[libc++][format] Implements LWG3892.

[Mordante]

Summary

This LWG issue is based on the discussion regarding

P2733R3 Fix handling of empty specifiers in std::format

This paper was disussed and changed a few times in LEWG during the
Issaquah meeting. The paper was not voted in, instead LEWG asked for
a DR against C++26.

This LWG issue contains the direction voted by LEWG. This issue has not
been voted in yet. However it fixes some of the defencies on the
container based formatting. Without this fix the range-default-formatter
for strings looks bad when used in containers.

The changes of this issue match the intended changes of P27333.

type fmt before after (if changed)

char {} a
char {:?} 'a'
array<char, 1> {} ['a']
array<char, 1> {::} [a]
array<char, 1> {::c} [a]
array<char, 1> {::?} ['a']
map<char, char> {} {a: a} -> {'a': 'a'}
map<char, char> {::} {'a': 'a'}
set<char> {} {'a'}
set<char> {::} {a}
set<char> {::c} {a}
set<char> {::?} {'a'}
tuple<char> {} ('a')
stack<char> {} ['a']
stack<char> {::} [a]
stack<char> {::c} [a]
stack<char> {::?} ['a']
array<array<char, 1>, 1> {} [[a]] -> {'a': 'a'}
array<array<char, 1>, 1> {::} [['a']]
array<array<char, 1>, 1> {:::} [[a]]
array<array<char, 1>, 1> {:::c} [[a]]
array<array<char, 1>, 1> {:::?} [['a']]
array<tuple<char>, 1> {} [(a)] -> [('a')]
tuple<tuple<char>> {} ((a)) -> (('a'))
tuple<array<char, 1>> {} ([a]) -> (['a'])

Note the optimization text as mentioned in the tuple formatter can't be
done. The call to parse may affect the formatter so its state needs to
be preserved.

[Mordante]

CI fixes

[ldionne]

LGTM, it would be great if @vitaut could have a quick look as well.

libcxx/test/support/format.functions.common.h

52

[EricWF]

Nice tests. Big fan of the work done in this patch.

I haven't reviewed it for correctness w.r.t. the format spec, but in general I think it looks good.

libcxx/include/__format/range_formatter.h

265

Is there a reason for always doing this rather than just guarding it in a macro?

[ldionne]

libcxx/test/std/utilities/format/format.range/format.range.fmtmap/format.functions.format.pass.cpp

43

This change seems like a leftover?

[Mordante]

Thanks for the reviews!

libcxx/include/__format/range_formatter.h

265

I'm not entirely sure what you mean. But we need to call __underlying_.parse(__parse_ctx); even when the range is empty.

Calling the parse member of the formatter is an observable effect by the parser, there are tests to validate the call is made.

[Mordante]

Rebased and addresses review comments. Giving it a CI run.

libcxx/docs/Status/Cxx2bIssues.csv

 "`3876 <https://wg21.link/LWG3876>`__","Default constructor of ``std::layout_XX::mapping`` misses precondition","February 2023","","",""
 "`3877 <https://wg21.link/LWG3877>`__","Incorrect constraints on ``const``-qualified monadic overloads for ``std::expected``","February 2023","","",""
 "`3878 <https://wg21.link/LWG3878>`__","import ``std;`` should guarantee initialization of standard iostreams objects","February 2023","","",""
 "`3879 <https://wg21.link/LWG3879>`__","``erase_if`` for ``flat_{,multi}set`` is incorrectly specified","February 2023","","",""
 "`3880 <https://wg21.link/LWG3880>`__","Clarify ``operator+=`` complexity for ``{chunk,stride}_view::iterator``","February 2023","","","|ranges|"
 "`3881 <https://wg21.link/LWG3881>`__","Incorrect formatting of container adapters backed by ``std::string``","February 2023","|Complete|","17.0","|format|"
 "","","","","",""
 "`3343 <https://wg21.link/LWG3343>`__","Ordering of calls to ``unlock()`` and ``notify_all()`` in Effects element of ``notify_all_at_thread_exit()`` should be reversed","Not Yet Adopted","|Complete|","16.0",""
+"`3892 <https://wg21.link/LWG3892>`__","Incorrect formatting of nested ranges and tuples","Not Yet Adopted","|Complete|","17.0",""

libcxx/include/__format/formatter_tuple.h

set_brackets(basic_string_view<_CharT> __opening_bracket, basic_string_view<_CharT> __closing_bracket) noexcept {
     __opening_bracket_ = __opening_bracket;
     __closing_bracket_ = __closing_bracket;
   }
 
   template <class _ParseContext>
   _LIBCPP_HIDE_FROM_ABI constexpr typename _ParseContext::iterator parse(_ParseContext& __parse_ctx) {
     auto __begin = __parser_.__parse(__parse_ctx, __format_spec::__fields_tuple);
 
