libstdc++: Add nodiscard in <algorithm>

Add the [[nodiscard]] attribute to several functions in <algorithm>.
These all have no side effects and are only called for their return
value (e.g. std::count) or produce a result that must not be discarded
for correctness (e.g. std::remove).

I was intending to add the attribute to a number of other functions like
std::copy_if, std::unique_copy, std::set_union, and std::set_difference.
I stopped when I noticed that MSVC doesn't use it on those functions,
which I suspect is because they're often used with an insert iterator
(e.g. std::back_insert_iterator). In that case it doesn't matter if
you discard the result, because you have the container to tell you how
many elements were copied to the output range.

libstdc++-v3/ChangeLog:

	* include/bits/stl_algo.h (find_end, all_of, none_of, any_of)
	(find_if_not, is_partitioned, partition_point, remove)
	(remove_if, unique, lower_bound, upper_bound, equal_range)
	(binary_search, includes, is_sorted, is_sorted_until, minmax)
	(minmax_element, is_permutation, clamp, find_if, find_first_of)
	(adjacent_find, count, count_if, search, search_n, min_element)
	(max_element): Add nodiscard attribute.
	* include/bits/stl_algobase.h (min, max, lower_bound, equal)
	(lexicographical_compare, lexicographical_compare_three_way)
	(mismatch): Likewise.
	* include/bits/stl_heap.h (is_heap, is_heap_until): Likewise.
	* testsuite/25_algorithms/equal/debug/1_neg.cc: Add dg-warning.
	* testsuite/25_algorithms/equal/debug/2_neg.cc: Likewise.
	* testsuite/25_algorithms/equal/debug/3_neg.cc: Likewise.
	* testsuite/25_algorithms/find_first_of/concept_check_1.cc:
	Likewise.
	* testsuite/25_algorithms/is_permutation/2.cc: Likewise.
	* testsuite/25_algorithms/lexicographical_compare/71545.cc:
	Likewise.
	* testsuite/25_algorithms/lower_bound/33613.cc: Likewise.
	* testsuite/25_algorithms/lower_bound/debug/irreflexive.cc:
	Likewise.
	* testsuite/25_algorithms/lower_bound/debug/partitioned_neg.cc:
	Likewise.
	* testsuite/25_algorithms/lower_bound/debug/partitioned_pred_neg.cc:
	Likewise.
	* testsuite/25_algorithms/minmax/3.cc: Likewise.
	* testsuite/25_algorithms/search/78346.cc: Likewise.
	* testsuite/25_algorithms/search_n/58358.cc: Likewise.
	* testsuite/25_algorithms/unique/1.cc: Likewise.
	* testsuite/25_algorithms/unique/11480.cc: Likewise.
	* testsuite/25_algorithms/upper_bound/33613.cc: Likewise.
	* testsuite/25_algorithms/upper_bound/debug/partitioned_neg.cc:
	Likewise.
	* testsuite/25_algorithms/upper_bound/debug/partitioned_pred_neg.cc:
	Likewise.
	* testsuite/ext/concept_checks.cc: Likewise.
	* testsuite/ext/is_heap/47709.cc: Likewise.
	* testsuite/ext/is_sorted/cxx0x.cc: Likewise.

