[libc++] Rename __fwd/hash.h to __fwd/functional.h and add reference_wrapper (llvm#81445)

We forward declare `reference_wrapper` in multiple places already. This
moves the declaration to the canonical place and removes unnecessary
includes of `__functional/reference_wrapper.h`.

Author: philnik777

libcxx/include/CMakeLists.txt

@@ -431,7 +431,7 @@ set(files
   __fwd/bit_reference.h
   __fwd/complex.h
   __fwd/fstream.h
-  __fwd/hash.h
+  __fwd/functional.h
   __fwd/ios.h
   __fwd/istream.h
   __fwd/mdspan.h

libcxx/include/__filesystem/path.h

@@ -14,9 +14,8 @@
 #include <__algorithm/replace_copy.h>
 #include <__availability>
 #include <__config>
-#include <__functional/hash.h>
 #include <__functional/unary_function.h>
-#include <__fwd/hash.h>
+#include <__fwd/functional.h>
 #include <__iterator/back_insert_iterator.h>
 #include <__iterator/iterator_traits.h>
 #include <__type_traits/decay.h>

libcxx/include/__functional/bind.h

@@ -13,6 +13,7 @@
 #include <__config>
 #include <__functional/invoke.h>
 #include <__functional/weak_result_type.h>
+#include <__fwd/functional.h>
 #include <__type_traits/decay.h>
 #include <__type_traits/is_reference_wrapper.h>
 #include <__type_traits/is_void.h>

libcxx/include/__functional/hash.h

@@ -12,7 +12,7 @@
 #include <__config>
 #include <__functional/invoke.h>
 #include <__functional/unary_function.h>
-#include <__fwd/hash.h>
+#include <__fwd/functional.h>
 #include <__tuple/sfinae_helpers.h>
 #include <__type_traits/is_copy_constructible.h>
 #include <__type_traits/is_default_constructible.h>

libcxx/include/__functional/identity.h

@@ -11,7 +11,7 @@
 #define _LIBCPP___FUNCTIONAL_IDENTITY_H
 
 #include <__config>
-#include <__functional/reference_wrapper.h>
+#include <__fwd/functional.h>
 #include <__type_traits/integral_constant.h>
 #include <__utility/forward.h>
 

libcxx/include/__fwd/hash.h renamed to libcxx/include/__fwd/functional.h

@@ -6,8 +6,8 @@
 //
 //===---------------------------------------------------------------------===//
 
-#ifndef _LIBCPP___FWD_HASH_H
-#define _LIBCPP___FWD_HASH_H
+#ifndef _LIBCPP___FWD_FUNCTIONAL_H
+#define _LIBCPP___FWD_FUNCTIONAL_H
 
 #include <__config>
 
@@ -20,6 +20,9 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 template <class>
 struct _LIBCPP_TEMPLATE_VIS hash;
 
+template <class>
+class _LIBCPP_TEMPLATE_VIS reference_wrapper;
+
 _LIBCPP_END_NAMESPACE_STD
 
-#endif // _LIBCPP___FWD_HASH_H
+#endif // _LIBCPP___FWD_FUNCTIONAL_H

libcxx/include/__thread/id.h

@@ -12,7 +12,7 @@
 
 #include <__compare/ordering.h>
 #include <__config>
-#include <__fwd/hash.h>
+#include <__fwd/functional.h>
 #include <__fwd/ostream.h>
 #include <__thread/support.h>
 

libcxx/include/__thread/support/pthread.h

@@ -14,7 +14,6 @@
 #include <__chrono/convert_to_timespec.h>
 #include <__chrono/duration.h>
 #include <__config>
-#include <__fwd/hash.h>
 #include <ctime>
 #include <errno.h>
 #include <pthread.h>

libcxx/include/__type_traits/is_reference_wrapper.h

@@ -10,6 +10,7 @@
 #define _LIBCPP___TYPE_TRAITS_IS_REFERENCE_WRAPPER_H
 
 #include <__config>
+#include <__fwd/functional.h>
 #include <__type_traits/integral_constant.h>
 #include <__type_traits/remove_cv.h>
 
@@ -19,9 +20,6 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-template <class _Tp>
-class _LIBCPP_TEMPLATE_VIS reference_wrapper;
-
 template <class _Tp>
 struct __is_reference_wrapper_impl : public false_type {};
 template <class _Tp>

libcxx/include/__type_traits/unwrap_ref.h

@@ -10,6 +10,7 @@
 #define _LIBCPP___TYPE_TRAITS_UNWRAP_REF_H
 
 #include <__config>
+#include <__fwd/functional.h>
 #include <__type_traits/decay.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -23,17 +24,11 @@ struct __unwrap_reference {
   typedef _LIBCPP_NODEBUG _Tp type;
 };
 
