libstdc++: Add [[nodiscard]] to iterators and related utilities

This adds [[nodiscard]] throughout <iterator>, as proposed by P2377R0
(with some minor corrections).

The attribute is added for all modes from C++11 up, using
[[__nodiscard__]] or _GLIBCXX_NODISCARD where C++17 [[nodiscard]] can't
be used directly.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	* include/bits/iterator_concepts.h (iter_move): Add
	[[nodiscard]].
	* include/bits/range_access.h (begin, end, cbegin, cend)
	(rbegin, rend, crbegin, crend, size, data, ssize): Likewise.
	* include/bits/ranges_base.h (ranges::begin, ranges::end)
	(ranges::cbegin, ranges::cend, ranges::rbegin, ranges::rend)
	(ranges::crbegin, ranges::crend, ranges::size, ranges::ssize)
	(ranges::empty, ranges::data, ranges::cdata): Likewise.
	* include/bits/stl_iterator.h (reverse_iterator, __normal_iterator)
	(back_insert_iterator, front_insert_iterator, insert_iterator)
	(move_iterator, move_sentinel, common_iterator)
	(counted_iterator): Likewise.
	* include/bits/stl_iterator_base_funcs.h (distance, next, prev):
	Likewise.
	* include/bits/stream_iterator.h (istream_iterator)
	(ostream_iterartor): Likewise.
	* include/bits/streambuf_iterator.h (istreambuf_iterator)
	(ostreambuf_iterator): Likewise.
	* include/std/ranges (views::single, views::iota, views::all)
	(views::filter, views::transform, views::take, views::take_while)
	(views::drop, views::drop_while, views::join, views::lazy_split)
	(views::split, views::counted, views::common, views::reverse)
	(views::elements): Likewise.
	* testsuite/20_util/rel_ops.cc: Use -Wno-unused-result.
	* testsuite/24_iterators/move_iterator/greedy_ops.cc: Likewise.
	* testsuite/24_iterators/normal_iterator/greedy_ops.cc:
	Likewise.
	* testsuite/24_iterators/reverse_iterator/2.cc: Likewise.
	* testsuite/24_iterators/reverse_iterator/greedy_ops.cc:
	Likewise.
	* testsuite/21_strings/basic_string/range_access/char/1.cc:
	Cast result to void.
	* testsuite/21_strings/basic_string/range_access/wchar_t/1.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/range_access/char/1.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/range_access/wchar_t/1.cc:
	Likewise.
	* testsuite/23_containers/array/range_access.cc: Likewise.
	* testsuite/23_containers/deque/range_access.cc: Likewise.
	* testsuite/23_containers/forward_list/range_access.cc:
	Likewise.
	* testsuite/23_containers/list/range_access.cc: Likewise.
	* testsuite/23_containers/map/range_access.cc: Likewise.
	* testsuite/23_containers/multimap/range_access.cc: Likewise.
	* testsuite/23_containers/multiset/range_access.cc: Likewise.
	* testsuite/23_containers/set/range_access.cc: Likewise.
	* testsuite/23_containers/unordered_map/range_access.cc:
	Likewise.
	* testsuite/23_containers/unordered_multimap/range_access.cc:
	Likewise.
	* testsuite/23_containers/unordered_multiset/range_access.cc:
	Likewise.
	* testsuite/23_containers/unordered_set/range_access.cc:
	Likewise.
	* testsuite/23_containers/vector/range_access.cc: Likewise.
	* testsuite/24_iterators/customization_points/iter_move.cc:
	Likewise.
	* testsuite/24_iterators/istream_iterator/sentinel.cc:
	Likewise.
	* testsuite/24_iterators/istreambuf_iterator/sentinel.cc:
	Likewise.
	* testsuite/24_iterators/move_iterator/dr2061.cc: Likewise.
	* testsuite/24_iterators/operations/prev_neg.cc: Likewise.
	* testsuite/24_iterators/ostreambuf_iterator/2.cc: Likewise.
	* testsuite/24_iterators/range_access/range_access.cc:
	Likewise.
	* testsuite/24_iterators/range_operations/100768.cc: Likewise.
	* testsuite/26_numerics/valarray/range_access2.cc: Likewise.
	* testsuite/28_regex/range_access.cc: Likewise.
	* testsuite/experimental/string_view/range_access/char/1.cc:
	Likewise.
	* testsuite/experimental/string_view/range_access/wchar_t/1.cc:
	Likewise.
	* testsuite/ext/vstring/range_access.cc: Likewise.
	* testsuite/std/ranges/adaptors/take.cc: Likewise.
	* testsuite/std/ranges/p2259.cc: Likewise.