24 files changed, 95 insertions, 72 deletions

diff --git a/libstdc++-v3/include/bits/stl_algo.h b/libstdc++-v3/include/bits/stl_algo.h
index 7a0cf6b..1a996aa 100644
--- a/libstdc++-v3/include/bits/stl_algo.h
+++ b/libstdc++-v3/include/bits/stl_algo.h
@@ -320,7 +320,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  [__first1,__last1-(__last2-__first2))
   */
   template<typename _ForwardIterator1, typename _ForwardIterator2>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline _ForwardIterator1
     find_end(_ForwardIterator1 __first1, _ForwardIterator1 __last1,
 	     _ForwardIterator2 __first2, _ForwardIterator2 __last2)
@@ -370,7 +370,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   */
   template<typename _ForwardIterator1, typename _ForwardIterator2,
 	   typename _BinaryPredicate>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline _ForwardIterator1
     find_end(_ForwardIterator1 __first1, _ForwardIterator1 __last1,
 	     _ForwardIterator2 __first2, _ForwardIterator2 __last2,
@@ -405,7 +405,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  @p [__first,__last), and false otherwise.
   */
   template<typename _InputIterator, typename _Predicate>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline bool
     all_of(_InputIterator __first, _InputIterator __last, _Predicate __pred)
     { return __last == std::find_if_not(__first, __last, __pred); }
@@ -423,7 +423,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  @p [__first,__last), and false otherwise.
   */
   template<typename _InputIterator, typename _Predicate>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline bool
     none_of(_InputIterator __first, _InputIterator __last, _Predicate __pred)
     { return __last == _GLIBCXX_STD_A::find_if(__first, __last, __pred); }
@@ -442,7 +442,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  otherwise.
   */
   template<typename _InputIterator, typename _Predicate>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline bool
     any_of(_InputIterator __first, _InputIterator __last, _Predicate __pred)
     { return !std::none_of(__first, __last, __pred); }
@@ -458,7 +458,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  such that @p __pred(*i) is false, or @p __last if no such iterator exists.
   */
   template<typename _InputIterator, typename _Predicate>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline _InputIterator
     find_if_not(_InputIterator __first, _InputIterator __last,
 		_Predicate __pred)
@@ -483,7 +483,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  do not.
   */
   template<typename _InputIterator, typename _Predicate>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline bool
     is_partitioned(_InputIterator __first, _InputIterator __last,
 		   _Predicate __pred)
@@ -505,7 +505,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *           and @p none_of(mid, __last, __pred) are both true.
   */
   template<typename _ForwardIterator, typename _Predicate>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     _ForwardIterator
     partition_point(_ForwardIterator __first, _ForwardIterator __last,
 		    _Predicate __pred)
@@ -783,7 +783,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  are still present, but their value is unspecified.
   */
   template<typename _ForwardIterator, typename _Tp>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline _ForwardIterator
     remove(_ForwardIterator __first, _ForwardIterator __last,
 	   const _Tp& __value)
@@ -817,7 +817,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  are still present, but their value is unspecified.
   */
   template<typename _ForwardIterator, typename _Predicate>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline _ForwardIterator
     remove_if(_ForwardIterator __first, _ForwardIterator __last,
 	      _Predicate __pred)
@@ -886,7 +886,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  are still present, but their value is unspecified.
   */
   template<typename _ForwardIterator>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline _ForwardIterator
     unique(_ForwardIterator __first, _ForwardIterator __last)
     {
@@ -917,7 +917,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  are still present, but their value is unspecified.
   */
   template<typename _ForwardIterator, typename _BinaryPredicate>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline _ForwardIterator
     unique(_ForwardIterator __first, _ForwardIterator __last,
 	   _BinaryPredicate __binary_pred)
@@ -1958,7 +1958,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
    *  the function used for the initial sort.
   */
   template<typename _ForwardIterator, typename _Tp, typename _Compare>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline _ForwardIterator
     lower_bound(_ForwardIterator __first, _ForwardIterator __last,
 		const _Tp& __val, _Compare __comp)
@@ -2014,7 +2014,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
    *  @ingroup binary_search_algorithms
   */
   template<typename _ForwardIterator, typename _Tp>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline _ForwardIterator
     upper_bound(_ForwardIterator __first, _ForwardIterator __last,
 		const _Tp& __val)
@@ -2045,7 +2045,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
    *  the function used for the initial sort.
   */
   template<typename _ForwardIterator, typename _Tp, typename _Compare>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline _ForwardIterator
     upper_bound(_ForwardIterator __first, _ForwardIterator __last,
 		const _Tp& __val, _Compare __comp)
@@ -2118,7 +2118,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
    *  but does not actually call those functions.
   */
   template<typename _ForwardIterator, typename _Tp>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline pair<_ForwardIterator, _ForwardIterator>
     equal_range(_ForwardIterator __first, _ForwardIterator __last,
 		const _Tp& __val)
@@ -2155,7 +2155,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
    *  but does not actually call those functions.
   */
   template<typename _ForwardIterator, typename _Tp, typename _Compare>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline pair<_ForwardIterator, _ForwardIterator>
     equal_range(_ForwardIterator __first, _ForwardIterator __last,
 		const _Tp& __val, _Compare __comp)
@@ -2189,7 +2189,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
    *  that, use std::find or a container's specialized find member functions.
   */
   template<typename _ForwardIterator, typename _Tp>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     bool
     binary_search(_ForwardIterator __first, _ForwardIterator __last,
 		  const _Tp& __val)
@@ -2223,7 +2223,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
    *  the function used for the initial sort.
   */
   template<typename _ForwardIterator, typename _Tp, typename _Compare>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     bool
     binary_search(_ForwardIterator __first, _ForwardIterator __last,
 		  const _Tp& __val, _Compare __comp)
@@ -2803,7 +2803,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
    *  returned.
   */
   template<typename _InputIterator1, typename _InputIterator2>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline bool
     includes(_InputIterator1 __first1, _InputIterator1 __last1,
 	     _InputIterator2 __first2, _InputIterator2 __last2)
@@ -2849,7 +2849,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
   */
   template<typename _InputIterator1, typename _InputIterator2,
 	   typename _Compare>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline bool
     includes(_InputIterator1 __first1, _InputIterator1 __last1,
 	     _InputIterator2 __first2, _InputIterator2 __last2,
@@ -3186,7 +3186,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
    *  @return  True if the elements are sorted, false otherwise.
   */
   template<typename _ForwardIterator>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline bool
     is_sorted(_ForwardIterator __first, _ForwardIterator __last)
     { return std::is_sorted_until(__first, __last) == __last; }
@@ -3201,7 +3201,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
    *  @return  True if the elements are sorted, false otherwise.
   */
   template<typename _ForwardIterator, typename _Compare>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline bool
     is_sorted(_ForwardIterator __first, _ForwardIterator __last,
 	      _Compare __comp)
@@ -3232,7 +3232,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
    *           for which the range [__first, i) is sorted.
   */
   template<typename _ForwardIterator>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline _ForwardIterator
     is_sorted_until(_ForwardIterator __first, _ForwardIterator __last)
     {
@@ -3257,7 +3257,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
    *           for which the range [__first, i) is sorted.
   */
   template<typename _ForwardIterator, typename _Compare>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline _ForwardIterator
     is_sorted_until(_ForwardIterator __first, _ForwardIterator __last,
 		    _Compare __comp)
@@ -3283,7 +3283,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
    *  __b) otherwise.
   */
   template<typename _Tp>
-    _GLIBCXX14_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX14_CONSTEXPR
     inline pair<const _Tp&, const _Tp&>
     minmax(const _Tp& __a, const _Tp& __b)
     {
@@ -3304,7 +3304,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
    *  __b) otherwise.
   */
   template<typename _Tp, typename _Compare>
-    _GLIBCXX14_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX14_CONSTEXPR
     inline pair<const _Tp&, const _Tp&>
     minmax(const _Tp& __a, const _Tp& __b, _Compare __comp)
     {
@@ -3384,7 +3384,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
    *           such that no other element in the range is larger.
   */
   template<typename _ForwardIterator>
-    _GLIBCXX14_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX14_CONSTEXPR
     inline pair<_ForwardIterator, _ForwardIterator>
     minmax_element(_ForwardIterator __first, _ForwardIterator __last)
     {
@@ -3412,7 +3412,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
    *           such that no other element in the range is larger.
   */
   template<typename _ForwardIterator, typename _Compare>
-    _GLIBCXX14_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX14_CONSTEXPR
     inline pair<_ForwardIterator, _ForwardIterator>
     minmax_element(_ForwardIterator __first, _ForwardIterator __last,
 		   _Compare __comp)
@@ -3430,7 +3430,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
     }
 
