[libc++] Verify that LWG4140 is implemented

[frederick-vs-ja]

According to the commit history, the constructors removed by LWG4140 have never been added to libc++.

Existence of non-public or deleted default constructor is observable, this patch tests that there's no such default constructor at all.

Closes #118356.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: A. Jiang (frederick-vs-ja)

Changes

According to the commit history, the constructors removed by LWG4140 have never been added to libc++.

Existence of non-public or deleted default constructor is observable, this patch tests that there's no such default constructor at all.

Closes #118356.

---

Full diff: https://github.com/llvm/llvm-project/pull/128624.diff

3 Files Affected:

• (modified) libcxx/docs/Status/Cxx2cIssues.csv (+1-1)
• (added) libcxx/test/std/containers/sequences/vector.bool/reference/ctor_no_default.compile.pass.cpp (+26)
• (added) libcxx/test/std/utilities/template.bitset/bitset.members/reference.ctor_no_default.compile.pass.cpp (+41)

diff --git a/libcxx/docs/Status/Cxx2cIssues.csv b/libcxx/docs/Status/Cxx2cIssues.csv
index 1ec23dfabd5ea..731cee8df2cf3 100644
--- a/libcxx/docs/Status/Cxx2cIssues.csv
+++ b/libcxx/docs/Status/Cxx2cIssues.csv
@@ -98,7 +98,7 @@
 "`LWG4126 <https://wg21.link/LWG4126>`__","Some feature-test macros for fully freestanding features are not yet marked freestanding","2024-11 (Wrocław)","","",""
 "`LWG4134 <https://wg21.link/LWG4134>`__","Issue with Philox algorithm specification","2024-11 (Wrocław)","","",""
 "`LWG4135 <https://wg21.link/LWG4135>`__","The helper lambda of ``std::erase`` for list should specify return type as ``bool``","2024-11 (Wrocław)","","",""
-"`LWG4140 <https://wg21.link/LWG4140>`__","Useless default constructors for bit reference types","2024-11 (Wrocław)","","",""
+"`LWG4140 <https://wg21.link/LWG4140>`__","Useless default constructors for bit reference types","2024-11 (Wrocław)","|Complete|","",""
 "`LWG4141 <https://wg21.link/LWG4141>`__","Improve prohibitions on ""additional storage""","2024-11 (Wrocław)","","",""
 "`LWG4142 <https://wg21.link/LWG4142>`__","``format_parse_context::check_dynamic_spec`` should require at least one type","2024-11 (Wrocław)","","",""
 "`LWG4144 <https://wg21.link/LWG4144>`__","Disallow ``unique_ptr<T&, D>``","2024-11 (Wrocław)","","",""
diff --git a/libcxx/test/std/containers/sequences/vector.bool/reference/ctor_no_default.compile.pass.cpp b/libcxx/test/std/containers/sequences/vector.bool/reference/ctor_no_default.compile.pass.cpp
new file mode 100644
index 0000000000000..a55caf28c8706
--- /dev/null
+++ b/libcxx/test/std/containers/sequences/vector.bool/reference/ctor_no_default.compile.pass.cpp
@@ -0,0 +1,26 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <vector>
+
+#include <vector>
+#include <type_traits>
+
+#include "test_allocator.h"
+#include "test_macros.h"
+
+static_assert(!std::is_default_constructible<std::vector<bool>::reference>::value, "");
+static_assert(!std::is_default_constructible<std::vector<bool, test_allocator<bool> >::reference>::value, "");
+
+#if TEST_STD_VER >= 11
+void test_no_ambiguity_among_default_constructors(std::enable_if<false>);
+void test_no_ambiguity_among_default_constructors(std::vector<bool>::reference);
+void test_no_ambiguity_among_default_constructors(std::vector<bool, test_allocator<bool>>::reference);
+
+ASSERT_SAME_TYPE(decltype(test_no_ambiguity_among_default_constructors({})), void);
+#endif
diff --git a/libcxx/test/std/utilities/template.bitset/bitset.members/reference.ctor_no_default.compile.pass.cpp b/libcxx/test/std/utilities/template.bitset/bitset.members/reference.ctor_no_default.compile.pass.cpp
new file mode 100644
index 0000000000000..68d6380c09b50
--- /dev/null
+++ b/libcxx/test/std/utilities/template.bitset/bitset.members/reference.ctor_no_default.compile.pass.cpp
@@ -0,0 +1,41 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <bitset>
+
+#include <bitset>
+#include <type_traits>
+
+#include "test_macros.h"
+
+static_assert(!std::is_default_constructible<std::bitset<0>::reference>::value, "");
+static_assert(!std::is_default_constructible<std::bitset<1>::reference>::value, "");
+static_assert(!std::is_default_constructible<std::bitset<8>::reference>::value, "");
+static_assert(!std::is_default_constructible<std::bitset<12>::reference>::value, "");
+static_assert(!std::is_default_constructible<std::bitset<16>::reference>::value, "");
+static_assert(!std::is_default_constructible<std::bitset<24>::reference>::value, "");
+static_assert(!std::is_default_constructible<std::bitset<32>::reference>::value, "");
+static_assert(!std::is_default_constructible<std::bitset<48>::reference>::value, "");
+static_assert(!std::is_default_constructible<std::bitset<64>::reference>::value, "");
+static_assert(!std::is_default_constructible<std::bitset<96>::reference>::value, "");
+
+#if TEST_STD_VER >= 11
+void test_no_ambiguity_among_default_constructors(std::enable_if<false>);
+void test_no_ambiguity_among_default_constructors(std::bitset<0>::reference);
+void test_no_ambiguity_among_default_constructors(std::bitset<1>::reference);
+void test_no_ambiguity_among_default_constructors(std::bitset<8>::reference);
+void test_no_ambiguity_among_default_constructors(std::bitset<12>::reference);
+void test_no_ambiguity_among_default_constructors(std::bitset<16>::reference);
+void test_no_ambiguity_among_default_constructors(std::bitset<24>::reference);
+void test_no_ambiguity_among_default_constructors(std::bitset<32>::reference);
+void test_no_ambiguity_among_default_constructors(std::bitset<48>::reference);
+void test_no_ambiguity_among_default_constructors(std::bitset<64>::reference);
+void test_no_ambiguity_among_default_constructors(std::bitset<96>::reference);
+
+ASSERT_SAME_TYPE(decltype(test_no_ambiguity_among_default_constructors({})), void);
+#endif

[mordante]

Thanks! LGTM!