[libc++][expected] Applied `[[nodiscard]]` #170245

H-G-Hristov · 2025-12-02T05:28:43Z

[[nodiscard]] should be applied to functions where discarding the return value is most likely a correctness issue.

It was already discussed not to mark the type std::expected as [[nodiscard]] see:
#139651
#130820
Also:
#154943

github-actions · 2025-12-02T05:40:10Z

✅ With the latest revision this PR passed the C/C++ code formatter.

libcxx/test/libcxx/utilities/expected/expected.expected/value_or.mandates.verify.cpp

[[nodiscard]] should be applied to functions where discarding the return value is most likely a correctness issue. - https://libcxx.llvm.org/CodingGuidelines.html - https://wg21.link/expected.bad.void - https://wg21.link/expected.bad - https://wg21.link/expected.expected - https://wg21.link/expected.void - https://wg21.link/expected.unexpected

llvmbot · 2025-12-03T05:51:29Z

@llvm/pr-subscribers-libcxx

Author: Hristo Hristov (H-G-Hristov)

Changes

[[nodiscard]] should be applied to functions where discarding the return value is most likely a correctness issue.

It was already discussed not to mark the type std::expected as [[nodiscard]] see:
#139651
#130820
Also:
#154943

Patch is 87.59 KiB, truncated to 20.00 KiB below, full version: https://github.com/llvm/llvm-project/pull/170245.diff

14 Files Affected:

(modified) libcxx/include/__expected/bad_expected_access.h (+8-6)
(modified) libcxx/include/__expected/expected.h (+62-62)
(modified) libcxx/include/__expected/unexpected.h (+4-4)
(modified) libcxx/test/libcxx/utilities/expected/expected.expected/and_then.mandates.verify.cpp (+8-8)
(modified) libcxx/test/libcxx/utilities/expected/expected.expected/error_or.mandates.verify.cpp (+4-4)
(modified) libcxx/test/libcxx/utilities/expected/expected.expected/or_else.mandates.verify.cpp (+8-8)
(modified) libcxx/test/libcxx/utilities/expected/expected.expected/transform_error.mandates.verify.cpp (+8-8)
(modified) libcxx/test/libcxx/utilities/expected/expected.expected/value_or.mandates.verify.cpp (+8-4)
(modified) libcxx/test/libcxx/utilities/expected/expected.void/and_then.mandates.verify.cpp (+8-8)
(modified) libcxx/test/libcxx/utilities/expected/expected.void/error_or.mandates.verify.cpp (+4-4)
(modified) libcxx/test/libcxx/utilities/expected/expected.void/or_else.mandates.verify.cpp (+8-8)
(modified) libcxx/test/libcxx/utilities/expected/expected.void/transform_error.mandates.verify.cpp (+8-8)
(modified) libcxx/test/libcxx/utilities/expected/expected.void/value.lwg3940.verify.cpp (+5-4)
(added) libcxx/test/libcxx/utilities/expected/nodiscard.verify.cpp (+175)

diff --git a/libcxx/include/__expected/bad_expected_access.h b/libcxx/include/__expected/bad_expected_access.h
index 1b734389e8311..b1958101d5178 100644
--- a/libcxx/include/__expected/bad_expected_access.h
+++ b/libcxx/include/__expected/bad_expected_access.h
@@ -43,9 +43,11 @@ class _LIBCPP_EXPORTED_FROM_ABI bad_expected_access<void> : public exception {
 
 public:
 #  if _LIBCPP_AVAILABILITY_HAS_BAD_EXPECTED_ACCESS_KEY_FUNCTION
-  const char* what() const noexcept override;
+  [[nodiscard]] const char* what() const noexcept override;
 #  else
-  _LIBCPP_HIDE_FROM_ABI_VIRTUAL const char* what() const noexcept override { return "bad access to std::expected"; }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI_VIRTUAL const char* what() const noexcept override {
+    return "bad access to std::expected";
+  }
 #  endif
 };
 _LIBCPP_DIAGNOSTIC_POP
@@ -55,10 +57,10 @@ class bad_expected_access : public bad_expected_access<void> {
 public:
   _LIBCPP_HIDE_FROM_ABI explicit bad_expected_access(_Err __e) : __unex_(std::move(__e)) {}
 