-    // [format.tuple]/7
-    //   ... For each element e in underlying_, if e.set_debug_format()
-    //   is a valid expression, calls e.set_debug_format().
-    // TODO FMT this can be removed when P2733 is accepted.
-    std::__for_each_index_sequence(make_index_sequence<sizeof...(_Args)>(), [&]<size_t _Index> {
-      std::__set_debug_format(std::get<_Index>(__underlying_));
-    });
-
     auto __end = __parse_ctx.end();
-    if (__begin == __end)
-      return __begin;
-
+    if (__begin != __end) {
       if (*__begin == _CharT('m')) {
         if constexpr (sizeof...(_Args) == 2) {
           set_separator(_LIBCPP_STATICALLY_WIDEN(_CharT, ": "));
           set_brackets({}, {});
           ++__begin;
         } else
           std::__throw_format_error("The format specifier m requires a pair or a two-element tuple");
       } else if (*__begin == _CharT('n')) {
         set_brackets({}, {});
         ++__begin;
       }
+    }
 
     if (__begin != __end && *__begin != _CharT('}'))
       std::__throw_format_error("The format-spec should consume the input or end with a '}'");
 
+    __parse_ctx.advance_to(__begin);
+
+    // [format.tuple]/7
+    //   ... For each element e in underlying_, if e.set_debug_format()
+    //   is a valid expression, calls e.set_debug_format().
+    std::__for_each_index_sequence(make_index_sequence<sizeof...(_Args)>(), [&]<size_t _Index> {
+      auto& __formatter = std::get<_Index>(__underlying_);
+      __formatter.parse(__parse_ctx);
+      // Unlike the range_formatter we don't guard against evil parsers. Since
+      // this format-spec never has a format-spec for the underlying type
+      // adding the test would give additional overhead.
+      std::__set_debug_format(__formatter);
+    });
+
     return __begin;
   }
 
   template <class _FormatContext>
   typename _FormatContext::iterator _LIBCPP_HIDE_FROM_ABI
   format(conditional_t<(formattable<const _Args, _CharT> && ...), const _Tuple&, _Tuple&> __tuple,
          _FormatContext& __ctx) const {
     __format_spec::__parsed_specifications<_CharT> __specs = __parser_.__get_parsed_std_specifications(__ctx);
struct _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FORMAT __formatter_tuple {
 
   template <class _FormatContext>
   _LIBCPP_HIDE_FROM_ABI typename _FormatContext::iterator __format_tuple(auto&& __tuple, _FormatContext& __ctx) const {
     __ctx.advance_to(std::ranges::copy(__opening_bracket_, __ctx.out()).out);
 
     std::__for_each_index_sequence(make_index_sequence<sizeof...(_Args)>(), [&]<size_t _Index> {
       if constexpr (_Index)
         __ctx.advance_to(std::ranges::copy(__separator_, __ctx.out()).out);
-
-        // During review Victor suggested to make the exposition only
-        // __underlying_ member a local variable. Currently the Standard
-        // requires nested debug-enabled formatter specializations not to
-        // output escaped output. P2733 fixes that bug, once accepted the
-        // code below can be used.
-        // (Note when a paper allows parsing a tuple-underlying-spec the
-        // exposition only member needs to be a class member. Earlier
-        // revisions of P2286 proposed that, but this was not pursued,
-        // due to time constrains and complexity of the matter.)
-        // TODO FMT This can be updated after P2733 is accepted.
-#  if 0
-      // P2286 uses an exposition only member in the formatter
-      //   tuple<formatter<remove_cvref_t<_Args>, _CharT>...> __underlying_;
-      // This was used in earlier versions of the paper since
-      // __underlying_.parse(...) was called. This is no longer the case
-      // so we can reduce the scope of the formatter.
-      //
-      // It does require the underlying's parse effect to be moved here too.
-      using _Arg = tuple_element<_Index, decltype(__tuple)>;
-      formatter<remove_cvref_t<_Args>, _CharT> __underlying;
-
-      // [format.tuple]/7
-      //   ... For each element e in underlying_, if e.set_debug_format()
-      //   is a valid expression, calls e.set_debug_format().
-      std::__set_debug_format(__underlying);
-#  else
       __ctx.advance_to(std::get<_Index>(__underlying_).format(std::get<_Index>(__tuple), __ctx));
-#  endif
     });
 
     return std::ranges::copy(__closing_bracket_, __ctx.out()).out;
   }
 
   __format_spec::__parser<_CharT> __parser_{.__alignment_ = __format_spec::__alignment::__left};
 
 private:

libcxx/include/__format/range_formatter.h

struct _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FORMAT range_formatter {
 
   _LIBCPP_HIDE_FROM_ABI constexpr formatter<_Tp, _CharT>& underlying() noexcept { return __underlying_; }
   _LIBCPP_HIDE_FROM_ABI constexpr const formatter<_Tp, _CharT>& underlying() const noexcept { return __underlying_; }
 
   template <class _ParseContext>
   _LIBCPP_HIDE_FROM_ABI constexpr typename _ParseContext::iterator parse(_ParseContext& __parse_ctx) {
     auto __begin = __parser_.__parse(__parse_ctx, __format_spec::__fields_range);
     auto __end   = __parse_ctx.end();
-    if (__begin == __end)
-      return __begin;
+    // Note the cases where __begin == __end in this code only happens when the
+    // replacement-field has no terminating }, or when the parse is manually
+    // called with a format-spec. The former is an error and the latter means
+    // using a formatter without the format functions or print.
+    if (__begin == __end) [[unlikely]]
+      return __parse_empty_range_underlying_spec(__parse_ctx, __begin);
 