   template<typename _Tp>
-    _GLIBCXX14_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX14_CONSTEXPR
     inline pair<_Tp, _Tp>
     minmax(initializer_list<_Tp> __l)
     {
@@ -3442,7 +3442,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
     }
 
   template<typename _Tp, typename _Compare>
-    _GLIBCXX14_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX14_CONSTEXPR
     inline pair<_Tp, _Tp>
     minmax(initializer_list<_Tp> __l, _Compare __comp)
     {
@@ -3469,7 +3469,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
   */
   template<typename _ForwardIterator1, typename _ForwardIterator2,
 	   typename _BinaryPredicate>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline bool
     is_permutation(_ForwardIterator1 __first1, _ForwardIterator1 __last1,
 		   _ForwardIterator2 __first2, _BinaryPredicate __pred)
@@ -3563,7 +3563,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
    *          otherwise, returns false.
   */
   template<typename _ForwardIterator1, typename _ForwardIterator2>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline bool
     is_permutation(_ForwardIterator1 __first1, _ForwardIterator1 __last1,
 		   _ForwardIterator2 __first2, _ForwardIterator2 __last2)
@@ -3592,7 +3592,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
   */
   template<typename _ForwardIterator1, typename _ForwardIterator2,
 	   typename _BinaryPredicate>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline bool
     is_permutation(_ForwardIterator1 __first1, _ForwardIterator1 __last1,
 		   _ForwardIterator2 __first2, _ForwardIterator2 __last2,
@@ -3619,7 +3619,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
    *  @pre `_Tp` is LessThanComparable and `(__hi < __lo)` is false.
    */
   template<typename _Tp>
-    constexpr const _Tp&
+    [[nodiscard]] constexpr const _Tp&
     clamp(const _Tp& __val, const _Tp& __lo, const _Tp& __hi)
     {
       __glibcxx_assert(!(__hi < __lo));
@@ -3639,7 +3639,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
    *  @pre `__comp(__hi, __lo)` is false.
    */
   template<typename _Tp, typename _Compare>
-    constexpr const _Tp&
+    [[nodiscard]] constexpr const _Tp&
     clamp(const _Tp& __val, const _Tp& __lo, const _Tp& __hi, _Compare __comp)
     {
       __glibcxx_assert(!__comp(__hi, __lo));
@@ -3861,7 +3861,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
    *  such that @p __pred(*i) is true, or @p __last if no such iterator exists.
   */
   template<typename _InputIterator, typename _Predicate>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline _InputIterator
     find_if(_InputIterator __first, _InputIterator __last,
 	    _Predicate __pred)
@@ -3893,7 +3893,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
    *  otherwise returns @p __last1.
   */
   template<typename _InputIterator, typename _ForwardIterator>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     _InputIterator
     find_first_of(_InputIterator __first1, _InputIterator __last1,
 		  _ForwardIterator __first2, _ForwardIterator __last2)
@@ -3935,7 +3935,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
   */
   template<typename _InputIterator, typename _ForwardIterator,
 	   typename _BinaryPredicate>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     _InputIterator
     find_first_of(_InputIterator __first1, _InputIterator __last1,
 		  _ForwardIterator __first2, _ForwardIterator __last2,
@@ -3967,7 +3967,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
    *  or @p __last if no such iterator exists.
   */
   template<typename _ForwardIterator>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline _ForwardIterator
     adjacent_find(_ForwardIterator __first, _ForwardIterator __last)
     {
@@ -3993,7 +3993,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
    *  exists.
   */
   template<typename _ForwardIterator, typename _BinaryPredicate>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline _ForwardIterator
     adjacent_find(_ForwardIterator __first, _ForwardIterator __last,
 		  _BinaryPredicate __binary_pred)
@@ -4019,7 +4019,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
    *  for which @c *i == @p __value
   */
   template<typename _InputIterator, typename _Tp>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline typename iterator_traits<_InputIterator>::difference_type
     count(_InputIterator __first, _InputIterator __last, const _Tp& __value)
     {
@@ -4043,7 +4043,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
    *  for which @p __pred(*i) is true.
   */
   template<typename _InputIterator, typename _Predicate>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline typename iterator_traits<_InputIterator>::difference_type
     count_if(_InputIterator __first, _InputIterator __last, _Predicate __pred)
     {
@@ -4084,7 +4084,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
    *  @p [__first1,__last1-(__last2-__first2))
   */
   template<typename _ForwardIterator1, typename _ForwardIterator2>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline _ForwardIterator1
     search(_ForwardIterator1 __first1, _ForwardIterator1 __last1,