-  _LIBCPP_HIDE_FROM_ABI _Err& error() & noexcept { return __unex_; }
-  _LIBCPP_HIDE_FROM_ABI const _Err& error() const& noexcept { return __unex_; }
-  _LIBCPP_HIDE_FROM_ABI _Err&& error() && noexcept { return std::move(__unex_); }
-  _LIBCPP_HIDE_FROM_ABI const _Err&& error() const&& noexcept { return std::move(__unex_); }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _Err& error() & noexcept { return __unex_; }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI const _Err& error() const& noexcept { return __unex_; }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _Err&& error() && noexcept { return std::move(__unex_); }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI const _Err&& error() const&& noexcept { return std::move(__unex_); }
 
 private:
   _Err __unex_;
diff --git a/libcxx/include/__expected/expected.h b/libcxx/include/__expected/expected.h
index be37e8ab66ac4..b6a9211ae3cdc 100644
--- a/libcxx/include/__expected/expected.h
+++ b/libcxx/include/__expected/expected.h
@@ -798,25 +798,25 @@ class expected : private __expected_base<_Tp, _Err> {
     return std::addressof(this->__val());
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr const _Tp& operator*() const& noexcept {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const _Tp& operator*() const& noexcept {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
         this->__has_val(), "expected::operator* requires the expected to contain a value");
     return this->__val();
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr _Tp& operator*() & noexcept {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp& operator*() & noexcept {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
         this->__has_val(), "expected::operator* requires the expected to contain a value");
     return this->__val();
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr const _Tp&& operator*() const&& noexcept {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const _Tp&& operator*() const&& noexcept {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
         this->__has_val(), "expected::operator* requires the expected to contain a value");
     return std::move(this->__val());
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr _Tp&& operator*() && noexcept {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp&& operator*() && noexcept {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
         this->__has_val(), "expected::operator* requires the expected to contain a value");
     return std::move(this->__val());
@@ -824,9 +824,9 @@ class expected : private __expected_base<_Tp, _Err> {
 
   _LIBCPP_HIDE_FROM_ABI constexpr explicit operator bool() const noexcept { return this->__has_val(); }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr bool has_value() const noexcept { return this->__has_val(); }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool has_value() const noexcept { return this->__has_val(); }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr const _Tp& value() const& {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const _Tp& value() const& {
     static_assert(is_copy_constructible_v<_Err>, "error_type has to be copy constructible");
     if (!this->__has_val()) {
       std::__throw_bad_expected_access<_Err>(std::as_const(error()));
@@ -834,7 +834,7 @@ class expected : private __expected_base<_Tp, _Err> {
     return this->__val();
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr _Tp& value() & {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp& value() & {
     static_assert(is_copy_constructible_v<_Err>, "error_type has to be copy constructible");
     if (!this->__has_val()) {
       std::__throw_bad_expected_access<_Err>(std::as_const(error()));
@@ -842,7 +842,7 @@ class expected : private __expected_base<_Tp, _Err> {
     return this->__val();
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr const _Tp&& value() const&& {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const _Tp&& value() const&& {
     static_assert(is_copy_constructible_v<_Err> && is_constructible_v<_Err, decltype(std::move(error()))>,
                   "error_type has to be both copy constructible and constructible from decltype(std::move(error()))");
     if (!this->__has_val()) {
@@ -851,7 +851,7 @@ class expected : private __expected_base<_Tp, _Err> {
     return std::move(this->__val());
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr _Tp&& value() && {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp&& value() && {
     static_assert(is_copy_constructible_v<_Err> && is_constructible_v<_Err, decltype(std::move(error()))>,
                   "error_type has to be both copy constructible and constructible from decltype(std::move(error()))");
     if (!this->__has_val()) {
@@ -860,46 +860,46 @@ class expected : private __expected_base<_Tp, _Err> {
     return std::move(this->__val());
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr const _Err& error() const& noexcept {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const _Err& error() const& noexcept {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
         !this->__has_val(), "expected::error requires the expected to contain an error");
     return this->__unex();
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr _Err& error() & noexcept {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Err& error() & noexcept {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
         !this->__has_val(), "expected::error requires the expected to contain an error");
     return this->__unex();
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr const _Err&& error() const&& noexcept {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const _Err&& error() const&& noexcept {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
         !this->__has_val(), "expected::error requires the expected to contain an error");
     return std::move(this->__unex());
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr _Err&& error() && noexcept {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Err&& error() && noexcept {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
         !this->__has_val(), "expected::error requires the expected to contain an error");
     return std::move(this->__unex());
   }
 
