libstdc++: Add [[nodiscard]] to std::span members

All std::span member functions are pure functions that have no side
effects. They are only useful for their return value, so they should all
warn if that value is not used.

libstdc++-v3/ChangeLog:

	* include/std/span (span, as_bytes, as_writable_bytes): Add
	[[nodiscard]] attribute on all non-void functions.
	* testsuite/23_containers/span/back_assert_neg.cc: Suppress
	nodiscard warning.
	* testsuite/23_containers/span/back_neg.cc: Likewise.
	* testsuite/23_containers/span/first_2_assert_neg.cc: Likewise.
	* testsuite/23_containers/span/first_assert_neg.cc: Likewise.
	* testsuite/23_containers/span/first_neg.cc: Likewise.
	* testsuite/23_containers/span/front_assert_neg.cc: Likewise.
	* testsuite/23_containers/span/front_neg.cc: Likewise.
	* testsuite/23_containers/span/index_op_assert_neg.cc: Likewise.
	* testsuite/23_containers/span/index_op_neg.cc: Likewise.
	* testsuite/23_containers/span/last_2_assert_neg.cc: Likewise.
	* testsuite/23_containers/span/last_assert_neg.cc: Likewise.
	* testsuite/23_containers/span/last_neg.cc: Likewise.
	* testsuite/23_containers/span/subspan_2_assert_neg.cc:
	Likewise.
	* testsuite/23_containers/span/subspan_3_assert_neg.cc:
	Likewise.
	* testsuite/23_containers/span/subspan_4_assert_neg.cc:
	Likewise.
	* testsuite/23_containers/span/subspan_5_assert_neg.cc:
	Likewise.
	* testsuite/23_containers/span/subspan_6_assert_neg.cc:
	Likewise.
	* testsuite/23_containers/span/subspan_assert_neg.cc: Likewise.
	* testsuite/23_containers/span/subspan_neg.cc: Likewise.
	* testsuite/23_containers/span/nodiscard.cc: New test.

21 files changed, 103 insertions, 23 deletions

diff --git a/libstdc++-v3/include/std/span b/libstdc++-v3/include/std/span
index d5644a19..90d08f1 100644
--- a/libstdc++-v3/include/std/span
+++ b/libstdc++-v3/include/std/span
@@ -246,20 +246,24 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       // observers
 
+      [[nodiscard]]
       constexpr size_type
       size() const noexcept
       { return this->_M_extent._M_extent(); }
 
+      [[nodiscard]]
       constexpr size_type
       size_bytes() const noexcept
       { return this->_M_extent._M_extent() * sizeof(element_type); }
 
-      [[nodiscard]] constexpr bool
+      [[nodiscard]]
+      constexpr bool
       empty() const noexcept
       { return size() == 0; }
 
       // element access
 
+      [[nodiscard]]
       constexpr reference
       front() const noexcept
       {
@@ -267,6 +271,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	return *this->_M_ptr;
       }
 
+      [[nodiscard]]
       constexpr reference
       back() const noexcept
       {
@@ -274,6 +279,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	return *(this->_M_ptr + (size() - 1));
       }
 
+      [[nodiscard]]
       constexpr reference
       operator[](size_type __idx) const noexcept
       {
@@ -281,41 +287,50 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	return *(this->_M_ptr + __idx);
       }
 
+      [[nodiscard]]
       constexpr pointer
       data() const noexcept
       { return this->_M_ptr; }
 
       // iterator support
 
+      [[nodiscard]]
       constexpr iterator
       begin() const noexcept
       { return iterator(this->_M_ptr); }
 
+      [[nodiscard]]
       constexpr iterator
       end() const noexcept
       { return iterator(this->_M_ptr + this->size()); }
 
+      [[nodiscard]]
       constexpr reverse_iterator
       rbegin() const noexcept
       { return reverse_iterator(this->end()); }
 
+      [[nodiscard]]
       constexpr reverse_iterator
       rend() const noexcept
       { return reverse_iterator(this->begin()); }
 
