libstdc++: Support N3644 "Null Forward Iterators" for testsuite iterators

Comparing value-initialized forward_iterator_wrapper<T> objects fails an
assertion, but should be valid in C++14 and later.

	* testsuite/util/testsuite_iterators.h (forward_iterator_wrapper): Add
	equality comparisons that support value-initialized iterators.

libstdc++-v3/ChangeLog

 2020-02-27  Jonathan Wakely  <jwakely@redhat.com>
 
+	* testsuite/util/testsuite_iterators.h (forward_iterator_wrapper): Add
+	equality comparisons that support value-initialized iterators.
+
 	* include/bits/boost_concept_check.h (__function_requires): Add
 	_GLIBCXX14_CONSTEXPR.
 	* testsuite/25_algorithms/min/concept_checks.cc: New test.

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

@@ -337,6 +337,26 @@ namespace __gnu_test
       ++*this;
       return tmp;
     }
+
+#if __cplusplus >= 201402L
+    bool
+    operator==(const forward_iterator_wrapper& it) const noexcept
+    {
+      // Since C++14 value-initialized forward iterators are comparable.
+      if (this->SharedInfo == nullptr || it.SharedInfo == nullptr)
+	return this->SharedInfo == it.SharedInfo && this->ptr == it.ptr;
+
+      const input_iterator_wrapper<T>& base_this = *this;
+      const input_iterator_wrapper<T>& base_that = it;
+      return base_this == base_that;
+    }
+
+    bool
+    operator!=(const forward_iterator_wrapper& it) const noexcept
+    {
+      return !(*this == it);
+    }
+#endif
   };
 
   /**