 	   _ForwardIterator2 __first2, _ForwardIterator2 __last2)
@@ -4118,7 +4118,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
    *  equal to @p __val.
   */
   template<typename _ForwardIterator, typename _Integer, typename _Tp>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline _ForwardIterator
     search_n(_ForwardIterator __first, _ForwardIterator __last,
 	     _Integer __count, const _Tp& __val)
@@ -4153,7 +4153,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
   */
   template<typename _ForwardIterator, typename _Integer, typename _Tp,
 	   typename _BinaryPredicate>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline _ForwardIterator
     search_n(_ForwardIterator __first, _ForwardIterator __last,
 	     _Integer __count, const _Tp& __val,
@@ -4178,7 +4178,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
    *  @return @p __searcher(__first,__last).first
   */
   template<typename _ForwardIterator, typename _Searcher>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline _ForwardIterator
     search(_ForwardIterator __first, _ForwardIterator __last,
 	   const _Searcher& __searcher)
@@ -5571,7 +5571,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
    *  @return  Iterator referencing the first instance of the smallest value.
   */
   template<typename _ForwardIterator>
-    _GLIBCXX14_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX14_CONSTEXPR
     _ForwardIterator
     inline min_element(_ForwardIterator __first, _ForwardIterator __last)
     {
@@ -5596,7 +5596,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
    *  according to __comp.
   */
   template<typename _ForwardIterator, typename _Compare>
-    _GLIBCXX14_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX14_CONSTEXPR
     inline _ForwardIterator
     min_element(_ForwardIterator __first, _ForwardIterator __last,
 		_Compare __comp)
@@ -5635,7 +5635,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
    *  @return  Iterator referencing the first instance of the largest value.
   */
   template<typename _ForwardIterator>
-    _GLIBCXX14_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX14_CONSTEXPR
     inline _ForwardIterator
     max_element(_ForwardIterator __first, _ForwardIterator __last)
     {
@@ -5660,7 +5660,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
    *  according to __comp.
   */
   template<typename _ForwardIterator, typename _Compare>
-    _GLIBCXX14_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX14_CONSTEXPR
     inline _ForwardIterator
     max_element(_ForwardIterator __first, _ForwardIterator __last,
 		_Compare __comp)
diff --git a/libstdc++-v3/include/bits/stl_algobase.h b/libstdc++-v3/include/bits/stl_algobase.h
index 74ff42d..d831e0e 100644
--- a/libstdc++-v3/include/bits/stl_algobase.h
+++ b/libstdc++-v3/include/bits/stl_algobase.h
@@ -228,7 +228,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  preprocessor macro.
   */
   template<typename _Tp>
-    _GLIBCXX14_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX14_CONSTEXPR
     inline const _Tp&
     min(const _Tp& __a, const _Tp& __b)
     {
@@ -252,7 +252,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  preprocessor macro.
   */
   template<typename _Tp>
-    _GLIBCXX14_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX14_CONSTEXPR
     inline const _Tp&
     max(const _Tp& __a, const _Tp& __b)
     {
@@ -276,7 +276,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  once, unlike a preprocessor macro.
   */
   template<typename _Tp, typename _Compare>
-    _GLIBCXX14_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX14_CONSTEXPR
     inline const _Tp&
     min(const _Tp& __a, const _Tp& __b, _Compare __comp)
     {
@@ -298,7 +298,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  once, unlike a preprocessor macro.
   */
   template<typename _Tp, typename _Compare>
-    _GLIBCXX14_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX14_CONSTEXPR
     inline const _Tp&
     max(const _Tp& __a, const _Tp& __b, _Compare __comp)
     {
@@ -1522,7 +1522,7 @@ _GLIBCXX_END_NAMESPACE_CONTAINER
    *  @ingroup binary_search_algorithms
   */
   template<typename _ForwardIterator, typename _Tp>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline _ForwardIterator
     lower_bound(_ForwardIterator __first, _ForwardIterator __last,
 		const _Tp& __val)
@@ -1571,7 +1571,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
    *  ranges are equal.
   */
   template<typename _II1, typename _II2>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline bool
     equal(_II1 __first1, _II1 __last1, _II2 __first2)
     {
@@ -1602,7 +1602,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
    *  ranges are equal.
   */
   template<typename _IIter1, typename _IIter2, typename _BinaryPredicate>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline bool
     equal(_IIter1 __first1, _IIter1 __last1,
 	  _IIter2 __first2, _BinaryPredicate __binary_pred)
@@ -1689,7 +1689,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
    *  ranges are equal.
   */
   template<typename _II1, typename _II2>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline bool