13 files changed, 28 insertions, 28 deletions

diff --git a/libstdc++-v3/testsuite/23_containers/array/range_access.cc b/libstdc++-v3/testsuite/23_containers/array/range_access.cc
index 38116888a6c..807ccdb2b01 100644
--- a/libstdc++-v3/testsuite/23_containers/array/range_access.cc
+++ b/libstdc++-v3/testsuite/23_containers/array/range_access.cc
@@ -25,6 +25,6 @@ void
 test01()
 {
   std::array<int, 3> a{{1, 2, 3}};
-  std::begin(a);
-  std::end(a);
+  (void) std::begin(a);
+  (void) std::end(a);
 }
diff --git a/libstdc++-v3/testsuite/23_containers/deque/range_access.cc b/libstdc++-v3/testsuite/23_containers/deque/range_access.cc
index fed56a48414..990fc0d573b 100644
--- a/libstdc++-v3/testsuite/23_containers/deque/range_access.cc
+++ b/libstdc++-v3/testsuite/23_containers/deque/range_access.cc
@@ -25,6 +25,6 @@ void
 test01()
 {
   std::deque<int> d{1, 2, 3};
-  std::begin(d);
-  std::end(d);
+  (void) std::begin(d);
+  (void) std::end(d);
 }
diff --git a/libstdc++-v3/testsuite/23_containers/forward_list/range_access.cc b/libstdc++-v3/testsuite/23_containers/forward_list/range_access.cc
index 0b70ff02131..6398240e126 100644
--- a/libstdc++-v3/testsuite/23_containers/forward_list/range_access.cc
+++ b/libstdc++-v3/testsuite/23_containers/forward_list/range_access.cc
@@ -25,6 +25,6 @@ void
 test01()
 {
   std::forward_list<int> fl{1, 2, 3};
-  std::begin(fl);
-  std::end(fl);
+  (void) std::begin(fl);
+  (void) std::end(fl);
 }
diff --git a/libstdc++-v3/testsuite/23_containers/list/range_access.cc b/libstdc++-v3/testsuite/23_containers/list/range_access.cc
index de845d446f0..15047ec755d 100644
--- a/libstdc++-v3/testsuite/23_containers/list/range_access.cc
+++ b/libstdc++-v3/testsuite/23_containers/list/range_access.cc
@@ -25,6 +25,6 @@ void
 test01()
 {
   std::list<int> l{1, 2, 3};
-  std::begin(l);
-  std::end(l);
+  (void) std::begin(l);
+  (void) std::end(l);
 }
diff --git a/libstdc++-v3/testsuite/23_containers/map/range_access.cc b/libstdc++-v3/testsuite/23_containers/map/range_access.cc
index dc0bdbb9a74..3c6b00128ca 100644
--- a/libstdc++-v3/testsuite/23_containers/map/range_access.cc
+++ b/libstdc++-v3/testsuite/23_containers/map/range_access.cc
@@ -25,6 +25,6 @@ void
 test01()
 {
   std::map<int, double> m{{1, 1.0}, {2, 2.0}, {3, 3.0}};
-  std::begin(m);
-  std::end(m);
+  (void) std::begin(m);
+  (void) std::end(m);
 }
diff --git a/libstdc++-v3/testsuite/23_containers/multimap/range_access.cc b/libstdc++-v3/testsuite/23_containers/multimap/range_access.cc
index 32ffa3b2651..020a7a9e627 100644
--- a/libstdc++-v3/testsuite/23_containers/multimap/range_access.cc
+++ b/libstdc++-v3/testsuite/23_containers/multimap/range_access.cc
@@ -25,6 +25,6 @@ void
 test01()
 {
   std::multimap<int, double> mm{{1, 1.0}, {2, 2.0}, {3, 3.0}};
-  std::begin(mm);
-  std::end(mm);
+  (void) std::begin(mm);
+  (void) std::end(mm);
 }
diff --git a/libstdc++-v3/testsuite/23_containers/multiset/range_access.cc b/libstdc++-v3/testsuite/23_containers/multiset/range_access.cc
index 2d08ba40a59..7e66bfa24aa 100644
--- a/libstdc++-v3/testsuite/23_containers/multiset/range_access.cc
+++ b/libstdc++-v3/testsuite/23_containers/multiset/range_access.cc
@@ -25,6 +25,6 @@ void
 test01()
 {
   std::multiset<int> ms{1, 2, 3};
