5 files changed, 47 insertions, 15 deletions
diff --git a/libcxx/include/__config b/libcxx/include/__config
index b9203e640a3e..51b100fa5569 100644
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -134,6 +134,15 @@
 #    define _LIBCPP_ABI_DO_NOT_EXPORT_VECTOR_BASE_COMMON
 // According to the Standard, `bitset::operator[] const` returns bool
 #    define _LIBCPP_ABI_BITSET_VECTOR_BOOL_CONST_SUBSCRIPT_RETURN_BOOL
+// Fix the implementation of CityHash used for std::hash<fundamental-type>.
+// This is an ABI break because `std::hash` will return a different result,
+// which means that hashing the same object in translation units built against
+// different versions of libc++ can return inconsistent results. This is especially
+// tricky since std::hash is used in the implementation of unordered containers.
+//
+// The incorrect implementation of CityHash has the problem that it drops some
+// bits on the floor.
+#    define _LIBCPP_ABI_FIX_CITYHASH_IMPLEMENTATION
 // Remove the base 10 implementation of std::to_chars from the dylib.
 // The implementation moved to the header, but we still export the symbols from
 // the dylib for backwards compatibility.
@@ -629,7 +638,11 @@ typedef __char32_t char32_t;
 #  else
 #    define _LIBCPP_HIDE_FROM_ABI _LIBCPP_HIDDEN _LIBCPP_EXCLUDE_FROM_EXPLICIT_INSTANTIATION
 #  endif
-#    define _LIBCPP_HIDE_FROM_ABI_VIRTUAL _LIBCPP_HIDDEN _LIBCPP_EXCLUDE_FROM_EXPLICIT_INSTANTIATION
+#  define _LIBCPP_HIDE_FROM_ABI_VIRTUAL _LIBCPP_HIDDEN _LIBCPP_EXCLUDE_FROM_EXPLICIT_INSTANTIATION
+
+// This macro provides a HIDE_FROM_ABI equivalent that can be applied to extern
+// "C" function, as those lack mangling.
+#  define _LIBCPP_HIDE_FROM_ABI_C _LIBCPP_HIDDEN _LIBCPP_EXCLUDE_FROM_EXPLICIT_INSTANTIATION
 
 #  ifdef _LIBCPP_BUILDING_LIBRARY
 #    if _LIBCPP_ABI_VERSION > 1
@@ -1223,12 +1236,12 @@ _LIBCPP_BEGIN_NAMESPACE_STD _LIBCPP_END_NAMESPACE_STD
 // functions are declared by the C library.
 #  define _LIBCPP_HAS_NO_C8RTOMB_MBRTOC8
 // GNU libc 2.36 and newer declare c8rtomb() and mbrtoc8() in C++ modes if
-// __cpp_char8_t is defined or if C2X extensions are enabled. Unfortunately,
-// determining the latter depends on internal GNU libc details. If the
-// __cpp_char8_t feature test macro is not defined, then a char8_t typedef
-// will be declared as well.
-#  if defined(_LIBCPP_GLIBC_PREREQ) && defined(__GLIBC_USE)
-#    if _LIBCPP_GLIBC_PREREQ(2, 36) && (defined(__cpp_char8_t) || __GLIBC_USE(ISOC2X))
+// __cpp_char8_t is defined or if C2X extensions are enabled. Determining
+// the latter depends on internal GNU libc details that are not appropriate
+// to depend on here, so any declarations present when __cpp_char8_t is not
+// defined are ignored.
+#  if defined(_LIBCPP_GLIBC_PREREQ)
+#    if _LIBCPP_GLIBC_PREREQ(2, 36) && defined(__cpp_char8_t)
 #      undef _LIBCPP_HAS_NO_C8RTOMB_MBRTOC8
 #    endif
 #  endif
diff --git a/libcxx/include/__format/format_functions.h b/libcxx/include/__format/format_functions.h
index 185148ccba53..0f0001272d40 100644
--- a/libcxx/include/__format/format_functions.h
+++ b/libcxx/include/__format/format_functions.h
@@ -258,10 +258,12 @@ __handle_replacement_field(const _CharT* __begin, const _CharT* __end,
 
