1 files changed, 113 insertions, 0 deletions
diff --git a/libcxx/include/__format/format_parse_context.h b/libcxx/include/__format/format_parse_context.h
new file mode 100644
index 000000000000..db39c1b54830
--- /dev/null
+++ b/libcxx/include/__format/format_parse_context.h
@@ -0,0 +1,113 @@
+// -*- 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_FORMAT_PARSE_CONTEXT_H
+#define _LIBCPP___FORMAT_FORMAT_PARSE_CONTEXT_H
+
+#include <__config>
+#include <__format/format_error.h>
+#include <string_view>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER > 17
+
+// TODO FMT Remove this once we require compilers with proper C++20 support.
+// If the compiler has no concepts support, the format header will be disabled.
+// Without concepts support enable_if needs to be used and that too much effort
+// to support compilers with partial C++20 support.
+#if !defined(_LIBCPP_HAS_NO_CONCEPTS) &&                                       \
+    !defined(_LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED)
+
+template <class _CharT>
+class _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FORMAT basic_format_parse_context {
+public:
+  using char_type = _CharT;
+  using const_iterator = typename basic_string_view<_CharT>::const_iterator;
+  using iterator = const_iterator;
+
+  _LIBCPP_HIDE_FROM_ABI
+  constexpr explicit basic_format_parse_context(basic_string_view<_CharT> __fmt,
+                                                size_t __num_args = 0) noexcept
+      : __begin_(__fmt.begin()),
+        __end_(__fmt.end()),
+        __indexing_(__unknown),
+        __next_arg_id_(0),
+        __num_args_(__num_args) {}
+
+  basic_format_parse_context(const basic_format_parse_context&) = delete;
+  basic_format_parse_context&
+  operator=(const basic_format_parse_context&) = delete;
+
+  _LIBCPP_HIDE_FROM_ABI constexpr const_iterator begin() const noexcept {
+    return __begin_;
+  }
+  _LIBCPP_HIDE_FROM_ABI constexpr const_iterator end() const noexcept {
+    return __end_;
+  }
+  _LIBCPP_HIDE_FROM_ABI constexpr void advance_to(const_iterator __it) {
+    __begin_ = __it;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr size_t next_arg_id() {
+    if (__indexing_ == __manual)
+      __throw_format_error("Using automatic argument numbering in manual "
+                           "argument numbering mode");
+
+    if (__indexing_ == __unknown)
+      __indexing_ = __automatic;
+    return __next_arg_id_++;
+  }
+  _LIBCPP_HIDE_FROM_ABI constexpr void check_arg_id(size_t __id) {
+    if (__indexing_ == __automatic)
+      __throw_format_error("Using manual argument numbering in automatic "
+                           "argument numbering mode");
+
+    if (__indexing_ == __unknown)
+      __indexing_ = __manual;
+
+    // Throws an exception to make the expression a non core constant
+    // expression as required by:
+    // [format.parse.ctx]/11
+    //   Remarks: Call expressions where id >= num_args_ are not core constant
+    //   expressions ([expr.const]).
+    // Note: the Throws clause [format.parse.ctx]/10 doesn't specify the
+    // behavior when id >= num_args_.
+    if (is_constant_evaluated() && __id >= __num_args_)
+      __throw_format_error("Argument index outside the valid range");
+  }
+
+private:
+  iterator __begin_;
+  iterator __end_;
+  enum _Indexing { __unknown, __manual, __automatic };
+  _Indexing __indexing_;
+  size_t __next_arg_id_;
+  size_t __num_args_;
+};
+
+using format_parse_context = basic_format_parse_context<char>;
+using wformat_parse_context = basic_format_parse_context<wchar_t>;
+
+#endif // !defined(_LIBCPP_HAS_NO_CONCEPTS) && !defined(_LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED)
+
+#endif //_LIBCPP_STD_VER > 17
+
+_LIBCPP_END_NAMESPACE_STD
+
+_LIBCPP_POP_MACROS
+
+#endif // _LIBCPP___FORMAT_FORMAT_PARSE_CONTEXT_H