = ranges::begin(v);
+}
+
+int
+main()
+{
+  test01();
+}

libstdc++-v3/testsuite/std/ranges/subrange/lwg3286.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 run { target c++2a } }
+
+#include <ranges>
+#include <testsuite_iterators.h>
+#include <testsuite_hooks.h>
+
+using __gnu_test::test_input_range;
+
+namespace ranges = std::ranges;
+
+struct my_range
+{
+  static inline int x[] = {1,2,3};
+  static inline test_input_range<int> r{x};
+
+  bool called_begin = false;
+
+  auto
+  begin()
+  {
+    called_begin = true;
+    return r.begin();
+  }
+
+  auto
+  end()
+  {
+    return r.end();
+  }
+
+  ranges::range_difference_t<decltype(r)>
+  size()
+  {
+    VERIFY( !called_begin );
+    return 3;
+  }
+};
+
+void
+test01()
+{
+  my_range r;
+  static_assert(!ranges::forward_range<my_range>);
+  static_assert(ranges::sized_range<my_range>);
+  ranges::subrange sr = r;
+  VERIFY( ranges::size(sr) == 3 );
+}
+
+int
+main()
+{
+  test01();
+}