   template <class _Up = remove_cv_t<_Tp>>
-  _LIBCPP_HIDE_FROM_ABI constexpr _Tp value_or(_Up&& __v) const& {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp value_or(_Up&& __v) const& {
     static_assert(is_copy_constructible_v<_Tp>, "value_type has to be copy constructible");
     static_assert(is_convertible_v<_Up, _Tp>, "argument has to be convertible to value_type");
     return this->__has_val() ? this->__val() : static_cast<_Tp>(std::forward<_Up>(__v));
   }
 
   template <class _Up = remove_cv_t<_Tp>>
-  _LIBCPP_HIDE_FROM_ABI constexpr _Tp value_or(_Up&& __v) && {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp value_or(_Up&& __v) && {
     static_assert(is_move_constructible_v<_Tp>, "value_type has to be move constructible");
     static_assert(is_convertible_v<_Up, _Tp>, "argument has to be convertible to value_type");
     return this->__has_val() ? std::move(this->__val()) : static_cast<_Tp>(std::forward<_Up>(__v));
   }
 
   template <class _Up = _Err>
-  _LIBCPP_HIDE_FROM_ABI constexpr _Err error_or(_Up&& __error) const& {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Err error_or(_Up&& __error) const& {
     static_assert(is_copy_constructible_v<_Err>, "error_type has to be copy constructible");
     static_assert(is_convertible_v<_Up, _Err>, "argument has to be convertible to error_type");
     if (has_value())
@@ -908,7 +908,7 @@ class expected : private __expected_base<_Tp, _Err> {
   }
 
   template <class _Up = _Err>
-  _LIBCPP_HIDE_FROM_ABI constexpr _Err error_or(_Up&& __error) && {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Err error_or(_Up&& __error) && {
     static_assert(is_move_constructible_v<_Err>, "error_type has to be move constructible");
     static_assert(is_convertible_v<_Up, _Err>, "argument has to be convertible to error_type");
     if (has_value())
@@ -919,7 +919,7 @@ class expected : private __expected_base<_Tp, _Err> {
   // [expected.void.monadic], monadic
   template <class _Func>
     requires is_constructible_v<_Err, _Err&>
-  _LIBCPP_HIDE_FROM_ABI constexpr auto and_then(_Func&& __f) & {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto and_then(_Func&& __f) & {
     using _Up = remove_cvref_t<invoke_result_t<_Func, _Tp&>>;
     static_assert(__is_std_expected<_Up>::value, "The result of f(value()) must be a specialization of std::expected");
     static_assert(is_same_v<typename _Up::error_type, _Err>,
@@ -932,7 +932,7 @@ class expected : private __expected_base<_Tp, _Err> {
 
   template <class _Func>
     requires is_constructible_v<_Err, const _Err&>
-  _LIBCPP_HIDE_FROM_ABI constexpr auto and_then(_Func&& __f) const& {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto and_then(_Func&& __f) const& {
     using _Up = remove_cvref_t<invoke_result_t<_Func, const _Tp&>>;
     static_assert(__is_std_expected<_Up>::value, "The result of f(value()) must be a specialization of std::expected");
     static_assert(is_same_v<typename _Up::error_type, _Err>,
@@ -945,7 +945,7 @@ class expected : private __expected_base<_Tp, _Err> {
 
