From: Jonathan Wakely <jwakely@redhat.com>
Date: Sat, 4 Nov 2023 08:30:54 +0000 (+0000)
Subject: libstdc++: Add [[nodiscard]] to std::span members
X-Git-Tag: basepoints/gcc-15~4796
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=a92a434024c59f57dc24328d946f97a5e71cee94;p=thirdparty%2Fgcc.git

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.
---

diff --git a/libstdc++-v3/include/std/span b/libstdc++-v3/include/std/span
index d5644a196a2a..90d08f18d2c0 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 b42aea3b4b40..71480246fedd 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 d3ea4f405f43..dce512aa1c83 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 876a6b773d63..d885268c96c3 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 f1d8cdccb9be..829818c65c24 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 4a8cd1841721..c47b8db72817 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 12cbd4a80aef..5dd9c9636774 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 b5525db8f1d9..b2c5e9cae320 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 c75924e5f38b..9a62bdd3233e 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 6e8cd4903b35..a2cdb8a44c7d 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 a464fa516b07..86439392e2cf 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 f36db455519d..a32246ada478 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 2e86233f19a4..c85668ab1284 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 000000000000..3bccd1545c3a
--- /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 12b79fffc5d5..3151f544a249 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 3130a5035348..4bb29fe99d67 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 2a57ddf48b8e..eee4d558feb4 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 48aaeaffbbfc..705359d504ee 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 2c90c5e67f83..21715a1a343e 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 4873c7579965..ee35e874b6d3 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 3c69f236755a..8c1d2bbfeef4 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 }