-template <class _Tp>
-class reference_wrapper;
-
 template <class _Tp>
 struct __unwrap_reference<reference_wrapper<_Tp> > {
   typedef _LIBCPP_NODEBUG _Tp& type;
 };
 
-template <class _Tp>
-struct decay;
-
 #if _LIBCPP_STD_VER >= 20
 template <class _Tp>
 struct unwrap_reference : __unwrap_reference<_Tp> {};

libcxx/include/experimental/propagate_const

@@ -108,7 +108,7 @@
 */
 
 #include <__functional/operations.h>
-#include <__fwd/hash.h>
+#include <__fwd/functional.h>
 #include <__type_traits/conditional.h>
 #include <__type_traits/decay.h>
 #include <__type_traits/enable_if.h>

libcxx/include/filesystem

@@ -564,6 +564,7 @@ inline constexpr bool std::ranges::enable_view<std::filesystem::recursive_direct
 #if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
 #  include <concepts>
 #  include <cstdlib>
+#  include <cstring>
 #  include <iosfwd>
 #  include <new>
 #  include <system_error>

libcxx/include/libcxx.imp

@@ -426,7 +426,7 @@
   { include: [ "<__fwd/bit_reference.h>", "private", "<vector>", "public" ] },
   { include: [ "<__fwd/complex.h>", "private", "<complex>", "public" ] },
   { include: [ "<__fwd/fstream.h>", "private", "<iosfwd>", "public" ] },
-  { include: [ "<__fwd/hash.h>", "private", "<functional>", "public" ] },
+  { include: [ "<__fwd/functional.h>", "private", "<functional>", "public" ] },
   { include: [ "<__fwd/ios.h>", "private", "<iosfwd>", "public" ] },
   { include: [ "<__fwd/istream.h>", "private", "<iosfwd>", "public" ] },
   { include: [ "<__fwd/mdspan.h>", "private", "<mdspan>", "public" ] },

libcxx/include/module.modulemap.in

@@ -1365,7 +1365,7 @@ module std_private_functional_hash                       [system] {
   export std_private_type_traits_underlying_type
   export std_private_utility_pair
 }
-module std_private_functional_hash_fwd                   [system] { header "__fwd/hash.h" }
+module std_private_functional_fwd                        [system] { header "__fwd/functional.h" }
 module std_private_functional_identity                   [system] { header "__functional/identity.h" }
 module std_private_functional_invoke                     [system] {
   header "__functional/invoke.h"

libcxx/include/optional

@@ -186,8 +186,8 @@ namespace std {
 #include <__exception/exception.h>
 #include <__functional/hash.h>
 #include <__functional/invoke.h>
-#include <__functional/reference_wrapper.h>
 #include <__functional/unary_function.h>
+#include <__fwd/functional.h>
 #include <__memory/addressof.h>
 #include <__memory/construct_at.h>
 #include <__tuple/sfinae_helpers.h>

libcxx/include/type_traits

@@ -418,7 +418,7 @@ namespace std
 */
 
 #include <__config>
-#include <__fwd/hash.h> // This is https://llvm.org/PR56938
+#include <__fwd/functional.h> // This is https://llvm.org/PR56938
 #include <__type_traits/add_const.h>
 #include <__type_traits/add_cv.h>
 #include <__type_traits/add_lvalue_reference.h>

libcxx/test/libcxx/transitive_includes/cxx23.csv

@@ -178,7 +178,6 @@ experimental/utility utility
 filesystem compare
 filesystem cstddef
 filesystem cstdint
-filesystem cstring
 filesystem ctime
 filesystem iomanip
 filesystem limits

libcxx/test/libcxx/transitive_includes/cxx26.csv

@@ -178,7 +178,6 @@ experimental/utility utility
 filesystem compare
 filesystem cstddef
 filesystem cstdint
-filesystem cstring
 filesystem ctime
 filesystem iomanip
 filesystem limits

libcxx/utils/generate_iwyu_mapping.py

@@ -34,8 +34,6 @@ def IWYU_mapping(header: str) -> typing.Optional[typing.List[str]]:
         return ["atomic", "mutex", "semaphore", "thread"]
     elif header == "__tree":
         return ["map", "set"]
-    elif header == "__fwd/hash.h":
-        return ["functional"]
     elif header == "__fwd/pair.h":
         return ["utility"]
     elif header == "__fwd/subrange.h":