     equal(_II1 __first1, _II1 __last1, _II2 __first2, _II2 __last2)
     {
@@ -1722,7 +1722,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
    *  ranges are equal.
   */
   template<typename _IIter1, typename _IIter2, typename _BinaryPredicate>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline bool
     equal(_IIter1 __first1, _IIter1 __last1,
 	  _IIter2 __first2, _IIter2 __last2, _BinaryPredicate __binary_pred)
@@ -1754,7 +1754,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
    *  then this is an inline call to @c memcmp.
   */
   template<typename _II1, typename _II2>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline bool
     lexicographical_compare(_II1 __first1, _II1 __last1,
 			    _II2 __first2, _II2 __last2)
@@ -1789,7 +1789,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
    *  comp parameter instead of @c <.
   */
   template<typename _II1, typename _II2, typename _Compare>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline bool
     lexicographical_compare(_II1 __first1, _II1 __last1,
 			    _II2 __first2, _II2 __last2, _Compare __comp)
@@ -1843,7 +1843,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
    *		returns.
   */
   template<typename _InputIter1, typename _InputIter2, typename _Comp>
-    constexpr auto
+    [[nodiscard]] constexpr auto
     lexicographical_compare_three_way(_InputIter1 __first1,
 				      _InputIter1 __last1,
 				      _InputIter2 __first2,
@@ -1932,7 +1932,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
    *  to by the iterators are not equal.
   */
   template<typename _InputIterator1, typename _InputIterator2>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline pair<_InputIterator1, _InputIterator2>
     mismatch(_InputIterator1 __first1, _InputIterator1 __last1,
 	     _InputIterator2 __first2)
@@ -1967,7 +1967,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
   */
   template<typename _InputIterator1, typename _InputIterator2,
 	   typename _BinaryPredicate>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline pair<_InputIterator1, _InputIterator2>
     mismatch(_InputIterator1 __first1, _InputIterator1 __last1,
 	     _InputIterator2 __first2, _BinaryPredicate __binary_pred)
@@ -2014,7 +2014,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
    *  to by the iterators are not equal.
   */
   template<typename _InputIterator1, typename _InputIterator2>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline pair<_InputIterator1, _InputIterator2>
     mismatch(_InputIterator1 __first1, _InputIterator1 __last1,
 	     _InputIterator2 __first2, _InputIterator2 __last2)
@@ -2051,7 +2051,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
   */
   template<typename _InputIterator1, typename _InputIterator2,
 	   typename _BinaryPredicate>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline pair<_InputIterator1, _InputIterator2>
     mismatch(_InputIterator1 __first1, _InputIterator1 __last1,
 	     _InputIterator2 __first2, _InputIterator2 __last2,
diff --git a/libstdc++-v3/include/bits/stl_heap.h b/libstdc++-v3/include/bits/stl_heap.h
index 98817cf..9c1214a 100644
--- a/libstdc++-v3/include/bits/stl_heap.h
+++ b/libstdc++-v3/include/bits/stl_heap.h
@@ -492,7 +492,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  the range [__first, i) is a heap.
   */
   template<typename _RandomAccessIterator>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline _RandomAccessIterator
     is_heap_until(_RandomAccessIterator __first, _RandomAccessIterator __last)
     {
@@ -521,7 +521,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  the range [__first, i) is a heap.  Comparisons are made using __comp.
   */
   template<typename _RandomAccessIterator, typename _Compare>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline _RandomAccessIterator
     is_heap_until(_RandomAccessIterator __first, _RandomAccessIterator __last,
 		  _Compare __comp)
@@ -546,7 +546,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  @ingroup heap_algorithms
   */
   template<typename _RandomAccessIterator>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline bool
     is_heap(_RandomAccessIterator __first, _RandomAccessIterator __last)
     { return std::is_heap_until(__first, __last) == __last; }
@@ -560,7 +560,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  @ingroup heap_algorithms
   */
   template<typename _RandomAccessIterator, typename _Compare>
-    _GLIBCXX20_CONSTEXPR
+    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline bool
     is_heap(_RandomAccessIterator __first, _RandomAccessIterator __last,
 	    _Compare __comp)
diff --git a/libstdc++-v3/testsuite/25_algorithms/equal/debug/1_neg.cc b/libstdc++-v3/testsuite/25_algorithms/equal/debug/1_neg.cc
index 9d18e7d..f932859 100644
--- a/libstdc++-v3/testsuite/25_algorithms/equal/debug/1_neg.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/equal/debug/1_neg.cc
@@ -27,6 +27,7 @@ test01()
   std::vector<int> vect;
   vect.push_back(1);
   std::equal(vect.end(), vect.begin(), vect.begin());
+  // { dg-warning "ignoring return value" "" { target c++17 } 29 }
 }
 