   if constexpr (same_as<_Ctx, __compile_time_basic_format_context<_CharT>>) {
     __arg_t __type = __ctx.arg(__r.__value);
-    if (__type == __arg_t::__handle)
+    if (__type == __arg_t::__none)
+      std::__throw_format_error("Argument index out of bounds");
+    else if (__type == __arg_t::__handle)
       __ctx.__handle(__r.__value).__parse(__parse_ctx);
-    else
-        __format::__compile_time_visit_format_arg(__parse_ctx, __ctx, __type);
+    else if (__parse)
+      __format::__compile_time_visit_format_arg(__parse_ctx, __ctx, __type);
   } else
     _VSTD::__visit_format_arg(
         [&](auto __arg) {
diff --git a/libcxx/include/__functional/hash.h b/libcxx/include/__functional/hash.h
index dfd8ea2553dc..39382aa9bff4 100644
--- a/libcxx/include/__functional/hash.h
+++ b/libcxx/include/__functional/hash.h
@@ -140,7 +140,11 @@ struct __murmur2_or_cityhash<_Size, 64>
     if (__len >= 4) {
       const uint32_t __a = std::__loadword<uint32_t>(__s);
       const uint32_t __b = std::__loadword<uint32_t>(__s + __len - 4);
+#ifdef _LIBCPP_ABI_FIX_CITYHASH_IMPLEMENTATION
       return __hash_len_16(__len + (static_cast<_Size>(__a) << 3), __b);
+#else
+      return __hash_len_16(__len + (__a << 3), __b);
+#endif
     }
     if (__len > 0) {
       const unsigned char __a = static_cast<unsigned char>(__s[0]);
diff --git a/libcxx/include/__ranges/join_view.h b/libcxx/include/__ranges/join_view.h
index 869540fc99c2..9d56e13b8439 100644
--- a/libcxx/include/__ranges/join_view.h
+++ b/libcxx/include/__ranges/join_view.h
@@ -40,7 +40,10 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if _LIBCPP_STD_VER > 17
+// Note: `join_view` is still marked experimental because there is an ABI-breaking change that affects `join_view` in
+// the pipeline (https://isocpp.org/files/papers/D2770R0.html).
+// TODO: make `join_view` non-experimental once D2770 is implemented.
+#if _LIBCPP_STD_VER > 17 && defined(_LIBCPP_ENABLE_EXPERIMENTAL)
 
 namespace ranges {
   template<class>
@@ -445,7 +448,7 @@ struct __segmented_iterator_traits<ranges::__join_view_iterator<_View, _Const>>
   }
 };
 
-#endif // _LIBCPP_STD_VER > 17
+#endif // #if _LIBCPP_STD_VER > 17 && defined(_LIBCPP_ENABLE_EXPERIMENTAL)
 
 _LIBCPP_END_NAMESPACE_STD
 
diff --git a/libcxx/include/module.modulemap.in b/libcxx/include/module.modulemap.in
index 1f1d67dbb7fc..8898e88156ef 100644
--- a/libcxx/include/module.modulemap.in
+++ b/libcxx/include/module.modulemap.in
@@ -755,13 +755,19 @@ module std [system] {
       module derived_from               { private header "__concepts/derived_from.h" }
       module destructible               { private header "__concepts/destructible.h" }
       module different_from             { private header "__concepts/different_from.h" }
-      module equality_comparable        { private header "__concepts/equality_comparable.h" }
+      module equality_comparable {
+        private header "__concepts/equality_comparable.h"
+        export type_traits.common_reference
+      }
       module invocable                  { private header "__concepts/invocable.h" }
       module movable                    { private header "__concepts/movable.h" }
       module predicate                  { private header "__concepts/predicate.h" }
       module regular                    { private header "__concepts/regular.h" }
       module relation                   { private header "__concepts/relation.h" }
-      module same_as                    { private header "__concepts/same_as.h" }
+      module same_as {
+        private header "__concepts/same_as.h"
+        export type_traits.is_same
+      }
       module semiregular                { private header "__concepts/semiregular.h" }
       module swappable                  { private header "__concepts/swappable.h" }
       module totally_ordered            { private header "__concepts/totally_ordered.h" }
@@ -979,7 +985,11 @@ module std [system] {
       module back_insert_iterator  { private header "__iterator/back_insert_iterator.h" }
       module bounded_iter          { private header "__iterator/bounded_iter.h" }
       module common_iterator       { private header "__iterator/common_iterator.h" }
-      module concepts              { private header "__iterator/concepts.h" }
+      module concepts {
+        private header "__iterator/concepts.h"
+        export concepts.equality_comparable
+        export type_traits.common_reference
+      }
       module counted_iterator      { private header "__iterator/counted_iterator.h" }
       module data                  { private header "__iterator/data.h" }
       module default_sentinel      { private header "__iterator/default_sentinel.h" }