     // The n field overrides a possible m type, therefore delay applying the
     // effect of n until the type has been procesed.
     bool __clear_brackets = (*__begin == _CharT('n'));
     if (__clear_brackets) {
       ++__begin;
-      if (__begin == __end) {
+      if (__begin == __end) [[unlikely]] {
         // Since there is no more data, clear the brackets before returning.
         set_brackets({}, {});
-        return __begin;
+        return __parse_empty_range_underlying_spec(__parse_ctx, __begin);
       }
     }
 
     __parse_type(__begin, __end);
     if (__clear_brackets)
       set_brackets({}, {});
-    if (__begin == __end)
-      return __begin;
+    if (__begin == __end) [[unlikely]]
+      return __parse_empty_range_underlying_spec(__parse_ctx, __begin);
 
     bool __has_range_underlying_spec = *__begin == _CharT(':');
+    if (__has_range_underlying_spec) {
+      // range-underlying-spec:
+      //   :  format-spec
+      ++__begin;
+    } else if (__begin != __end && *__begin != _CharT('}'))
+      // When there is no underlaying range the current parse should have
+      // consumed the format-spec. If not, the not consumed input will be
+      // processed by the underlying. For example {:-} for a range in invalid,
+      // the sign field is not present. Without this check the underlying_ will
+      // get -} as input which my be valid.
+      std::__throw_format_error("The format-spec should consume the input or end with a '}'");
+
+    __parse_ctx.advance_to(__begin);
+    __begin = __underlying_.parse(__parse_ctx);
+
+    // This test should not be required if __has_range_underlying_spec is false.
+    // However this test makes sure the underlying formatter left the parser in
+    // a valid state. (Note this is not a full protection against evil parsers.
+    // For example
+    //   } this is test for the next argument {}
+    //   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
+    // could consume more than it should.
+    if (__begin != __end && *__begin != _CharT('}'))
+      std::__throw_format_error("The format-spec should consume the input or end with a '}'");
+
     if (__parser_.__type_ != __format_spec::__type::__default) {
       // [format.range.formatter]/6
       //   If the range-type is s or ?s, then there shall be no n option and no
       //   range-underlying-spec.
       if (__clear_brackets) {
         if (__parser_.__type_ == __format_spec::__type::__string)
           std::__throw_format_error("The n option and type s can't be used together");
         std::__throw_format_error("The n option and type ?s can't be used together");
       }
       if (__has_range_underlying_spec) {
         if (__parser_.__type_ == __format_spec::__type::__string)
           std::__throw_format_error("Type s and an underlying format specification can't be used together");
         std::__throw_format_error("Type ?s and an underlying format specification can't be used together");
       }
     } else if (!__has_range_underlying_spec)
       std::__set_debug_format(__underlying_);
 
-    if (__has_range_underlying_spec) {
-      // range-underlying-spec:
-      //   :  format-spec
-      ++__begin;
-      if (__begin == __end)
-        return __begin;
-
-      __parse_ctx.advance_to(__begin);
-      __begin = __underlying_.parse(__parse_ctx);
-    }
-
-    if (__begin != __end && *__begin != _CharT('}'))
-      std::__throw_format_error("The format-spec should consume the input or end with a '}'");
-
     return __begin;
   }
 
   template <ranges::input_range _Rp, class _FormatContext>
     requires formattable<ranges::range_reference_t<_Rp>, _CharT> &&
              same_as<remove_cvref_t<ranges::range_reference_t<_Rp>>, _Tp>
   _LIBCPP_HIDE_FROM_ABI typename _FormatContext::iterator format(_Rp&& __range, _FormatContext& __ctx) const {
     __format_spec::__parsed_specifications<_CharT> __specs = __parser_.__get_parsed_std_specifications(__ctx);
case _CharT('?'):
       if constexpr (same_as<_Tp, _CharT>) {
         __parser_.__type_ = __format_spec::__type::__debug;
         ++__begin;
       } else
         std::__throw_format_error("The range-format-spec type ?s requires formatting a character type");
     }
   }
 
+  template <class _ParseContext>
+  _LIBCPP_HIDE_FROM_ABI constexpr typename _ParseContext::iterator
+  __parse_empty_range_underlying_spec(_ParseContext& __parse_ctx, typename _ParseContext::iterator __begin) {
+    __parse_ctx.advance_to(__begin);
+    [[maybe_unused]] typename _ParseContext::iterator __result = __underlying_.parse(__parse_ctx);
+    _LIBCPP_ASSERT(__result == __begin,
+                   "the underlying's parse function should not advance the input beyond the end of the input");
+    return __begin;
+  }
+
   formatter<_Tp, _CharT> __underlying_;
   basic_string_view<_CharT> __separator_       = _LIBCPP_STATICALLY_WIDEN(_CharT, ", ");
   basic_string_view<_CharT> __opening_bracket_ = _LIBCPP_STATICALLY_WIDEN(_CharT, "[");
   basic_string_view<_CharT> __closing_bracket_ = _LIBCPP_STATICALLY_WIDEN(_CharT, "]");
 };
 
 #endif //_LIBCPP_STD_VER >= 23
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___FORMAT_RANGE_FORMATTER_H

libcxx/test/std/utilities/format/format.range/format.range.fmtdef/format.pass.cpp

