libstdc++: Add a testsuite range type that has a sized sentinel

This adds a testsuite range type whose end() is a sized sentinel to
<testsuite_iterators.h>, which will be used in the tests that verify LWG 3355.

libstdc++-v3/ChangeLog:

	* testsuite/util/testsuite_iterators.h (test_range::get_iterator): Make
	protected instead of private.
	(test_sized_range_sized_sent): New.

libstdc++-v3/ChangeLog

 2020-03-04  Patrick Palka  <ppalka@redhat.com>
 
+	* testsuite/util/testsuite_iterators.h (test_range::get_iterator): Make
+	protected instead of private.
+	(test_sized_range_sized_sent): New.
+
 	* testsuite/util/testsuite_iterators.h (input_iterator_wrapper_nocopy):
 	New testsuite iterator.
 	* testsuite/24_iterators/counted_iterator/lwg3389.cc: use it.

libstdc++-v3/testsuite/util/testsuite_iterators.h

@@ -735,6 +735,7 @@ namespace __gnu_test
 	  { return i.ptr - s.end; }
 	};
 
+    protected:
       auto
       get_iterator(T* p)
       {
@@ -812,6 +813,37 @@ namespace __gnu_test
     using test_output_sized_range
       = test_sized_range<T, output_iterator_wrapper>;
 
+  // A type meeting the minimum std::sized_range requirements, and whose end()
+  // returns a sized sentinel.
+  template<typename T, template<typename> class Iter>
+    struct test_sized_range_sized_sent : test_sized_range<T, Iter>
+    {
+      using test_sized_range<T, Iter>::test_sized_range;
+
+      template<typename I>
+	struct sentinel
+	{
+	  T* end;
+
+	  friend bool operator==(const sentinel& s, const I& i) noexcept
+	  { return s.end == i.ptr; }
+
+	  friend std::iter_difference_t<I>
+	  operator-(const sentinel& s, const I& i) noexcept
+	  { return s.end - i.ptr; }
+
+	  friend std::iter_difference_t<I>
+	  operator-(const I& i, const sentinel& s) noexcept
+	  { return i.ptr - s.end; }
+	};
+
+      auto end() &
+      {
+	using I = decltype(this->get_iterator(this->bounds.last));
+	return sentinel<I>{this->bounds.last};
+      }
+    };
+
 // test_range and test_sized_range do not own their elements, so they model
 // std::ranges::borrowed_range.  This file does not define specializations of
 // std::ranges::enable_borrowed_range, so that individual tests can decide