libstdc++: Fix complexity of drop_view::begin() const [PR112641]

Views are required to have a amortized O(1) begin(), but our drop_view's
const begin overload is O(n) for non-common ranges with a non-sized
sentinel.  This patch reimplements it so that it's O(1) always.  See
also LWG 4009.

	PR libstdc++/112641

libstdc++-v3/ChangeLog:

	* include/std/ranges (drop_view::begin): Reimplement const
	overload so that it's O(1) always.
	* testsuite/std/ranges/adaptors/drop.cc (test10): New test.

Reviewed-by: Jonathan Wakely <[email protected]>
(cherry picked from commit 7f622ee)

Author: Patrick Palka

libstdc++-v3/include/std/ranges

@@ -2530,8 +2530,8 @@ namespace views::__adaptor
       begin() const
 	requires random_access_range<const _Vp> && sized_range<const _Vp>
       {
-	return ranges::next(ranges::begin(_M_base), _M_count,
-			    ranges::end(_M_base));
+	return ranges::begin(_M_base) + ranges::min(ranges::distance(_M_base),
+						    _M_count);
       }
 
       constexpr auto

libstdc++-v3/testsuite/std/ranges/adaptors/drop.cc

@@ -275,6 +275,17 @@ test09()
   static_assert(!requires { views::all | drop; });
 }
 
+constexpr bool
+test10()
+{
+  // PR libstdc++/112641 - drop_view::begin const may have O(n) complexity
+  const auto s = ranges::subrange(views::iota(size_t(1)), size_t(-1));
+  const auto r = ranges::drop_view(s, s.size() - 1);
+  const auto b = r.begin(); // time out
+  VERIFY( *b == size_t(-1) );
+  return true;
+}
+
 int
 main()
 {
@@ -287,4 +298,5 @@ main()
   test07();
   test08();
   test09();
+  static_assert(test10());
 }