 //     format(maybe-const-r& elems, FormatContext& ctx) const;
 
 #include <array>
 #include <cassert>
 #include <concepts>
 #include <format>
 #include <iterator>
 
+#include "assert_macros.h"
+#include "format.functions.common.h"
 #include "test_format_context.h"
 #include "test_macros.h"
 #include "make_string.h"
 
 #define SV(S) MAKE_STRING_VIEW(CharT, S)
 
 template <class StringViewT>
 void test_format(StringViewT expected, std::array<int, 2> arg) {
   using CharT      = typename StringViewT::value_type;
   using String     = std::basic_string<CharT>;
   using OutIt      = std::back_insert_iterator<String>;
   using FormatCtxT = std::basic_format_context<OutIt, CharT>;
 
   std::formatter<std::array<int, 2>, CharT> formatter;
 
   String result;
   OutIt out             = std::back_inserter(result);
   FormatCtxT format_ctx = test_format_context_create<OutIt, CharT>(out, std::make_format_args<FormatCtxT>(arg));
   formatter.format(arg, format_ctx);
   assert(result == expected);
 }
 
 template <class CharT>
+void test_assure_parse_is_called(std::basic_string_view<CharT> fmt) {
+  using String     = std::basic_string<CharT>;
+  using OutIt      = std::back_insert_iterator<String>;
+  using FormatCtxT = std::basic_format_context<OutIt, CharT>;
+  std::array<parse_call_validator, 2> arg;
+
+  String result;
+  OutIt out             = std::back_inserter(result);
+  FormatCtxT format_ctx = test_format_context_create<OutIt, CharT>(out, std::make_format_args<FormatCtxT>(arg));
+
+  std::formatter<decltype(arg), CharT> formatter;
+  std::basic_format_parse_context<CharT> ctx{fmt};
+
+  formatter.parse(ctx);
+  formatter.format(arg, format_ctx);
+}
+
+template <class CharT>
+void test_assure_parse_is_called() {
+  using String     = std::basic_string<CharT>;
+  using OutIt      = std::back_insert_iterator<String>;
+  using FormatCtxT = std::basic_format_context<OutIt, CharT>;
+  std::array<parse_call_validator, 2> arg;
+
+  String result;
+  OutIt out             = std::back_inserter(result);
+  FormatCtxT format_ctx = test_format_context_create<OutIt, CharT>(out, std::make_format_args<FormatCtxT>(arg));
+
+  { // parse not called
+    [[maybe_unused]] const std::formatter<decltype(arg), CharT> formatter;
+    TEST_THROWS_TYPE(parse_call_validator::parse_function_not_called, formatter.format(arg, format_ctx));
+  }
+
+  test_assure_parse_is_called(SV("5"));
+  test_assure_parse_is_called(SV("n"));
+  test_assure_parse_is_called(SV("5n"));
+}
+
+template <class CharT>
 void test_fmt() {
   test_format(SV("[1, 42]"), std::array<int, 2>{{1, 42}});
   test_format(SV("[0, 99]"), std::array<int, 2>{{0, 99}});
+
+  test_assure_parse_is_called<CharT>();
 }
 
 void test() {
   test_fmt<char>();
 #ifndef TEST_HAS_NO_WIDE_CHARACTERS
   test_fmt<wchar_t>();
 #endif
 }
 
 int main(int, char**) {
   test();
 
   return 0;
 }

libcxx/test/std/utilities/format/format.range/format.range.fmtmap/format.functions.format.pass.cpp

 #include "test_format_string.h"
 #include "test_macros.h"
 #include "assert_macros.h"
 #include "concat_macros.h"
 
 auto test = []<class CharT, class... Args>(
                 std::basic_string_view<CharT> expected, test_format_string<CharT, Args...> fmt, Args&&... args) {
   std::basic_string<CharT> out = std::format(fmt, std::forward<Args>(args)...);
-  TEST_REQUIRE(out == expected,
-               TEST_WRITE_CONCATENATED(
-                   "\nFormat string   ", fmt, "\nExpected output ", expected, "\nActual output   ", out, '\n'));
+  TEST_REQUIRE(
+      out == expected,
+          "\nFormat string   ", fmt.get(), "\nExpected output ", expected, "\nActual output   ", out, '\n')); // XXX
 };
 
 auto test_exception = []<class CharT, class... Args>(std::string_view, std::basic_string_view<CharT>, Args&&...) {
   // After P2216 most exceptions thrown by std::format become ill-formed.
   // Therefore this tests does nothing.
 };
 
 int main(int, char**) {
   format_tests<char>(test, test_exception);
 
 #ifndef TEST_HAS_NO_WIDE_CHARACTERS
   format_tests<wchar_t>(test, test_exception);
 #endif
 
   return 0;
 }

libcxx/test/std/utilities/format/format.range/format.range.fmtmap/format.functions.tests.h

 //
 // Char
 //
 
 template <class CharT, class TestFunction, class ExceptionTest>
 void test_char(TestFunction check, ExceptionTest check_exception) {
   std::map<CharT, CharT> input{{CharT('a'), CharT('A')}, {CharT('c'), CharT('C')}, {CharT('b'), CharT('B')}};
 
