[gcc r11-8545] libstdc++: Fix access issue in iota_view::_Sentinel [PR100690]

[Patrick Palka <ppalka@gcc.gnu.org>]

https://gcc.gnu.org/g:c6ccaf17ddbe8adb76475826be02580790ab28ca

commit r11-8545-gc6ccaf17ddbe8adb76475826be02580790ab28ca
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu May 20 23:39:05 2021 -0400

    libstdc++: Fix access issue in iota_view::_Sentinel [PR100690]
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/100690
            * include/std/ranges (iota_view::_Sentinel::_M_distance_from):
            Split out this member function from ...
            (iota_view::_Sentinel::operator-): ... here, for sake of access
            control.
            * testsuite/std/ranges/iota/iota_view.cc (test05): New test.
    
    (cherry picked from commit 317a38cd468d565dc8ce45c6da0dbccf38808f70)

Diff:
---
 libstdc++-v3/include/std/ranges                     |  8 ++++++--
 libstdc++-v3/testsuite/std/ranges/iota/iota_view.cc | 11 +++++++++++
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 1f4093a17b5..9ba28afbfce 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -499,6 +499,10 @@ namespace ranges
 	_M_equal(const _Iterator& __x) const
 	{ return __x._M_value == _M_bound; }
 
+	constexpr auto
+	_M_distance_from(const _Iterator& __x) const
+	{ return _M_bound - __x._M_value; }
+
 	_Bound _M_bound = _Bound();
 
       public:
@@ -515,12 +519,12 @@ namespace ranges
 	friend constexpr iter_difference_t<_Winc>
 	operator-(const _Iterator& __x, const _Sentinel& __y)
 	  requires sized_sentinel_for<_Bound, _Winc>
-	{ return __x._M_value - __y._M_bound; }
+	{ return -__y._M_distance_from(__x); }
 
 	friend constexpr iter_difference_t<_Winc>
 	operator-(const _Sentinel& __x, const _Iterator& __y)
 	  requires sized_sentinel_for<_Bound, _Winc>
-	{ return -(__y - __x); }
+	{ return __x._M_distance_from(__y); }
       };
 
       _Winc _M_value = _Winc();
diff --git a/libstdc++-v3/testsuite/std/ranges/iota/iota_view.cc b/libstdc++-v3/testsuite/std/ranges/iota/iota_view.cc
index be8695120ad..362ef1f7f78 100644
--- a/libstdc++-v3/testsuite/std/ranges/iota/iota_view.cc
+++ b/libstdc++-v3/testsuite/std/ranges/iota/iota_view.cc
@@ -80,6 +80,16 @@ test04()
 // Verify we optimize away the 'bound' data member of an unbounded iota_view.
 static_assert(sizeof(std::ranges::iota_view<char>) == 1);
 
+void
+test05()
+{
+  // PR libstdc++/100690
+  int x[] = {42, 42, 42};
+  auto r = std::views::iota(std::ranges::begin(x), std::ranges::cbegin(x) + 3);
+  VERIFY( r.end() - r.begin() == 3 );
+  VERIFY( r.begin() - r.end() == -3 );
+}
+
 int
 main()
 {
@@ -87,4 +97,5 @@ main()
   test02();
   test03();
   test04();
+  test05();
 }