 #if __cplusplus > 202002L
+      [[nodiscard]]
       constexpr const_iterator
       cbegin() const noexcept
       { return begin(); }
 
+      [[nodiscard]]
       constexpr const_iterator
       cend() const noexcept
       { return end(); }
 
+      [[nodiscard]]
       constexpr const_reverse_iterator
       crbegin() const noexcept
       { return rbegin(); }
 
+      [[nodiscard]]
       constexpr const_reverse_iterator
       crend() const noexcept
       { return rend(); }
@@ -324,6 +339,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       // subviews
 
       template<size_t _Count>
+	[[nodiscard]]
 	constexpr span<element_type, _Count>
 	first() const noexcept
 	{
@@ -335,6 +351,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  return _Sp{ this->data(), _Count };
 	}
 
+      [[nodiscard]]
       constexpr span<element_type, dynamic_extent>
       first(size_type __count) const noexcept
       {
@@ -343,6 +360,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       }
 
       template<size_t _Count>
+	[[nodiscard]]
 	constexpr span<element_type, _Count>
 	last() const noexcept
 	{
@@ -354,6 +372,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  return _Sp{ this->data() + (this->size() - _Count), _Count };
 	}
 
+      [[nodiscard]]
       constexpr span<element_type, dynamic_extent>
       last(size_type __count) const noexcept
       {
@@ -362,6 +381,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       }
 
       template<size_t _Offset, size_t _Count = dynamic_extent>
+	[[nodiscard]]
 	constexpr auto
 	subspan() const noexcept
 	-> span<element_type, _S_subspan_extent<_Offset, _Count>()>
@@ -393,6 +413,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	    }
 	}
 
+      [[nodiscard]]
       constexpr span<element_type, dynamic_extent>
       subspan(size_type __offset, size_type __count = dynamic_extent) const
       noexcept
@@ -434,6 +455,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       -> span<remove_reference_t<ranges::range_reference_t<_Range&>>>;
 
   template<typename _Type, size_t _Extent>
+    [[nodiscard]]
     inline
     span<const byte, _Extent == dynamic_extent
 	? dynamic_extent : _Extent * sizeof(_Type)>
@@ -451,7 +473,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     inline
     span<byte, _Extent == dynamic_extent
        ? dynamic_extent : _Extent * sizeof(_Type)>
-    as_writable_bytes(span<_Type, _Extent> __sp) noexcept
+    as_writable_bytes [[nodiscard]] (span<_Type, _Extent> __sp) noexcept
     {
       auto data = reinterpret_cast<byte*>(__sp.data());
       auto size = __sp.size_bytes();
diff --git a/libstdc++-v3/testsuite/23_containers/span/back_assert_neg.cc b/libstdc++-v3/testsuite/23_containers/span/back_assert_neg.cc
index b42aea3..7148024 100644
--- a/libstdc++-v3/testsuite/23_containers/span/back_assert_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/span/back_assert_neg.cc
@@ -25,5 +25,5 @@
 int main()
 {
   std::span<int, std::dynamic_extent> s;
-  s.back();
+  (void) s.back();
 }
diff --git a/libstdc++-v3/testsuite/23_containers/span/back_neg.cc b/libstdc++-v3/testsuite/23_containers/span/back_neg.cc
index d3ea4f4..dce512a 100644
--- a/libstdc++-v3/testsuite/23_containers/span/back_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/span/back_neg.cc
@@ -24,7 +24,7 @@ test01(bool b)
 {
   std::span<int, 0> s;
   if (b || !s.empty())
-    s.back();
+    (void) s.back();
   return true;
 }
 