-  check(SV("{a: A, b: B, c: C}"), SV("{}"), input);
+  check(SV("{'a': 'A', 'b': 'B', 'c': 'C'}"), SV("{}"), input);
 
   // ***** underlying has no format-spec
 
   // *** align-fill & width ***
-  check(SV("{a: A, b: B, c: C}     "), SV("{:23}"), input);
-  check(SV("{a: A, b: B, c: C}*****"), SV("{:*<23}"), input);
-  check(SV("__{a: A, b: B, c: C}___"), SV("{:_^23}"), input);
-  check(SV("#####{a: A, b: B, c: C}"), SV("{:#>23}"), input);
-
-  check(SV("{a: A, b: B, c: C}     "), SV("{:{}}"), input, 23);
-  check(SV("{a: A, b: B, c: C}*****"), SV("{:*<{}}"), input, 23);
-  check(SV("__{a: A, b: B, c: C}___"), SV("{:_^{}}"), input, 23);
-  check(SV("#####{a: A, b: B, c: C}"), SV("{:#>{}}"), input, 23);
+  check(SV("{'a': 'A', 'b': 'B', 'c': 'C'}     "), SV("{:35}"), input);
+  check(SV("{'a': 'A', 'b': 'B', 'c': 'C'}*****"), SV("{:*<35}"), input);
+  check(SV("__{'a': 'A', 'b': 'B', 'c': 'C'}___"), SV("{:_^35}"), input);
+  check(SV("#####{'a': 'A', 'b': 'B', 'c': 'C'}"), SV("{:#>35}"), input);
+
+  check(SV("{'a': 'A', 'b': 'B', 'c': 'C'}     "), SV("{:{}}"), input, 35);
+  check(SV("{'a': 'A', 'b': 'B', 'c': 'C'}*****"), SV("{:*<{}}"), input, 35);
+  check(SV("__{'a': 'A', 'b': 'B', 'c': 'C'}___"), SV("{:_^{}}"), input, 35);
+  check(SV("#####{'a': 'A', 'b': 'B', 'c': 'C'}"), SV("{:#>{}}"), input, 35);
 
   check_exception("The format-spec range-fill field contains an invalid character", SV("{:}<}"), input);
   check_exception("The format-spec range-fill field contains an invalid character", SV("{:{<}"), input);
   check_exception("The format-spec range-fill field contains an invalid character", SV("{::<}"), input);
 
   // *** sign ***
   check_exception("The format-spec should consume the input or end with a '}'", SV("{:-}"), input);
   check_exception("The format-spec should consume the input or end with a '}'", SV("{:+}"), input);
   check_exception("The format-spec should consume the input or end with a '}'", SV("{: }"), input);
 
   // *** alternate form ***
   check_exception("The format-spec should consume the input or end with a '}'", SV("{:#}"), input);
 
   // *** zero-padding ***
   check_exception("A format-spec width field shouldn't have a leading zero", SV("{:0}"), input);
 
   // *** precision ***
   check_exception("The format-spec should consume the input or end with a '}'", SV("{:.}"), input);
 
   // *** locale-specific form ***
   check_exception("The format-spec should consume the input or end with a '}'", SV("{:L}"), input);
 
   // *** n
-  check(SV("__a: A, b: B, c: C___"), SV("{:_^21n}"), input);
+  check(SV("__'a': 'A', 'b': 'B', 'c': 'C'___"), SV("{:_^33n}"), input);
 
   // *** type ***
-  check(SV("__{a: A, b: B, c: C}___"), SV("{:_^23m}"), input); // the m type does the same as the default.
+  check(SV("__{'a': 'A', 'b': 'B', 'c': 'C'}___"), SV("{:_^35m}"), input); // the m type does the same as the default.
   check_exception("The range-format-spec type s requires formatting a character type", SV("{:s}"), input);
   check_exception("The range-format-spec type ?s requires formatting a character type", SV("{:?s}"), input);
 
   for (std::basic_string_view<CharT> fmt : fmt_invalid_types<CharT>("s"))
     check_exception("The format-spec should consume the input or end with a '}'", fmt, input);
 
   // ***** Only underlying has a format-spec
 

 //
 
 #ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
 template <class TestFunction, class ExceptionTest>
 void test_char_to_wchar(TestFunction check, ExceptionTest check_exception) {
   std::map<char, char> input{{'a', 'A'}, {'c', 'C'}, {'b', 'B'}};
 
   using CharT = wchar_t;
-  check(SV("{a: A, b: B, c: C}"), SV("{}"), input);
+  check(SV("{'a': 'A', 'b': 'B', 'c': 'C'}"), SV("{}"), input);
 