 int
diff --git a/libstdc++-v3/testsuite/25_algorithms/equal/debug/2_neg.cc b/libstdc++-v3/testsuite/25_algorithms/equal/debug/2_neg.cc
index a24d883..2e9134a 100644
--- a/libstdc++-v3/testsuite/25_algorithms/equal/debug/2_neg.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/equal/debug/2_neg.cc
@@ -27,6 +27,7 @@ test01()
   std::vector<int> v1, v2;
   v1.push_back(1);
   std::equal(v1.begin(), v1.end(), v2.begin());
+  // { dg-warning "ignoring return value" "" { target c++17 } 29 }
 }
 
 int
diff --git a/libstdc++-v3/testsuite/25_algorithms/equal/debug/3_neg.cc b/libstdc++-v3/testsuite/25_algorithms/equal/debug/3_neg.cc
index 9301e25..425e7aa 100644
--- a/libstdc++-v3/testsuite/25_algorithms/equal/debug/3_neg.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/equal/debug/3_neg.cc
@@ -33,6 +33,7 @@ test01()
   l2.push_back(2);
 
   std::equal(++l1.begin(), l1.end(), ++l2.begin());
+  // { dg-warning "ignoring return value" "" { target c++17 } 35 }
 }
 
 int
diff --git a/libstdc++-v3/testsuite/25_algorithms/find_first_of/concept_check_1.cc b/libstdc++-v3/testsuite/25_algorithms/find_first_of/concept_check_1.cc
index f68cc80..a970cd4 100644
--- a/libstdc++-v3/testsuite/25_algorithms/find_first_of/concept_check_1.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/find_first_of/concept_check_1.cc
@@ -37,6 +37,7 @@ class2 b;
 void test01()
 {
   std::find_first_of(&a, &a, &b, &b, comp);
+  // { dg-warning "ignoring return value" "" { target c++17 } 39 }
 }
 
 int main()
diff --git a/libstdc++-v3/testsuite/25_algorithms/is_permutation/2.cc b/libstdc++-v3/testsuite/25_algorithms/is_permutation/2.cc
index 90ad2ff..ab0610c 100644
--- a/libstdc++-v3/testsuite/25_algorithms/is_permutation/2.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/is_permutation/2.cc
@@ -103,6 +103,7 @@ void test02()
   int arr[] = { 11, 22, 33 };
   using namespace std;
   is_permutation(begin(arr0), end(arr0), begin(arr), end(arr), thrower);
+  // { dg-warning "ignoring return value" "" { target c++17 } 105 }
 }
 