-  std::begin(ms);
-  std::end(ms);
+  (void) std::begin(ms);
+  (void) std::end(ms);
 }
diff --git a/libstdc++-v3/testsuite/23_containers/set/range_access.cc b/libstdc++-v3/testsuite/23_containers/set/range_access.cc
index 35703e2a7be..66099b39df4 100644
--- a/libstdc++-v3/testsuite/23_containers/set/range_access.cc
+++ b/libstdc++-v3/testsuite/23_containers/set/range_access.cc
@@ -25,6 +25,6 @@ void
 test01()
 {
   std::set<int> s{1, 2, 3};
-  std::begin(s);
-  std::end(s);
+  (void) std::begin(s);
+  (void) std::end(s);
 }
diff --git a/libstdc++-v3/testsuite/23_containers/unordered_map/range_access.cc b/libstdc++-v3/testsuite/23_containers/unordered_map/range_access.cc
index edb06d06894..ad20c1da942 100644
--- a/libstdc++-v3/testsuite/23_containers/unordered_map/range_access.cc
+++ b/libstdc++-v3/testsuite/23_containers/unordered_map/range_access.cc
@@ -25,6 +25,6 @@ void
 test01()
 {
   std::unordered_map<int, double> um{{1, 1.0}, {2, 2.0}, {3, 3.0}};
-  std::begin(um);
-  std::end(um);
+  (void) std::begin(um);
+  (void) std::end(um);
 }
diff --git a/libstdc++-v3/testsuite/23_containers/unordered_multimap/range_access.cc b/libstdc++-v3/testsuite/23_containers/unordered_multimap/range_access.cc
index e19e825a042..4bb126e34f9 100644
--- a/libstdc++-v3/testsuite/23_containers/unordered_multimap/range_access.cc
+++ b/libstdc++-v3/testsuite/23_containers/unordered_multimap/range_access.cc
@@ -25,6 +25,6 @@ void
 test01()
 {
   std::unordered_multimap<int, double> umm{{1, 1.0}, {2, 2.0}, {3, 3.0}};
-  std::begin(umm);
-  std::end(umm);
+  (void) std::begin(umm);
+  (void) std::end(umm);
 }
diff --git a/libstdc++-v3/testsuite/23_containers/unordered_multiset/range_access.cc b/libstdc++-v3/testsuite/23_containers/unordered_multiset/range_access.cc
index 574012bee32..899b74af35c 100644
--- a/libstdc++-v3/testsuite/23_containers/unordered_multiset/range_access.cc
+++ b/libstdc++-v3/testsuite/23_containers/unordered_multiset/range_access.cc
@@ -25,6 +25,6 @@ void
 test01()
 {
   std::unordered_multiset<int> ums{1, 2, 3};
-  std::begin(ums);
-  std::end(ums);
+  (void) std::begin(ums);
+  (void) std::end(ums);
 }
diff --git a/libstdc++-v3/testsuite/23_containers/unordered_set/range_access.cc b/libstdc++-v3/testsuite/23_containers/unordered_set/range_access.cc
index 033653a37ec..4f9e8857113 100644
--- a/libstdc++-v3/testsuite/23_containers/unordered_set/range_access.cc
+++ b/libstdc++-v3/testsuite/23_containers/unordered_set/range_access.cc
@@ -25,6 +25,6 @@ void
 test01()
 {
   std::unordered_set<int> us{1, 2, 3};
-  std::begin(us);
-  std::end(us);
+  (void) std::begin(us);
+  (void) std::end(us);
 }
diff --git a/libstdc++-v3/testsuite/23_containers/vector/range_access.cc b/libstdc++-v3/testsuite/23_containers/vector/range_access.cc
index a36975cac72..0dd761b71f1 100644
--- a/libstdc++-v3/testsuite/23_containers/vector/range_access.cc
+++ b/libstdc++-v3/testsuite/23_containers/vector/range_access.cc
@@ -25,10 +25,10 @@ void
 test01()
 {
   std::vector<double> v{1.0, 2.0, 3.0};
-  std::begin(v);
-  std::end(v);
+  (void) std::begin(v);
+  (void) std::end(v);
 
   std::vector<bool> vb{true, false, true};
-  std::begin(vb);
-  std::end(vb);
+  (void) std::begin(vb);
+  (void) std::end(vb);
 }