[libcxx-commits] [libcxx] 823efd8 - [libc++] Implement P3798R1: The unexpected in `std::expected` (#219236)
via libcxx-commits
libcxx-commits at lists.llvm.org
Sat Aug 29 04:32:07 PDT 2026
Author: Alex Kremer
Date: 2026-08-29T19:32:01+08:00
New Revision: 823efd84c5be6eb3b5585caea3b80b9403a1ce1b
URL: https://github.com/llvm/llvm-project/commit/823efd84c5be6eb3b5585caea3b80b9403a1ce1b
DIFF: https://github.com/llvm/llvm-project/commit/823efd84c5be6eb3b5585caea3b80b9403a1ce1b.diff
LOG: [libc++] Implement P3798R1: The unexpected in `std::expected` (#219236)
Closes https://github.com/llvm/llvm-project/issues/204394
Re-application of https://github.com/llvm/llvm-project/pull/204826
(45a65bb48b5925707f43d08e30df2263a5e4e268)
Implements P3798R1 and related tests.
Added:
libcxx/test/std/utilities/expected/expected.expected/observers/has_error.pass.cpp
libcxx/test/std/utilities/expected/expected.void/observers/has_error.pass.cpp
Modified:
libcxx/docs/FeatureTestMacroTable.rst
libcxx/docs/ReleaseNotes/24.rst
libcxx/docs/Status/Cxx29Papers.csv
libcxx/include/__expected/expected.h
libcxx/include/version
libcxx/test/libcxx/utilities/expected/nodiscard.verify.cpp
libcxx/test/std/language.support/support.limits/support.limits.general/expected.version.compile.pass.cpp
libcxx/test/std/language.support/support.limits/support.limits.general/version.version.compile.pass.cpp
libcxx/utils/generate_feature_test_macro_components.py
Removed:
################################################################################
diff --git a/libcxx/docs/FeatureTestMacroTable.rst b/libcxx/docs/FeatureTestMacroTable.rst
index cabd30847b2af..bde7e9cf75f57 100644
--- a/libcxx/docs/FeatureTestMacroTable.rst
+++ b/libcxx/docs/FeatureTestMacroTable.rst
@@ -558,6 +558,8 @@ Status
---------------------------------------------------------- -----------------
**C++29**
----------------------------------------------------------------------------
+ ``__cpp_lib_expected`` ``202606L``
+ ---------------------------------------------------------- -----------------
``__cpp_lib_map_lookup`` *unimplemented*
---------------------------------------------------------- -----------------
``__cpp_lib_mdspan_copy`` *unimplemented*
diff --git a/libcxx/docs/ReleaseNotes/24.rst b/libcxx/docs/ReleaseNotes/24.rst
index 9cf08b5732970..b9e638213191a 100644
--- a/libcxx/docs/ReleaseNotes/24.rst
+++ b/libcxx/docs/ReleaseNotes/24.rst
@@ -47,6 +47,7 @@ Implemented Papers
- P3961R1: Less double indirection in ``function_ref`` (RU-220) (`Github <https://llvm.org/PR189606>`__)
- P1901R2: Enabling the Use of ``weak_ptr`` as Keys in Unordered Associative Containers (`Github <https://llvm.org/PR105372>`__)
- P0528R3: The Curious Case of Padding Bits, Featuring Atomic Compare-and-Exchange (`Github <https://llvm.org/PR76180>`__)
+- P3798R1: The unexpected in ``std::expected`` (`Github <https://llvm.org/PR204394>`__)
Improvements and New Features
-----------------------------
diff --git a/libcxx/docs/Status/Cxx29Papers.csv b/libcxx/docs/Status/Cxx29Papers.csv
index 3c48fe89be33d..1a413c1da63f9 100644
--- a/libcxx/docs/Status/Cxx29Papers.csv
+++ b/libcxx/docs/Status/Cxx29Papers.csv
@@ -3,7 +3,7 @@
"`P4101R1 <https://wg21.link/P4101R1>`__","Consteval-only Values for C++26","2026-06 (Brno)","","","`#204391 <https://github.com/llvm/llvm-project/issues/204391>`__","Voted as a Defect Report."
"`P2414R12 <https://wg21.link/P2414R12>`__","Pointer lifetime-end zap proposed solutions","2026-06 (Brno)","","","`#204392 <https://github.com/llvm/llvm-project/issues/204392>`__","Voted as a Defect Report."