 int main()
diff --git a/libstdc++-v3/testsuite/25_algorithms/lexicographical_compare/71545.cc b/libstdc++-v3/testsuite/25_algorithms/lexicographical_compare/71545.cc
index fb9a0ca..9f666ca 100644
--- a/libstdc++-v3/testsuite/25_algorithms/lexicographical_compare/71545.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/lexicographical_compare/71545.cc
@@ -33,4 +33,5 @@ int main()
   X x[1];
   int i[1];
   std::lexicographical_compare(x, x+1, i, i+1);
+  // { dg-warning "ignoring return value" "" { target c++17 } 35 }
 }
diff --git a/libstdc++-v3/testsuite/25_algorithms/lower_bound/33613.cc b/libstdc++-v3/testsuite/25_algorithms/lower_bound/33613.cc
index 618091a..fa42b82 100644
--- a/libstdc++-v3/testsuite/25_algorithms/lower_bound/33613.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/lower_bound/33613.cc
@@ -33,4 +33,5 @@ bool ab(A, B);
 void test01(A* a, B b)
 {
   std::lower_bound(a, a, b, ab);
+  // { dg-warning "ignoring return value" "" { target c++17 } 35 }
 }
diff --git a/libstdc++-v3/testsuite/25_algorithms/lower_bound/debug/irreflexive.cc b/libstdc++-v3/testsuite/25_algorithms/lower_bound/debug/irreflexive.cc
index 691917e..41a6028 100644
--- a/libstdc++-v3/testsuite/25_algorithms/lower_bound/debug/irreflexive.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/lower_bound/debug/irreflexive.cc
@@ -44,4 +44,5 @@ void test01()
 {
   A as[] = { 0, 1, 2, 3 };
   std::lower_bound(as, as + 4, 1, A_int_comparer());
+  // { dg-warning "ignoring return value" "" { target c++17 } 46 }
 }
diff --git a/libstdc++-v3/testsuite/25_algorithms/lower_bound/debug/partitioned_neg.cc b/libstdc++-v3/testsuite/25_algorithms/lower_bound/debug/partitioned_neg.cc
index b72787e..bc01d81 100644
--- a/libstdc++-v3/testsuite/25_algorithms/lower_bound/debug/partitioned_neg.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/lower_bound/debug/partitioned_neg.cc
@@ -36,6 +36,7 @@ void test01()
 {
   A as[] = { 0, 1, 2, 0, 2, 3 };
   std::lower_bound(as, as + 6, A(1));
+  // { dg-warning "ignoring return value" "" { target c++17 } 38 }
 }
 
 int
diff --git a/libstdc++-v3/testsuite/25_algorithms/lower_bound/debug/partitioned_pred_neg.cc b/libstdc++-v3/testsuite/25_algorithms/lower_bound/debug/partitioned_pred_neg.cc
index 27210de..f860df9 100644
--- a/libstdc++-v3/testsuite/25_algorithms/lower_bound/debug/partitioned_pred_neg.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/lower_bound/debug/partitioned_pred_neg.cc
@@ -24,6 +24,7 @@ void test01()
 {
   int as[] = { 0, 1, 0, 2, 3 };
   std::lower_bound(as, as + 5, 1, std::less<int>());
+  // { dg-warning "ignoring return value" "" { target c++17 } 26 }
 }
 
 
diff --git a/libstdc++-v3/testsuite/25_algorithms/minmax/3.cc b/libstdc++-v3/testsuite/25_algorithms/minmax/3.cc
index 159421c..a6620b9 100644
--- a/libstdc++-v3/testsuite/25_algorithms/minmax/3.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/minmax/3.cc
@@ -43,6 +43,7 @@ int compare_counter::count = 0;
 void test01()
 {
   std::minmax({1, 2, 3, 4, 5, 6, 7, 8}, compare_counter());
+  // { dg-warning "ignoring return value" "" { target c++17 } 45 }
 
   // If N is the number of arguments in the minmax function call, 
   // 25.3.7 specifies that at most 3N/2 comparisons are allowed.
diff --git a/libstdc++-v3/testsuite/25_algorithms/search/78346.cc b/libstdc++-v3/testsuite/25_algorithms/search/78346.cc
index c1bc5a4..e6f3956 100644
--- a/libstdc++-v3/testsuite/25_algorithms/search/78346.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/search/78346.cc
@@ -109,6 +109,7 @@ test01()
 {
   value s[] = { 0, 1, 2, 3, 4, 5 };
   std::search(s, s+6, stashing_iterator(s), stashing_iterator(s+4));
+  // { dg-warning "ignoring return value" "" { target c++17 } 111 }
 }
 
 int
diff --git a/libstdc++-v3/testsuite/25_algorithms/search_n/58358.cc b/libstdc++-v3/testsuite/25_algorithms/search_n/58358.cc
index 49397b1..651fd86 100644
--- a/libstdc++-v3/testsuite/25_algorithms/search_n/58358.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/search_n/58358.cc
@@ -29,6 +29,7 @@ void test01()
   int count = 0;
   std::search_n(a.begin(), a.end(), 10, 1,
 		[&count](int t, int u) { ++count; return t == u; });
+  // { dg-warning "ignoring return value" "" { target c++17 } 30 }
   VERIFY( count <= 11 );
 }
 