diff --git a/libstdc++-v3/testsuite/23_containers/span/first_2_assert_neg.cc b/libstdc++-v3/testsuite/23_containers/span/first_2_assert_neg.cc
index 876a6b7..d885268 100644
--- a/libstdc++-v3/testsuite/23_containers/span/first_2_assert_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/span/first_2_assert_neg.cc
@@ -26,5 +26,5 @@ int main()
 {
   int a[4];
   std::span<int, std::dynamic_extent> s(a);
-  s.first(5);
+  (void) s.first(5);
 }
diff --git a/libstdc++-v3/testsuite/23_containers/span/first_assert_neg.cc b/libstdc++-v3/testsuite/23_containers/span/first_assert_neg.cc
index f1d8cdc..829818c 100644
--- a/libstdc++-v3/testsuite/23_containers/span/first_assert_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/span/first_assert_neg.cc
@@ -26,5 +26,5 @@ int main()
 {
   int a[4];
   std::span<int, std::dynamic_extent> s(a);
-  s.first<5>();
+  (void) s.first<5>();
 }
diff --git a/libstdc++-v3/testsuite/23_containers/span/first_neg.cc b/libstdc++-v3/testsuite/23_containers/span/first_neg.cc
index 4a8cd18..c47b8db 100644
--- a/libstdc++-v3/testsuite/23_containers/span/first_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/span/first_neg.cc
@@ -24,6 +24,6 @@ test01()
 {
   int a[4];
   std::span<int, 4> s(a);
-  s.first<5>(); // { dg-error "here" }
+  (void) s.first<5>(); // { dg-error "here" }
 }
 // { dg-error "static assertion failed" "" { target *-*-* } 0 }
diff --git a/libstdc++-v3/testsuite/23_containers/span/front_assert_neg.cc b/libstdc++-v3/testsuite/23_containers/span/front_assert_neg.cc
index 12cbd4a..5dd9c96 100644
--- a/libstdc++-v3/testsuite/23_containers/span/front_assert_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/span/front_assert_neg.cc
@@ -25,5 +25,5 @@
 int main()
 {
   std::span<int, std::dynamic_extent> s;
-  s.front();
+  (void) s.front();
 }
diff --git a/libstdc++-v3/testsuite/23_containers/span/front_neg.cc b/libstdc++-v3/testsuite/23_containers/span/front_neg.cc
index b5525db..b2c5e9c 100644
--- a/libstdc++-v3/testsuite/23_containers/span/front_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/span/front_neg.cc
@@ -24,7 +24,7 @@ test01(bool b)
 {
   std::span<int, 0> s;
   if (b || !s.empty())
-    s.front();
+    (void) s.front();
   return true;
 }
 
diff --git a/libstdc++-v3/testsuite/23_containers/span/index_op_assert_neg.cc b/libstdc++-v3/testsuite/23_containers/span/index_op_assert_neg.cc
index c75924e..9a62bdd 100644
--- a/libstdc++-v3/testsuite/23_containers/span/index_op_assert_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/span/index_op_assert_neg.cc
@@ -25,5 +25,5 @@
 int main()
 {
   std::span<int, std::dynamic_extent> s;
-  s[99];
+  (void) s[99];
 }
diff --git a/libstdc++-v3/testsuite/23_containers/span/index_op_neg.cc b/libstdc++-v3/testsuite/23_containers/span/index_op_neg.cc
index 6e8cd49..a2cdb8a4 100644
--- a/libstdc++-v3/testsuite/23_containers/span/index_op_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/span/index_op_neg.cc
@@ -24,7 +24,7 @@ test01(bool b)
 {
   std::span<int, 0> s;
   if (b || !s.empty())
-    s[99];
+    (void) s[99];
   return true;
 }
 