   template <class _Func>
     requires is_constructible_v<_Err, _Err&&>
-  _LIBCPP_HIDE_FROM_ABI constexpr auto and_then(_Func&& __f) && {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto and_then(_Func&& __f) && {
     using _Up = remove_cvref_t<invoke_result_t<_Func, _Tp&&>>;
     static_assert(
         __is_std_expected<_Up>::value, "The result of f(std::move(value())) must be a specialization of std::expected");
@@ -959,7 +959,7 @@ class expected : private __expected_base<_Tp, _Err> {
 
   template <class _Func>
     requires is_constructible_v<_Err, const _Err&&>
-  _LIBCPP_HIDE_FROM_ABI constexpr auto and_then(_Func&& __f) const&& {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto and_then(_Func&& __f) const&& {
     using _Up = remove_cvref_t<invoke_result_t<_Func, const _Tp&&>>;
     static_assert(
         __is_std_expected<_Up>::value, "The result of f(std::move(value())) must be a specialization of std::expected");
@@ -973,7 +973,7 @@ class expected : private __expected_base<_Tp, _Err> {
 
   template <class _Func>
     requires is_constructible_v<_Tp, _Tp&>
-  _LIBCPP_HIDE_FROM_ABI constexpr auto or_else(_Func&& __f) & {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto or_else(_Func&& __f) & {
     using _Gp = remove_cvref_t<invoke_result_t<_Func, _Err&>>;
     static_assert(__is_std_expected<_Gp>::value, "The result of f(error()) must be a specialization of std::expected");
     static_assert(is_same_v<typename _Gp::value_type, _Tp>,
@@ -986,7 +986,7 @@ class expected : private __expected_base<_Tp, _Err> {
 
   template <class _Func>
     requires is_constructible_v<_Tp, const _Tp&>
-  _LIBCPP_HIDE_FROM_ABI constexpr auto or_else(_Func&& __f) const& {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto or_else(_Func&& __f) const& {
     using _Gp = remove_cvref_t<invoke_result_t<_Func, const _Err&>>;
     static_assert(__is_std_expected<_Gp>::value, "The result of f(error()) must be a specialization of std::expected");
     static_assert(is_same_v<typename _Gp::value_type, _Tp>,
@@ -999,7 +999,7 @@ class expected : private __expected_base<_Tp, _Err> {
 
   template <class _Func>
     requires is_constructible_v<_Tp, _Tp&&>
-  _LIBCPP_HIDE_FROM_ABI constexpr auto or_else(_Func&& __f) && {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto or_else(_Func&& __f) && {
     using _Gp = remove_cvref_t<invoke_result_t<_Func, _Err&&>>;
     static_assert(
         __is_std_expected<_Gp>::value, "The result of f(std::move(error())) must be a specialization of std::expected");
@@ -1013,7 +1013,7 @@ class expected : private __expected_base<_Tp, _Err> {
 
   template <class _Func>
     requires is_constructible_v<_Tp, const _Tp&&>
-  _LIBCPP_HIDE_FROM_ABI constexpr auto or_else(_Func&& __f) const&& {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto or_else(_Func&& __f) const&& {
     using _Gp = remove_cvref_t<invoke_result_t<_Func, const _Err&&>>;
     static_assert(
         __is_std_expected<_Gp>::value, "The result of f(std::move(error())) must be a specialization of std::expected");
@@ -1027,7 +1027,7 @@ class expected : private __expected_base<_Tp, _Err> {
 
   template <class _Func>
     requires is_constructible_v<_Err, _Err&>
-  _LIBCPP_HIDE_FROM_ABI constexpr auto transform(_Func&& __f) & {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto transform(_Func&& __f) & {
     using _Up = remove_cv_t<invoke_result_t<_Func, _Tp&>>;
     if (!has_value()) {
       return expected<_Up, _Err>(unexpect, error());
@@ -1043,7 +1043,7 @@ class expected : private __expected_base<_Tp, _Err> {
 
