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

[Nathan Myers]

LGTM.

Furthermore, a vote for PR85973 to be fixed, maybe gated on -std=c++23.


On 3/2/26 10:59 AM, Arthur O'Dwyer wrote:
> 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.odwyer@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
> +}