   // ***** underlying has no format-spec
 
   // *** align-fill & width ***
-  check(SV("{a: A, b: B, c: C}     "), SV("{:23}"), input);
-  check(SV("{a: A, b: B, c: C}*****"), SV("{:*<23}"), input);
-  check(SV("__{a: A, b: B, c: C}___"), SV("{:_^23}"), input);
-  check(SV("#####{a: A, b: B, c: C}"), SV("{:#>23}"), input);
-
-  check(SV("{a: A, b: B, c: C}     "), SV("{:{}}"), input, 23);
-  check(SV("{a: A, b: B, c: C}*****"), SV("{:*<{}}"), input, 23);
-  check(SV("__{a: A, b: B, c: C}___"), SV("{:_^{}}"), input, 23);
-  check(SV("#####{a: A, b: B, c: C}"), SV("{:#>{}}"), input, 23);
+  check(SV("{'a': 'A', 'b': 'B', 'c': 'C'}     "), SV("{:35}"), input);
+  check(SV("{'a': 'A', 'b': 'B', 'c': 'C'}*****"), SV("{:*<35}"), input);
+  check(SV("__{'a': 'A', 'b': 'B', 'c': 'C'}___"), SV("{:_^35}"), input);
+  check(SV("#####{'a': 'A', 'b': 'B', 'c': 'C'}"), SV("{:#>35}"), input);
+
+  check(SV("{'a': 'A', 'b': 'B', 'c': 'C'}     "), SV("{:{}}"), input, 35);
+  check(SV("{'a': 'A', 'b': 'B', 'c': 'C'}*****"), SV("{:*<{}}"), input, 35);
+  check(SV("__{'a': 'A', 'b': 'B', 'c': 'C'}___"), SV("{:_^{}}"), input, 35);
+  check(SV("#####{'a': 'A', 'b': 'B', 'c': 'C'}"), SV("{:#>{}}"), input, 35);
 
   check_exception("The format-spec range-fill field contains an invalid character", SV("{:}<}"), input);
   check_exception("The format-spec range-fill field contains an invalid character", SV("{:{<}"), input);
   check_exception("The format-spec range-fill field contains an invalid character", SV("{::<}"), input);
 
   // *** sign ***
   check_exception("The format-spec should consume the input or end with a '}'", SV("{:-}"), input);
   check_exception("The format-spec should consume the input or end with a '}'", SV("{:+}"), input);
   check_exception("The format-spec should consume the input or end with a '}'", SV("{: }"), input);
 
   // *** alternate form ***
   check_exception("The format-spec should consume the input or end with a '}'", SV("{:#}"), input);
 
   // *** zero-padding ***
   check_exception("A format-spec width field shouldn't have a leading zero", SV("{:0}"), input);
 
   // *** precision ***
   check_exception("The format-spec should consume the input or end with a '}'", SV("{:.}"), input);
 
   // *** locale-specific form ***
   check_exception("The format-spec should consume the input or end with a '}'", SV("{:L}"), input);
 
   // *** n
-  check(SV("__a: A, b: B, c: C___"), SV("{:_^21n}"), input);
+  check(SV("__'a': 'A', 'b': 'B', 'c': 'C'___"), SV("{:_^33n}"), input);
 
   // *** type ***
-  check(SV("__{a: A, b: B, c: C}___"), SV("{:_^23m}"), input); // the m type does the same as the default.
+  check(SV("__{'a': 'A', 'b': 'B', 'c': 'C'}___"), SV("{:_^35m}"), input); // the m type does the same as the default.
   check_exception("The range-format-spec type s requires formatting a character type", SV("{:s}"), input);
   check_exception("The range-format-spec type ?s requires formatting a character type", SV("{:?s}"), input);
 
   for (std::basic_string_view<CharT> fmt : fmt_invalid_types<CharT>("s"))
     check_exception("The format-spec should consume the input or end with a '}'", fmt, input);
 
   // ***** Only underlying has a format-spec
   check(SV("{'a': 'A'     , 'b': 'B'     , 'c': 'C'     }"), SV("{::13}"), input);

 // String
 //
 
 template <class CharT, class TestFunction, class ExceptionTest>
 void test_string(TestFunction check, ExceptionTest check_exception) {
   std::map<std::basic_string<CharT>, std::basic_string<CharT>> input{
       {STR("hello"), STR("HELLO")}, {STR("world"), STR("WORLD")}};
 
-  check(SV(R"({hello: HELLO, world: WORLD})"), SV("{}"), input);
+  check(SV(R"({"hello": "HELLO", "world": "WORLD"})"), SV("{}"), input);
 
   // ***** underlying has no format-spec
 
   // *** align-fill & width ***
-  check(SV(R"({hello: HELLO, world: WORLD}     )"), SV("{:33}"), input);
-  check(SV(R"({hello: HELLO, world: WORLD}*****)"), SV("{:*<33}"), input);
-  check(SV(R"(__{hello: HELLO, world: WORLD}___)"), SV("{:_^33}"), input);
-  check(SV(R"(#####{hello: HELLO, world: WORLD})"), SV("{:#>33}"), input);
-
-  check(SV(R"({hello: HELLO, world: WORLD}     )"), SV("{:{}}"), input, 33);
-  check(SV(R"({hello: HELLO, world: WORLD}*****)"), SV("{:*<{}}"), input, 33);
-  check(SV(R"(__{hello: HELLO, world: WORLD}___)"), SV("{:_^{}}"), input, 33);
-  check(SV(R"(#####{hello: HELLO, world: WORLD})"), SV("{:#>{}}"), input, 33);
+  check(SV(R"({"hello": "HELLO", "world": "WORLD"}     )"), SV("{:41}"), input);
+  check(SV(R"({"hello": "HELLO", "world": "WORLD"}*****)"), SV("{:*<41}"), input);
+  check(SV(R"(__{"hello": "HELLO", "world": "WORLD"}___)"), SV("{:_^41}"), input);
+  check(SV(R"(#####{"hello": "HELLO", "world": "WORLD"})"), SV("{:#>41}"), input);
+
+  check(SV(R"({"hello": "HELLO", "world": "WORLD"}     )"), SV("{:{}}"), input, 41);
+  check(SV(R"({"hello": "HELLO", "world": "WORLD"}*****)"), SV("{:*<{}}"), input, 41);
+  check(SV(R"(__{"hello": "HELLO", "world": "WORLD"}___)"), SV("{:_^{}}"), input, 41);
+  check(SV(R"(#####{"hello": "HELLO", "world": "WORLD"})"), SV("{:#>{}}"), input, 41);
 