   template <class _Func>
     requires is_constructible_v<_Err, const _Err&>
-  _LIBCPP_HIDE_FROM_ABI constexpr auto transform(_Func&& __f) const& {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto transform(_Func&& __f) const& {
     using _Up = remove_cv_t<invoke_result_t<_Func, const _Tp&>>;
     if (!has_value()) {
       return expected<_Up, _Err>(unexpect, error());
@@ -1059,7 +1059,7 @@ class expected : private __expected_base<_Tp, _Err> {
 
   template <class _Func>
     requires is_constructible_v<_Err, _Err&&>
-  _LIBCPP_HIDE_FROM_ABI constexpr auto transform(_Func&& __f) && {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto transform(_Func&& __f) && {
     using _Up = remove_cv_t<invoke_result_t<_Func, _Tp&&>>;
     if (!has_value()) {
       return expected<_Up, _Err>(unexpect, std::move(error()));
@@ -1075,7 +1075,7 @@ class expected : private __expected_base<_Tp, _Err> {
 
   template <class _Func>
     requires is_constructible_v<_Err, const _Err&&>
-  _LIBCPP_HIDE_FROM_ABI constexpr auto transform(_Func&& __f) const&& {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto transform(_Func&& __f) const&& {
     using _Up = remove_cv_t<invoke_result_t<_Func, const _Tp&&>>;
     if (!has_value()) {
       return expected<_Up, _Err>(unexpect, std::move(error()));
@@ -1091,7 +1091,7 @@ class expected : private __expected_base<_Tp, _Err> {
 
   template <class _Func>
     requires is_constructible_v<_Tp, _Tp&>
-  _LIBCPP_HIDE_FROM_ABI constexpr auto transform_error(_Func&& __f) & {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto transform_error(_Func&& __f) & {
     using _Gp = remove_cv_t<invoke_result_t<_Func, _Err&>>;
     static_assert(__valid_std_unexpected<_Gp>::value,
                   "The result of f(error()) must be a valid template argument for unexpected");
@@ -1103,7 +1103,7 @@ class expected : private __expected_base<_Tp, _Err> {
 
   template <class _Func>
     requires is_constructible_v<_Tp, const _Tp&>
-  _LIBCPP_HIDE_FROM_ABI constexpr auto transform_error(_Func&& __f) const& {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto transform_error(_Func&& __f) const& {
     using _Gp = remove_cv_t<invoke_result_t<_Func, const _Err&>>;
     static_assert(__valid_std_unexpected<_Gp>::value,
                   "The result of f(error()) must be a valid template argument for unexpected");
@@ -1115,7 +1115,7 @@ class expected : private __expected_base<_Tp, _Err> {
 
   template <class _Func>
     requires is_constructible_v<_Tp, _Tp&&>
-  _LIBCPP_HIDE_FROM_ABI constexpr auto transform_error(_Func&& __f) && {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto transform_error(_Func&& __f) && {
     using _Gp = remove_cv_t<invoke_result_t<_Func, _Err&&>>;
     static_assert(__valid_std_unexpected<_Gp>::value,
                   "The result of f(std::move(error())) must be a valid template argument for unexpected");
@@ -1128,7 +1128,7 @@ class expected : private __expected_base<_Tp, _Err> {
 