diff --git a/libstdc++-v3/testsuite/25_algorithms/unique/1.cc b/libstdc++-v3/testsuite/25_algorithms/unique/1.cc
index 6826e26..698c370 100644
--- a/libstdc++-v3/testsuite/25_algorithms/unique/1.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/unique/1.cc
@@ -27,5 +27,6 @@ int main()
   using namespace std;
   list<int> menge;
   unique (menge.begin(), menge.end());
+  // { dg-warning "ignoring return value" "" { target c++17 } 29 }
   return 0;
 }
diff --git a/libstdc++-v3/testsuite/25_algorithms/unique/11480.cc b/libstdc++-v3/testsuite/25_algorithms/unique/11480.cc
index 9d1bd4e..a008fc5 100644
--- a/libstdc++-v3/testsuite/25_algorithms/unique/11480.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/unique/11480.cc
@@ -31,7 +31,7 @@ bool compare(int a, int b)
 // libstdc++/11480
 void test01()
 {
-  std::unique(a, a+10, compare);
+  std::unique(a, a+10, compare); // { dg-warning "ignoring return value" "" { target c++17 } }
   VERIFY( compare_count == 9 );
 }
 
diff --git a/libstdc++-v3/testsuite/25_algorithms/upper_bound/33613.cc b/libstdc++-v3/testsuite/25_algorithms/upper_bound/33613.cc
index 71ba2fd..19453a3 100644
--- a/libstdc++-v3/testsuite/25_algorithms/upper_bound/33613.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/upper_bound/33613.cc
@@ -33,4 +33,5 @@ bool ba(B, A);
 void test01(A* a, B b)
 {
   std::upper_bound(a, a, b, ba);
+  // { dg-warning "ignoring return value" "" { target c++17 } 35 }
 }
diff --git a/libstdc++-v3/testsuite/25_algorithms/upper_bound/debug/partitioned_neg.cc b/libstdc++-v3/testsuite/25_algorithms/upper_bound/debug/partitioned_neg.cc
index e6dfb89..630357d 100644
--- a/libstdc++-v3/testsuite/25_algorithms/upper_bound/debug/partitioned_neg.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/upper_bound/debug/partitioned_neg.cc
@@ -36,6 +36,7 @@ void test01()
 {
   A as[] = { 0, 2, 1, 3, 4, 5 };
   std::upper_bound(as, as + 6, A(1));
+  // { dg-warning "ignoring return value" "" { target c++17 } 38 }
 }
 
 int
diff --git a/libstdc++-v3/testsuite/25_algorithms/upper_bound/debug/partitioned_pred_neg.cc b/libstdc++-v3/testsuite/25_algorithms/upper_bound/debug/partitioned_pred_neg.cc
index ec5fea8..5beab7d 100644
--- a/libstdc++-v3/testsuite/25_algorithms/upper_bound/debug/partitioned_pred_neg.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/upper_bound/debug/partitioned_pred_neg.cc
@@ -24,6 +24,7 @@ void test01()
 {
   int as[] = { 0, 2, 1, 3, 4 };
   std::upper_bound(as, as + 5, 1, std::less<int>());
+  // { dg-warning "ignoring return value" "" { target c++17 } 26 }
 }
 
 
diff --git a/libstdc++-v3/testsuite/ext/concept_checks.cc b/libstdc++-v3/testsuite/ext/concept_checks.cc
index e6baad3..abb43a4 100644
--- a/libstdc++-v3/testsuite/ext/concept_checks.cc
+++ b/libstdc++-v3/testsuite/ext/concept_checks.cc
@@ -73,6 +73,10 @@ test2054( )
   upper_bound(Index.begin(), Index.end(), SearchTerm, aComparison);
   equal_range(Index.begin(), Index.end(), SearchTerm, aComparison);
   binary_search(Index.begin(), Index.end(), SearchTerm, aComparison);
+  // { dg-warning "ignoring return value" "" { target c++17 } 72 }
+  // { dg-warning "ignoring return value" "" { target c++17 } 73 }
+  // { dg-warning "ignoring return value" "" { target c++17 } 74 }
+  // { dg-warning "ignoring return value" "" { target c++17 } 75 }
 }
 
 int main()
diff --git a/libstdc++-v3/testsuite/ext/is_heap/47709.cc b/libstdc++-v3/testsuite/ext/is_heap/47709.cc
index d9bf2df..50bbcca 100644
--- a/libstdc++-v3/testsuite/ext/is_heap/47709.cc
+++ b/libstdc++-v3/testsuite/ext/is_heap/47709.cc
@@ -25,4 +25,5 @@ void foo()
 {
   std::vector<int> v;
   is_heap(v.begin(), v.end());
+  // { dg-warning "ignoring return value" "" { target c++17 } 27 }
 }
diff --git a/libstdc++-v3/testsuite/ext/is_sorted/cxx0x.cc b/libstdc++-v3/testsuite/ext/is_sorted/cxx0x.cc
index dfdd726..3592d68 100644
--- a/libstdc++-v3/testsuite/ext/is_sorted/cxx0x.cc
+++ b/libstdc++-v3/testsuite/ext/is_sorted/cxx0x.cc
@@ -24,4 +24,5 @@ void foo()
 {
   std::vector<int> v;
   is_sorted(v.begin(), v.end());
+  // { dg-warning "ignoring return value" "" { target c++17 } 26 }
 }