[libc++] Granularize range_format and format_kind declarations (llvm#148876)

While working on llvm#105430 I ran into an issue implementing
[[optional.syn]](https://eel.is/c++draft/optional.syn) because of a
circular include that looked like the following: `optional ->
__format/range_default_formatter.h -> __format/range_formatter.h ->
__format/format_context.h -> optional`. Only `format_kind` and
`range_format` are needed, and so they looked like candidates to be put
into an internal header.

Author: smallp-o-p

libcxx/include/CMakeLists.txt

@@ -403,6 +403,7 @@ set(files
   __format/indic_conjunct_break_table.h
   __format/parser_std_format_spec.h
   __format/range_default_formatter.h
+  __format/range_format.h
   __format/range_formatter.h
   __format/unicode.h
   __format/width_estimation_table.h

libcxx/include/__format/range_default_formatter.h

@@ -16,10 +16,10 @@
 
 #include <__algorithm/ranges_copy.h>
 #include <__chrono/statically_widen.h>
-#include <__concepts/same_as.h>
 #include <__config>
 #include <__format/concepts.h>
 #include <__format/formatter.h>
+#include <__format/range_format.h>
 #include <__format/range_formatter.h>
 #include <__iterator/back_insert_iterator.h>
 #include <__ranges/concepts.h>
@@ -42,51 +42,11 @@ concept __const_formattable_range =
 template <class _Rp, class _CharT>
 using __fmt_maybe_const _LIBCPP_NODEBUG = conditional_t<__const_formattable_range<_Rp, _CharT>, const _Rp, _Rp>;
 
-_LIBCPP_DIAGNOSTIC_PUSH
-_LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wshadow")
-_LIBCPP_GCC_DIAGNOSTIC_IGNORED("-Wshadow")
-// This shadows map, set, and string.
-enum class range_format { disabled, map, set, sequence, string, debug_string };
-_LIBCPP_DIAGNOSTIC_POP
-
 // There is no definition of this struct, it's purely intended to be used to
 // generate diagnostics.
 template <class _Rp>
 struct __instantiated_the_primary_template_of_format_kind;
 
-template <class _Rp>
-constexpr range_format format_kind = [] {
-  // [format.range.fmtkind]/1
-  // A program that instantiates the primary template of format_kind is ill-formed.
-  static_assert(sizeof(_Rp) != sizeof(_Rp), "create a template specialization of format_kind for your type");
-  return range_format::disabled;
-}();
-
-template <ranges::input_range _Rp>
-  requires same_as<_Rp, remove_cvref_t<_Rp>>
-inline constexpr range_format format_kind<_Rp> = [] {
-  // [format.range.fmtkind]/2
-
-  // 2.1 If same_as<remove_cvref_t<ranges::range_reference_t<R>>, R> is true,
-  // Otherwise format_kind<R> is range_format::disabled.
-  if constexpr (same_as<remove_cvref_t<ranges::range_reference_t<_Rp>>, _Rp>)
-    return range_format::disabled;
-  // 2.2 Otherwise, if the qualified-id R::key_type is valid and denotes a type:
-  else if constexpr (requires { typename _Rp::key_type; }) {
-    // 2.2.1 If the qualified-id R::mapped_type is valid and denotes a type ...
-    if constexpr (requires { typename _Rp::mapped_type; } &&
-                  // 2.2.1 ... If either U is a specialization of pair or U is a specialization
-                  // of tuple and tuple_size_v<U> == 2
-                  __fmt_pair_like<remove_cvref_t<ranges::range_reference_t<_Rp>>>)
-      return range_format::map;
-    else
-      // 2.2.2 Otherwise format_kind<R> is range_format::set.
-      return range_format::set;
-  } else
-    // 2.3 Otherwise, format_kind<R> is range_format::sequence.
-    return range_format::sequence;
-}();
-
 template <range_format _Kp, ranges::input_range _Rp, class _CharT>
 struct __range_default_formatter;
 

libcxx/include/__format/range_format.h

@@ -0,0 +1,71 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___FORMAT_RANGE_FORMAT_H
+#define _LIBCPP___FORMAT_RANGE_FORMAT_H
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#include <__concepts/same_as.h>
+#include <__config>
+#include <__format/concepts.h>
+#include <__ranges/concepts.h>
+#include <__type_traits/remove_cvref.h>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 23
+
+_LIBCPP_DIAGNOSTIC_PUSH
+_LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wshadow")
+_LIBCPP_GCC_DIAGNOSTIC_IGNORED("-Wshadow")
+// This shadows map, set, and string.
+enum class range_format { disabled, map, set, sequence, string, debug_string };
+_LIBCPP_DIAGNOSTIC_POP
+
+template <class _Rp>
+constexpr range_format format_kind = [] {
+  // [format.range.fmtkind]/1
+  // A program that instantiates the primary template of format_kind is ill-formed.
+  static_assert(sizeof(_Rp) != sizeof(_Rp), "create a template specialization of format_kind for your type");
+  return range_format::disabled;
+}();
+
+template <ranges::input_range _Rp>
+  requires same_as<_Rp, remove_cvref_t<_Rp>>
+inline constexpr range_format format_kind<_Rp> = [] {
+  // [format.range.fmtkind]/2
+
+  // 2.1 If same_as<remove_cvref_t<ranges::range_reference_t<R>>, R> is true,
+  // Otherwise format_kind<R> is range_format::disabled.
+  if constexpr (same_as<remove_cvref_t<ranges::range_reference_t<_Rp>>, _Rp>)
+    return range_format::disabled;
+  // 2.2 Otherwise, if the qualified-id R::key_type is valid and denotes a type:
+  else if constexpr (requires { typename _Rp::key_type; }) {
+    // 2.2.1 If the qualified-id R::mapped_type is valid and denotes a type ...
+    if constexpr (requires { typename _Rp::mapped_type; } &&
+                  // 2.2.1 ... If either U is a specialization of pair or U is a specialization
+                  // of tuple and tuple_size_v<U> == 2
+                  __fmt_pair_like<remove_cvref_t<ranges::range_reference_t<_Rp>>>)
+      return range_format::map;
+    else
+      // 2.2.2 Otherwise format_kind<R> is range_format::set.
+      return range_format::set;
+  } else
+    // 2.3 Otherwise, format_kind<R> is range_format::sequence.
+    return range_format::sequence;
+}();
+
+#endif // _LIBCPP_STD_VER >= 23
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif

libcxx/include/module.modulemap.in

@@ -1369,6 +1369,7 @@ module std [system] {
     module indic_conjunct_break_table         { header "__format/indic_conjunct_break_table.h" }
     module parser_std_format_spec             { header "__format/parser_std_format_spec.h" }
     module range_default_formatter            { header "__format/range_default_formatter.h" }
+    module range_format                       { header "__format/range_format.h" }
     module range_formatter                    { header "__format/range_formatter.h" }
     module unicode                            { header "__format/unicode.h" }
     module width_estimation_table             { header "__format/width_estimation_table.h" }