[PATCH] libstdc++: Make `std::expected` nodiscard [PR119197]

[Arthur O'Dwyer]

The new test includes two lines that currently do not warn because of
GCC compiler bug PR85973; the lines that do warn are the more
important cases.

        PR libstdc++/119197

libstdc++-v3/ChangeLog:

        * include/std/expected (std::expected): Add [[nodiscard]] to class.
        * testsuite/20_util/expected/119197.cc: New test.

Signed-off-by: Arthur O'Dwyer <arthur.j.odw...@gmail.com>
---
 libstdc++-v3/include/std/expected             |  4 ++--
 .../testsuite/20_util/expected/119197.cc      | 20 +++++++++++++++++++
 2 files changed, 22 insertions(+), 2 deletions(-)
 create mode 100644 libstdc++-v3/testsuite/20_util/expected/119197.cc

diff --git a/libstdc++-v3/include/std/expected
b/libstdc++-v3/include/std/expected
index 7ab4e4595fa..21522850d7a 100644
--- a/libstdc++-v3/include/std/expected
+++ b/libstdc++-v3/include/std/expected
@@ -367,7 +367,7 @@ namespace __expected
 /// @endcond

   template<typename _Tp, typename _Er>
-    class expected
+    class [[nodiscard]] expected
     {
       static_assert( ! is_reference_v<_Tp> );
       static_assert( ! is_function_v<_Tp> );
@@ -1360,7 +1360,7 @@ namespace __expected

   // Partial specialization for std::expected<cv void, E>
   template<typename _Tp, typename _Er> requires is_void_v<_Tp>
-    class expected<_Tp, _Er>
+    class [[nodiscard]] expected<_Tp, _Er>
     {
       static_assert( __expected::__can_be_unexpected<_Er> );

diff --git a/libstdc++-v3/testsuite/20_util/expected/119197.cc
b/libstdc++-v3/testsuite/20_util/expected/119197.cc
new file mode 100644
index 00000000000..dd4db6d68f3
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/expected/119197.cc
@@ -0,0 +1,20 @@
+// { dg-do compile { target c++23 } }
+
+// PR libstdc++/119197 - std::expected should be nodiscard
+
+#include <expected>
+
+std::expected<int, int> f();
+std::expected<void, int> g();
+
+void
+test_nodiscard()
+{
+  f();                         // { dg-warning "ignoring return" }
+  std::expected<int, int>(42); // c++/PR85973: should warn "ignoring temporary"
+  (void)f();                   // OK
+
+  g();                         // { dg-warning "ignoring return" }
+  std::expected<void, int>();  // c++/PR85973: should warn "ignoring temporary"
+  (void)g();                   // OK
+}
-- 
2.34.1