   template <class _Func>
     requires is_constructible_v<_Tp, const _Tp&&>
-  _LIBCPP_HIDE_FROM_ABI constexpr auto transform_error(_Func&& __f) const&& {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto transform_error(_Func&& __f) const&& {
     using _Gp = remove_cv_t<invoke_result_t<_Func, const _Err&&>>;
     static_assert(__valid_std_unexpected<_Gp>::value,
                   "The result of f(std::move(error())) must be a valid template argument for unexpected");
@@ -1145,8 +1145,8 @@ class expected : private __expected_base<_Tp, _Err> {
     requires(!is_void_v<_T2>)
 #  if _LIBCPP_STD_VER >= 26
             && requires {
-                 { *__x == *__y } -> __core_convertible_to<bool>;
-                 { __x.error() == __y.error() } -> __core_convertible_to<bool>;
+                 { *__x == *__y }->__core_convertible_to<bool>;
+                 { __x.error() == __y.error() }->__core_convertible_to<bool>;
                }
 #  endif
   {
@@ -1165,7 +1165,7 @@ class expected : private __expected_base<_Tp, _Err> {
   _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const _T2& __v)
 #  if _LIBCPP_STD_VER >= 26
     requires(!__is_std_expected<_T2>::value) && requires {
-      { *__x == __v } -> __core_convertible_to<bool>;
+      { *__x == __v }->__core_convertible_to<bool>;
     }
 #  endif
   {
@@ -1176,7 +1176,7 @@ class expected : private __expected_base<_Tp, _Err> {
   _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const unexpected<_E2>& __e)
 #  if _LIBCPP_STD_VER >= 26
     requires requires {
-      { __x.error() == __e.error() } -> __core_convertible_to<bool>;
+      { __x.error() == __e.error() }->__core_convertible_to<bool>;
     }
 #  endif
   {
@@ -1595,7 +1595,7 @@ class expected<_Tp, _Err> : private __expected_void_base<_Err> {
   // [expected.void.obs], observers
   _LIBCPP_HIDE_FROM_ABI constexpr explicit operator bool() const noexcept { return this->__has_val(); }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr bool has_value() const noexcept { return this->__has_val(); }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool has_value() const noexcept { return this->__has_val(); }
 
   _LIBCPP_HIDE_FROM_ABI constexpr void operator*() const noexcept {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
@@ -1616,32 +1616,32 @@ class expected<_Tp, _Err> : private __expected_void_base<_Err> {
     }
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr const _Err& error() const& noexcept {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const _Err& error() const& noexcept {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
         !this->__has_val(), "expected::error requires the expected to contain an error");
     return this->__unex();
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr _Err& error() & noexcept {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Err& error() & noexcept {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
         !this->__has_val(), "expected::error requires the expected to contain an error");
     return this->__unex();
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr const _Err&& error() const&& noexcept {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const _Err&& error() const&& noexcept {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
         !this->__has_val(), "expected::error requires the expected to contain an error");
     return std::move(this->__unex());
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr _Err&& error() && noexcept {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Err&& error() && noexcept {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
         !this->__has_val(), "expected::error requires the expected to contain an error");
     return std::move(this->__unex());
   }
 
   template <class _Up = _Err>
-  _LIBCPP_HIDE_FROM_ABI constexpr _Err error_or(_Up&& __error) const& {
+  [[nodiscard]] _LIBC...
[truncated]

H-G-Hristov force-pushed the hgh/libcxx/nodiscard-to-expected branch from e7baa25 to ff344f6 Compare December 2, 2025 05:38

frederick-vs-ja reviewed Dec 2, 2025

View reviewed changes

libcxx/test/libcxx/utilities/expected/expected.expected/value_or.mandates.verify.cpp Outdated Show resolved Hide resolved

H-G-Hristov force-pushed the hgh/libcxx/nodiscard-to-expected branch from ff344f6 to 65d0809 Compare December 2, 2025 06:11

H-G-Hristov force-pushed the hgh/libcxx/nodiscard-to-expected branch from 65d0809 to ef8aa11 Compare December 2, 2025 06:14

Simplified tests

e735741

H-G-Hristov marked this pull request as ready for review December 3, 2025 05:50

H-G-Hristov requested a review from a team as a code owner December 3, 2025 05:50

llvmbot added the libc++ libc++ C++ Standard Library. Not GNU libstdc++. Not libc++abi. label Dec 3, 2025

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[libc++][expected] Applied `[[nodiscard]]` #170245

[libc++][expected] Applied `[[nodiscard]]` #170245

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[libc++][expected] Applied `[[nodiscard]]` #170245

[libc++][expected] Applied `[[nodiscard]]` #170245

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