"`P3319R6 <https://wg21.link/P3319R6>`__","Add an ``iota`` object for ``simd`` (and more)","2026-06 (Brno)","","","`#204393 <https://github.com/llvm/llvm-project/issues/204393>`__",""
-"`P3798R1 <https://wg21.link/P3798R1>`__","The unexpected in ``std::expected``","2026-06 (Brno)","","","`#204394 <https://github.com/llvm/llvm-project/issues/204394>`__",""
+"`P3798R1 <https://wg21.link/P3798R1>`__","The unexpected in ``std::expected``","2026-06 (Brno)","|Complete|","24","`#204394 <https://github.com/llvm/llvm-project/issues/204394>`__",""
"`P3052R2 <https://wg21.link/P3052R2>`__","``view_interface::at()``","2026-06 (Brno)","","","`#204395 <https://github.com/llvm/llvm-project/issues/204395>`__",""
"`P4206R0 <https://wg21.link/P4206R0>`__","Revert string support in ``std::constant_wrapper``","2026-06 (Brno)","|Complete|","23","`#203336 <https://github.com/llvm/llvm-project/issues/203336>`__","Applied as a Defect Report."
"`P3395R6 <https://wg21.link/P3395R6>`__","Fix encoding issues and add a formatter for ``std::error_code``","2026-06 (Brno)","","","`#204396 <https://github.com/llvm/llvm-project/issues/204396>`__",""
diff --git a/libcxx/include/__expected/expected.h b/libcxx/include/__expected/expected.h
index 5588196939738..13d01ad53c62c 100644
--- a/libcxx/include/__expected/expected.h
+++ b/libcxx/include/__expected/expected.h
@@ -827,6 +827,10 @@ class expected : private __expected_base<_Tp, _Err> {
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool has_value() const noexcept { return this->__has_val(); }
+# if _LIBCPP_STD_VER >= 29
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool has_error() const noexcept { return !this->has_value(); }
+# endif
+
[[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()) {
@@ -1601,6 +1605,10 @@ class expected<_Tp, _Err> : private __expected_void_base<_Err> {
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool has_value() const noexcept { return this->__has_val(); }
+# if _LIBCPP_STD_VER >= 29
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool has_error() const noexcept { return !this->has_value(); }
+# endif
+
_LIBCPP_HIDE_FROM_ABI constexpr void operator*() const noexcept {
_LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
this->__has_val(), "expected::operator* requires the expected to contain a value");
diff --git a/libcxx/include/version b/libcxx/include/version
index 589ce789ffa7c..ec74903885830 100644
--- a/libcxx/include/version
+++ b/libcxx/include/version
@@ -112,7 +112,8 @@ __cpp_lib_erase_if 202002L <deque> <forward
__cpp_lib_exchange_function 201304L <utility>
__cpp_lib_execution 201902L <execution>
201603L // C++17
-__cpp_lib_expected 202211L <expected>
+__cpp_lib_expected 202606L <expected>
+ 202211L // C++23
__cpp_lib_filesystem 201703L <filesystem>
__cpp_lib_flat_map 202511L <flat_map>
__cpp_lib_flat_set 202511L <flat_set>
@@ -661,6 +662,8 @@ __cpp_lib_void_t 201411L <type_traits>
#endif
#if _LIBCPP_STD_VER >= 29
+# undef __cpp_lib_expected
+# define __cpp_lib_expected 202606L
// # define __cpp_lib_map_lookup 202606L
// # define __cpp_lib_mdspan_copy 202606L
// # define __cpp_lib_pointer_tag_pair 202606L
diff --git a/libcxx/test/libcxx/utilities/expected/nodiscard.verify.cpp b/libcxx/test/libcxx/utilities/expected/nodiscard.verify.cpp
index c9af7a91c67f2..2c5c831161d2c 100644
--- a/libcxx/test/libcxx/utilities/expected/nodiscard.verify.cpp
+++ b/libcxx/test/libcxx/utilities/expected/nodiscard.verify.cpp
@@ -15,6 +15,8 @@
#include <expected>
#include <utility>
+#include "test_macros.h"
+
void test() {
// [expected.bad.void]
@@ -47,6 +49,9 @@ void test() {
*std::move(exp); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
exp.has_value(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+#if TEST_STD_VER >= 29
+ exp.has_error(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+#endif
cExp.value(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
exp.value(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
@@ -110,6 +115,9 @@ void test() {
const std::expected<void, int> cVExp{};
vExp.has_value(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+#if TEST_STD_VER >= 29
+ vExp.has_error(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+#endif
// expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
cVExp.error();
diff --git a/libcxx/test/std/language.support/support.limits/support.limits.general/expected.version.compile.pass.cpp b/libcxx/test/std/language.support/support.limits/support.limits.general/expected.version.compile.pass.cpp
index 7c25c805a940e..38b36a8eb8653 100644
--- a/libcxx/test/std/language.support/support.limits/support.limits.general/expected.version.compile.pass.cpp
+++ b/libcxx/test/std/language.support/support.limits/support.limits.general/expected.version.compile.pass.cpp
@@ -132,8 +132,8 @@
# ifndef __cpp_lib_expected
# error "__cpp_lib_expected should be defined in c++29"
# endif
-# if __cpp_lib_expected != 202211L
-# error "__cpp_lib_expected should have the value 202211L in c++29"
+# if __cpp_lib_expected != 202606L
+# error "__cpp_lib_expected should have the value 202606L in c++29"
# endif
# if !defined(_LIBCPP_VERSION)
diff --git a/libcxx/test/std/language.support/support.limits/support.limits.general/version.version.compile.pass.cpp b/libcxx/test/std/language.support/support.limits/support.limits.general/version.version.compile.pass.cpp
index 769b79c7924d2..cab073405f01e 100644
--- a/libcxx/test/std/language.support/support.limits/support.limits.general/version.version.compile.pass.cpp
+++ b/libcxx/test/std/language.support/support.limits/support.limits.general/version.version.compile.pass.cpp
@@ -9241,8 +9241,8 @@
# ifndef __cpp_lib_expected
# error "__cpp_lib_expected should be defined in c++29"
# endif
-# if __cpp_lib_expected != 202211L
-# error "__cpp_lib_expected should have the value 202211L in c++29"
+# if __cpp_lib_expected != 202606L
+# error "__cpp_lib_expected should have the value 202606L in c++29"
# endif
# if !defined(_LIBCPP_VERSION) || _LIBCPP_HAS_FILESYSTEM
diff --git a/libcxx/test/std/utilities/expected/expected.expected/observers/has_error.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/observers/has_error.pass.cpp
new file mode 100644
index 0000000000000..b6190c054d387
--- /dev/null
+++ b/libcxx/test/std/utilities/expected/expected.expected/observers/has_error.pass.cpp
@@ -0,0 +1,40 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: std-at-least-c++29
+
+// constexpr bool has_error() const noexcept;
+
+#include <cassert>
+#include <concepts>
+#include <expected>
+
+#include "../../types.h"
+
+constexpr bool test() {
+ {
+ const std::expected<int, int> e(std::unexpect, 5);
+ static_assert(noexcept(e.has_error()));
+ std::same_as<bool> decltype(auto) has_err = e.has_error();
+ assert(has_err);
+ }
+
+ {
+ const std::expected<int, int> e(5);
+ assert(!e.has_error());
+ }
+
+ return true;
+}
+
+int main(int, char**) {
+ test();
+ static_assert(test());
+
+ return 0;
+}
diff --git a/libcxx/test/std/utilities/expected/expected.void/observers/has_error.pass.cpp b/libcxx/test/std/utilities/expected/expected.void/observers/has_error.pass.cpp
new file mode 100644
index 0000000000000..98bbf118c028e
--- /dev/null
+++ b/libcxx/test/std/utilities/expected/expected.void/observers/has_error.pass.cpp
@@ -0,0 +1,40 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: std-at-least-c++29
+
+// constexpr bool has_error() const noexcept;
+
+#include <cassert>
+#include <concepts>
+#include <expected>
+
+#include "../../types.h"
+
+constexpr bool test() {
+ {
+ const std::expected<void, int> e(std::unexpect, 5);
+ static_assert(noexcept(e.has_error()));
+ std::same_as<bool> decltype(auto) has_err = e.has_error();
+ assert(has_err);
+ }
+
+ {
+ const std::expected<void, int> e;
+ assert(!e.has_error());
+ }
+
+ return true;
+}
+
+int main(int, char**) {
+ test();
+ static_assert(test());
+
+ return 0;
+}
diff --git a/libcxx/utils/generate_feature_test_macro_components.py b/libcxx/utils/generate_feature_test_macro_components.py
old mode 100644
new mode 100755
index 6b4acf671a0c9..f8ccd37690ce4
--- a/libcxx/utils/generate_feature_test_macro_components.py
+++ b/libcxx/utils/generate_feature_test_macro_components.py
@@ -567,7 +567,7 @@ def add_version_header(tc):
"name": "__cpp_lib_expected",
"values": {
"c++23": 202211,
- # "c++29": 202606 # P3798R1 The unexpected in std::expected
+ "c++29": 202606,
},
"headers": ["expected"],
},
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