   check_exception("The format-spec range-fill field contains an invalid character", SV("{:}<}"), input);
   check_exception("The format-spec range-fill field contains an invalid character", SV("{:{<}"), input);
   check_exception("The format-spec range-fill field contains an invalid character", SV("{::<}"), input);
 
   // *** sign ***
   check_exception("The format-spec should consume the input or end with a '}'", SV("{:#}"), input);
 
   // *** alternate form ***
   check_exception("The format-spec should consume the input or end with a '}'", SV("{:#}"), input);
 
   // *** zero-padding ***
   check_exception("A format-spec width field shouldn't have a leading zero", SV("{:0}"), input);
 
   // *** precision ***
   check_exception("The format-spec should consume the input or end with a '}'", SV("{:.}"), input);
 
   // *** locale-specific form ***
   check_exception("The format-spec should consume the input or end with a '}'", SV("{:L}"), input);
 
   // *** n
-  check(SV(R"(__hello: HELLO, world: WORLD___)"), SV("{:_^31n}"), input);
+  check(SV(R"(__"hello": "HELLO", "world": "WORLD"___)"), SV("{:_^39n}"), input);
 
   // *** type ***
-  check(SV(R"(__{hello: HELLO, world: WORLD}___)"), SV("{:_^33m}"), input);
+  check(SV(R"(__{"hello": "HELLO", "world": "WORLD"}___)"), SV("{:_^41m}"), input);
   check_exception("The range-format-spec type s requires formatting a character type", SV("{:s}"), input);
   check_exception("The range-format-spec type ?s requires formatting a character type", SV("{:?s}"), input);
 
   for (std::basic_string_view<CharT> fmt : fmt_invalid_types<CharT>("s"))
     check_exception("The format-spec should consume the input or end with a '}'", fmt, input);
 
   // ***** Only underlying has a format-spec
   check(SV(R"({"hello": "HELLO"     , "world": "WORLD"     })"), SV("{::21}"), input);

libcxx/test/std/utilities/format/format.range/format.range.fmtmap/format.pass.cpp

 // the format.functions test.
 
 #include <cassert>
 #include <concepts>
 #include <format>
 #include <iterator>
 #include <map>
 
+#include "assert_macros.h"
+#include "format.functions.common.h"
 #include "test_format_context.h"
 #include "test_macros.h"
 #include "make_string.h"
 
 #define SV(S) MAKE_STRING_VIEW(CharT, S)
 
 template <class StringViewT>
 void test_format(StringViewT expected, std::map<int, int> arg) {
   using CharT      = typename StringViewT::value_type;
   using String     = std::basic_string<CharT>;
   using OutIt      = std::back_insert_iterator<String>;
   using FormatCtxT = std::basic_format_context<OutIt, CharT>;
 
   std::formatter<std::map<int, int>, CharT> formatter;
 
   String result;
   OutIt out             = std::back_inserter(result);
   FormatCtxT format_ctx = test_format_context_create<OutIt, CharT>(out, std::make_format_args<FormatCtxT>(arg));
   formatter.format(arg, format_ctx);
   assert(result == expected);
 }
 
 template <class CharT>
+void test_assure_parse_is_called(std::basic_string_view<CharT> fmt) {
+  using String     = std::basic_string<CharT>;
+  using OutIt      = std::back_insert_iterator<String>;
+  using FormatCtxT = std::basic_format_context<OutIt, CharT>;
+  std::map<parse_call_validator, parse_call_validator> arg{{parse_call_validator{}, parse_call_validator{}}};
+
+  String result;
+  OutIt out             = std::back_inserter(result);
+  FormatCtxT format_ctx = test_format_context_create<OutIt, CharT>(out, std::make_format_args<FormatCtxT>(arg));
+
+  std::formatter<decltype(arg), CharT> formatter;
+  std::basic_format_parse_context<CharT> ctx{fmt};
+
+  formatter.parse(ctx);
+  formatter.format(arg, format_ctx);
+}
+
+template <class CharT>
+void test_assure_parse_is_called() {
+  using String     = std::basic_string<CharT>;
+  using OutIt      = std::back_insert_iterator<String>;
+  using FormatCtxT = std::basic_format_context<OutIt, CharT>;
+  std::map<parse_call_validator, parse_call_validator> arg{{parse_call_validator{}, parse_call_validator{}}};
+
+  String result;
+  OutIt out             = std::back_inserter(result);
+  FormatCtxT format_ctx = test_format_context_create<OutIt, CharT>(out, std::make_format_args<FormatCtxT>(arg));
+
+  { // parse not called
+    [[maybe_unused]] const std::formatter<decltype(arg), CharT> formatter;
+    TEST_THROWS_TYPE(parse_call_validator::parse_function_not_called, formatter.format(arg, format_ctx));
+  }
+
+  test_assure_parse_is_called(SV("5"));
+  test_assure_parse_is_called(SV("n"));
+  test_assure_parse_is_called(SV("m"));
+  test_assure_parse_is_called(SV("5n"));
+  test_assure_parse_is_called(SV("5m"));
+}
+
+template <class CharT>
 void test_fmt() {
   test_format(SV("{1: 42}"), std::map<int, int>{{1, 42}});
   test_format(SV("{0: 99}"), std::map<int, int>{{0, 99}});
+
+  test_assure_parse_is_called<CharT>();
 }
 