diff --git a/libstdc++-v3/testsuite/23_containers/span/last_2_assert_neg.cc b/libstdc++-v3/testsuite/23_containers/span/last_2_assert_neg.cc
index a464fa5..8643939 100644
--- a/libstdc++-v3/testsuite/23_containers/span/last_2_assert_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/span/last_2_assert_neg.cc
@@ -26,5 +26,5 @@ int main()
 {
   int a[4];
   std::span<int, std::dynamic_extent> s(a);
-  s.last(5);
+  (void) s.last(5);
 }
diff --git a/libstdc++-v3/testsuite/23_containers/span/last_assert_neg.cc b/libstdc++-v3/testsuite/23_containers/span/last_assert_neg.cc
index f36db45..a32246a 100644
--- a/libstdc++-v3/testsuite/23_containers/span/last_assert_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/span/last_assert_neg.cc
@@ -26,5 +26,5 @@ int main()
 {
   int a[4];
   std::span<int, std::dynamic_extent> s(a);
-  s.last<5>();
+  (void) s.last<5>();
 }
diff --git a/libstdc++-v3/testsuite/23_containers/span/last_neg.cc b/libstdc++-v3/testsuite/23_containers/span/last_neg.cc
index 2e86233..c85668a 100644
--- a/libstdc++-v3/testsuite/23_containers/span/last_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/span/last_neg.cc
@@ -24,6 +24,6 @@ test01()
 {
   int a[2];
   std::span<int, 2> s(a);
-  s.last<3>(); // { dg-error "here" }
+  (void) s.last<3>(); // { dg-error "here" }
 }
 // { dg-error "static assertion failed" "" { target *-*-* } 0 }
diff --git a/libstdc++-v3/testsuite/23_containers/span/nodiscard.cc b/libstdc++-v3/testsuite/23_containers/span/nodiscard.cc
new file mode 100644
index 0000000..3bccd15
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/span/nodiscard.cc
@@ -0,0 +1,58 @@
+// { dg-do compile { target c++20 } }
+
+#include <span>
+
+void
+test_observers(std::span<int> s)
+{
+  s.size(); // { dg-warning "ignoring return value" }
+  s.size_bytes(); // { dg-warning "ignoring return value" }
+  s.empty(); // { dg-warning "ignoring return value" }
+}
+
+void
+test_element_access(std::span<float> s)
+{
+  s.front(); // { dg-warning "ignoring return value" }
+  s.back(); // { dg-warning "ignoring return value" }
+  s[1]; // { dg-warning "ignoring return value" }
+  s.data(); // { dg-warning "ignoring return value" }
+}
+
+struct S { };
+
+void
+test_iterators(std::span<S> s)
+{
+  s.begin(); // { dg-warning "ignoring return value" }
+  s.end(); // { dg-warning "ignoring return value" }
+  s.rbegin(); // { dg-warning "ignoring return value" }
+  s.rend(); // { dg-warning "ignoring return value" }
+
+#if __cplusplus > 202002L
+  s.cbegin(); // { dg-warning "ignoring return value" "" { target c++23 } }
+  s.cend(); // { dg-warning "ignoring return value" "" { target c++23 } }
+  s.crbegin(); // { dg-warning "ignoring return value" "" { target c++23 } }
+  s.crend(); // { dg-warning "ignoring return value" "" { target c++23 } }
+#endif
+}
+
+void
+test_subviews(std::span<long, 20> s)
+{
+  s.first<5>(); // { dg-warning "ignoring return value" }
+  s.first(6); // { dg-warning "ignoring return value" }
+  s.last<7>(); // { dg-warning "ignoring return value" }
+  s.last(8); // { dg-warning "ignoring return value" }
+  s.subspan<1>(); // { dg-warning "ignoring return value" }
+  s.subspan<2, 3>(); // { dg-warning "ignoring return value" }
+  s.subspan(4); // { dg-warning "ignoring return value" }
+  s.subspan(5, 6); // { dg-warning "ignoring return value" }
+}
+
+void
+test_non_members(std::span<S, 20> s)
+{
+  std::as_bytes(s); // { dg-warning "ignoring return value" }
+  std::as_writable_bytes(s); // { dg-warning "ignoring return value" }
+}
