[libc++] Remove the constexpr hash<vector<bool>> extension

libcxx/docs/ReleaseNotes/21.rst

@@ -64,13 +64,20 @@ Deprecations and Removals
 
 - ``std::is_pod`` and ``std::is_pod_v`` are deprecated in C++20 and later.
 
+- libc++ no long adds ``constexpr`` to ``std::hash<std::vector<bool, A>>::operator()``. The ``constexpr`` addition
+  since C++20 was an unintended extension. The ``_LIBCPP_ENABLE_REMOVED_CONSTEXPR_HASH_VECTOR_BOOL`` macro can be
+  defined to temporarily re-enable this extension. This macro will be honored for one release and ignored starting in
+  LLVM 22.
+
 Upcoming Deprecations and Removals
 ----------------------------------
 
 LLVM 22
 ~~~~~~~
 
-- TODO
+- TODO: The ``constexpr`` addition to ``std::hash<std::vector<bool, A>>::operator()`` will be removed entirely, and the
+  ``_LIBCPP_ENABLE_REMOVED_CONSTEXPR_HASH_VECTOR_BOOL`` macro that was used to re-enable this extension will be ignored
+  in LLVM 22.
 
 
 ABI Affecting Changes

libcxx/include/__vector/vector_bool.h

@@ -66,6 +66,13 @@ _LIBCPP_PUSH_MACROS
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+// TODO(LLVM 22): Remove the escape hatch
+#ifdef _LIBCPP_ENABLE_REMOVED_CONSTEXPR_HASH_VECTOR_BOOL
+#  define _LIBCPP_CONSTEXPR_HASH_VECTOR_BOOL _LIBCPP_CONSTEXPR_SINCE_CXX20
+#else
+#  define _LIBCPP_CONSTEXPR_HASH_VECTOR_BOOL
+#endif
+
 template <class _Allocator>
 struct hash<vector<bool, _Allocator> >;
 
@@ -512,7 +519,7 @@ class _LIBCPP_TEMPLATE_VIS vector<bool, _Allocator> {
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __move_assign_alloc(vector&, false_type) _NOEXCEPT {}
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_t __hash_code() const _NOEXCEPT;
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_HASH_VECTOR_BOOL size_t __hash_code() const _NOEXCEPT;
 
   friend class __bit_reference<vector>;
   friend class __bit_const_reference<vector>;
@@ -1093,7 +1100,7 @@ _LIBCPP_CONSTEXPR_SINCE_CXX20 bool vector<bool, _Allocator>::__invariants() cons
 }
 
 template <class _Allocator>
-_LIBCPP_CONSTEXPR_SINCE_CXX20 size_t vector<bool, _Allocator>::__hash_code() const _NOEXCEPT {
+_LIBCPP_CONSTEXPR_HASH_VECTOR_BOOL size_t vector<bool, _Allocator>::__hash_code() const _NOEXCEPT {
   size_t __h = 0;
   // do middle whole words
   size_type __n         = __size_;
@@ -1111,12 +1118,14 @@ _LIBCPP_CONSTEXPR_SINCE_CXX20 size_t vector<bool, _Allocator>::__hash_code() con
 template <class _Allocator>
 struct _LIBCPP_TEMPLATE_VIS hash<vector<bool, _Allocator> >
     : public __unary_function<vector<bool, _Allocator>, size_t> {
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_t
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_HASH_VECTOR_BOOL size_t
   operator()(const vector<bool, _Allocator>& __vec) const _NOEXCEPT {
     return __vec.__hash_code();
   }
 };
 
+#undef _LIBCPP_CONSTEXPR_HASH_VECTOR_BOOL
+
 _LIBCPP_END_NAMESPACE_STD
 
 _LIBCPP_POP_MACROS

libcxx/test/libcxx/containers/sequences/vector.bool/hash.constexpr.pass.cpp

@@ -8,11 +8,14 @@
 
 // REQUIRES: std-at-least-c++20
 
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_REMOVED_CONSTEXPR_HASH_VECTOR_BOOL
+
 // <vector>
 
 // template<class Allocator> struct hash<vector<bool, Allocator>>;
 
-// libc++ makes the operator() of this partial specialization constexpr since C++20, which is a conforming extension.
+// Since LLVM 16, libc++ has made the operator() of this partial specialization constexpr since C++20,
+// which is a conforming extension. However, such extension was unintended, and is being removed.
 
 #include <cassert>
 #include <functional>