[libc++][format] Adds range-default-formatter.

This adds an incomplete version where the specializations for the
format_kinds are disabled dummy formatters.

Implements part of
- P2585R0 Improving default container formatting

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D137271

2 files changed, 151 insertions, 0 deletions

diff --git a/libcxx/include/__format/concepts.h b/libcxx/include/__format/concepts.h
index 5407fee..fe4a7b9 100644
--- a/libcxx/include/__format/concepts.h
+++ b/libcxx/include/__format/concepts.h
@@ -15,6 +15,9 @@
 #include <__config>
 #include <__format/format_fwd.h>
 #include <__format/format_parse_context.h>
+#include <__type_traits/is_specialization.h>
+#include <__utility/pair.h>
+#include <tuple>
 #include <type_traits>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -56,6 +59,17 @@ concept __formattable =
 #  if _LIBCPP_STD_VER > 20
 template <class _Tp, class _CharT>
 concept formattable = __formattable<_Tp, _CharT>;
+
+// [tuple.like] defines a tuple-like exposition only concept. This concept is
+// not related to that. Therefore it uses a different name for the concept.
+//
+// TODO FMT Add a test to validate we fail when using that concept after P2165
+// has been implemented.
+template <class _Tp>
+concept __fmt_pair_like = __is_specialization_v<_Tp, pair> ||
+                          // Use a requires since tuple_size_v may fail to instantiate,
+                          (__is_specialization_v<_Tp, tuple> && requires { tuple_size_v<_Tp> == 2; });
+
 #  endif //_LIBCPP_STD_VER > 20
 #endif //_LIBCPP_STD_VER > 17
 
diff --git a/libcxx/include/__format/range_default_formatter.h b/libcxx/include/__format/range_default_formatter.h
new file mode 100644
index 0000000..56558f3
--- /dev/null
+++ b/libcxx/include/__format/range_default_formatter.h
@@ -0,0 +1,137 @@
+// -*- 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_DEFAULT_FORMATTER_H
+#define _LIBCPP___FORMAT_RANGE_DEFAULT_FORMATTER_H
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#include <__availability>
+#include <__concepts/same_as.h>
+#include <__config>
+#include <__format/concepts.h>
+#include <__format/formatter.h>
+#include <__ranges/concepts.h>
+#include <__type_traits/remove_cvref.h>
+#include <__utility/pair.h>
+#include <tuple>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER > 20
+
+template <class _Rp, class _CharT>
+concept __const_formattable_range =
+    ranges::input_range<const _Rp> && formattable<ranges::range_reference_t<const _Rp>, _CharT>;
+
+template <class _Rp, class _CharT>
+using __fmt_maybe_const = 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 _LIBCPP_TEMPLATE_VIS __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;
+}();
+
+// This is a non-standard work-around to fix instantiation of
+//   formatter<const _CharT[N], _CharT>
+// const _CharT[N] satisfies the ranges::input_range concept.
+// remove_cvref_t<const _CharT[N]> is _CharT[N] so it does not satisfy the
+// requirement of the above specialization. Instead it will instantiate the
+// primary template, which is ill-formed.
+//
+// An alternative solution is to remove the offending formatter.
+//
+// https://godbolt.org/z/bqjhhaexx
+//
+// The removal is proposed in LWG3833, but use the work-around until the issue
+// has been adopted.
+// TODO FMT Implement LWG3833.
+template <class _CharT, size_t N>
+inline constexpr range_format format_kind<const _CharT[N]> = range_format::disabled;
+
+template <range_format _Kp, ranges::input_range _Rp, class _CharT>
+struct _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FORMAT __range_default_formatter;
+
+// Required specializations
+
+template <ranges::input_range _Rp, class _CharT>
+struct _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FORMAT __range_default_formatter<range_format::sequence, _Rp, _CharT> {
+  __range_default_formatter() = delete; // TODO FMT Implement
+};
+
+template <ranges::input_range _Rp, class _CharT>
+struct _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FORMAT __range_default_formatter<range_format::map, _Rp, _CharT> {
+  __range_default_formatter() = delete; // TODO FMT Implement
+};
+
+template <ranges::input_range _Rp, class _CharT>
+struct _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FORMAT __range_default_formatter<range_format::set, _Rp, _CharT> {
+  __range_default_formatter() = delete; // TODO FMT Implement
+};
+
+template <range_format _Kp, ranges::input_range _Rp, class _CharT>
+  requires(_Kp == range_format::string || _Kp == range_format::debug_string)
+struct _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FORMAT __range_default_formatter<_Kp, _Rp, _CharT> {
+  __range_default_formatter() = delete; // TODO FMT Implement
+};
+
+// Dispatcher to select the specialization based on the type of the range.
+
+template <ranges::input_range _Rp, class _CharT>
+  requires(format_kind<_Rp> != range_format::disabled && formattable<ranges::range_reference_t<_Rp>, _CharT>)
+struct _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FORMAT formatter<_Rp, _CharT>
+    : __range_default_formatter<format_kind<_Rp>, _Rp, _CharT> {};
+
+#endif //_LIBCPP_STD_VER > 20
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___FORMAT_RANGE_DEFAULT_FORMATTER_H