diff --git a/libstdc++-v3/testsuite/23_containers/span/subspan_2_assert_neg.cc b/libstdc++-v3/testsuite/23_containers/span/subspan_2_assert_neg.cc
index 12b79ff..3151f54 100644
--- a/libstdc++-v3/testsuite/23_containers/span/subspan_2_assert_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/span/subspan_2_assert_neg.cc
@@ -26,5 +26,5 @@ int main()
 {
   int a[4];
   std::span<int, std::dynamic_extent> s(a);
-  s.subspan<2, 5>();
+  (void) s.subspan<2, 5>();
 }
diff --git a/libstdc++-v3/testsuite/23_containers/span/subspan_3_assert_neg.cc b/libstdc++-v3/testsuite/23_containers/span/subspan_3_assert_neg.cc
index 3130a50..4bb29fe 100644
--- a/libstdc++-v3/testsuite/23_containers/span/subspan_3_assert_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/span/subspan_3_assert_neg.cc
@@ -26,5 +26,5 @@ int main()
 {
   int a[4];
   std::span<int, std::dynamic_extent> s(a);
-  s.subspan<2, 3>();
+  (void) s.subspan<2, 3>();
 }
diff --git a/libstdc++-v3/testsuite/23_containers/span/subspan_4_assert_neg.cc b/libstdc++-v3/testsuite/23_containers/span/subspan_4_assert_neg.cc
index 2a57ddf..eee4d55 100644
--- a/libstdc++-v3/testsuite/23_containers/span/subspan_4_assert_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/span/subspan_4_assert_neg.cc
@@ -26,5 +26,5 @@ int main()
 {
   int a[4];
   std::span<int, std::dynamic_extent> s(a);
-  s.subspan(5, 0);
+  (void) s.subspan(5, 0);
 }
diff --git a/libstdc++-v3/testsuite/23_containers/span/subspan_5_assert_neg.cc b/libstdc++-v3/testsuite/23_containers/span/subspan_5_assert_neg.cc
index 48aaeaf..705359d 100644
--- a/libstdc++-v3/testsuite/23_containers/span/subspan_5_assert_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/span/subspan_5_assert_neg.cc
@@ -26,5 +26,5 @@ int main()
 {
   int a[4];
   std::span<int, std::dynamic_extent> s(a);
-  s.subspan(2, 5);
+  (void) s.subspan(2, 5);
 }
diff --git a/libstdc++-v3/testsuite/23_containers/span/subspan_6_assert_neg.cc b/libstdc++-v3/testsuite/23_containers/span/subspan_6_assert_neg.cc
index 2c90c5e..21715a1 100644
--- a/libstdc++-v3/testsuite/23_containers/span/subspan_6_assert_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/span/subspan_6_assert_neg.cc
@@ -26,5 +26,5 @@ int main()
 {
   int a[4];
   std::span<int, std::dynamic_extent> s(a);
-  s.subspan(2, 3);
+  (void) s.subspan(2, 3);
 }
diff --git a/libstdc++-v3/testsuite/23_containers/span/subspan_assert_neg.cc b/libstdc++-v3/testsuite/23_containers/span/subspan_assert_neg.cc
index 4873c75..ee35e87 100644
--- a/libstdc++-v3/testsuite/23_containers/span/subspan_assert_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/span/subspan_assert_neg.cc
@@ -26,5 +26,5 @@ int main()
 {
   int a[4];
   std::span<int, std::dynamic_extent> s(a);
-  s.subspan<5, 0>();
+  (void) s.subspan<5, 0>();
 }
diff --git a/libstdc++-v3/testsuite/23_containers/span/subspan_neg.cc b/libstdc++-v3/testsuite/23_containers/span/subspan_neg.cc
index 3c69f23..8c1d2bb 100644
--- a/libstdc++-v3/testsuite/23_containers/span/subspan_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/span/subspan_neg.cc
@@ -24,7 +24,7 @@ test01()
 {
   int a[4];
   std::span<int, 4> s(a);
-  s.subspan<5, 0>(); // { dg-error "here" }
+  (void) s.subspan<5, 0>(); // { dg-error "here" }
 }
 
 void
@@ -32,7 +32,7 @@ test02()
 {
   int a[4];
   std::span<int, 4> s(a);
-  s.subspan<3, 5>(); // { dg-error "here" }
+  (void) s.subspan<3, 5>(); // { dg-error "here" }
 }
 
 void
@@ -40,7 +40,7 @@ test03()
 {
   int a[4];
   std::span<int, 4> s(a);
-  s.subspan<3, 2>(); // { dg-error "here" }
+  (void) s.subspan<3, 2>(); // { dg-error "here" }
 }
 
 // { dg-error "static assertion failed" "" { target *-*-* } 0 }