 void test() {
   test_fmt<char>();
 #ifndef TEST_HAS_NO_WIDE_CHARACTERS
   test_fmt<wchar_t>();
 #endif
 }
 
 int main(int, char**) {
   test();
 
   return 0;
 }

libcxx/test/std/utilities/format/format.range/format.range.fmtset/format.functions.format.pass.cpp

 #include "assert_macros.h"
 #include "concat_macros.h"
 
 auto test = []<class CharT, class... Args>(
                 std::basic_string_view<CharT> expected, test_format_string<CharT, Args...> fmt, Args&&... args) {
   std::basic_string<CharT> out = std::format(fmt, std::forward<Args>(args)...);
   TEST_REQUIRE(out == expected,
                TEST_WRITE_CONCATENATED(
-                   "\nFormat string   ", fmt, "\nExpected output ", expected, "\nActual output   ", out, '\n'));
+                   "\nFormat string   ", fmt.get(), "\nExpected output ", expected, "\nActual output   ", out, '\n'));
 };
 
 auto test_exception = []<class CharT, class... Args>(std::string_view, std::basic_string_view<CharT>, Args&&...) {
   // After P2216 most exceptions thrown by std::format become ill-formed.
   // Therefore this tests does nothing.
 };
 
 int main(int, char**) {
   format_tests<char>(test, test_exception);
 
 #ifndef TEST_HAS_NO_WIDE_CHARACTERS
   format_tests<wchar_t>(test, test_exception);
 #endif
 
   return 0;
 }

libcxx/test/std/utilities/format/format.range/format.range.fmtset/format.functions.tests.h

 }
 
 //
 // Pair
 //
 
 template <class CharT, class TestFunction, class ExceptionTest>
 void test_pair_tuple(TestFunction check, ExceptionTest check_exception, auto&& input) {
-  check(SV("{(1, a), (42, *)}"), SV("{}"), input);
+  check(SV("{(1, 'a'), (42, '*')}"), SV("{}"), input);
 
   // ***** underlying has no format-spec
 
   // *** align-fill & width ***
-  check(SV("{(1, a), (42, *)}     "), SV("{:22}"), input);
-  check(SV("{(1, a), (42, *)}*****"), SV("{:*<22}"), input);
-  check(SV("__{(1, a), (42, *)}___"), SV("{:_^22}"), input);
-  check(SV("#####{(1, a), (42, *)}"), SV("{:#>22}"), input);
-
-  check(SV("{(1, a), (42, *)}     "), SV("{:{}}"), input, 22);
-  check(SV("{(1, a), (42, *)}*****"), SV("{:*<{}}"), input, 22);
-  check(SV("__{(1, a), (42, *)}___"), SV("{:_^{}}"), input, 22);
-  check(SV("#####{(1, a), (42, *)}"), SV("{:#>{}}"), input, 22);
+  check(SV("{(1, 'a'), (42, '*')}     "), SV("{:26}"), input);
+  check(SV("{(1, 'a'), (42, '*')}*****"), SV("{:*<26}"), input);
+  check(SV("__{(1, 'a'), (42, '*')}___"), SV("{:_^26}"), input);
+  check(SV("#####{(1, 'a'), (42, '*')}"), SV("{:#>26}"), input);
+
+  check(SV("{(1, 'a'), (42, '*')}     "), SV("{:{}}"), input, 26);
+  check(SV("{(1, 'a'), (42, '*')}*****"), SV("{:*<{}}"), input, 26);
+  check(SV("__{(1, 'a'), (42, '*')}___"), SV("{:_^{}}"), input, 26);
+  check(SV("#####{(1, 'a'), (42, '*')}"), SV("{:#>{}}"), input, 26);
 
   check_exception("The format-spec range-fill field contains an invalid character", SV("{:}<}"), input);
   check_exception("The format-spec range-fill field contains an invalid character", SV("{:{<}"), input);
   check_exception("The format-spec range-fill field contains an invalid character", SV("{::<}"), input);
 
   // *** sign ***
   check_exception("The format-spec should consume the input or end with a '}'", SV("{:-}"), input);
   check_exception("The format-spec should consume the input or end with a '}'", SV("{:+}"), input);
   check_exception("The format-spec should consume the input or end with a '}'", SV("{: }"), input);
 
   // *** alternate form ***
   check_exception("The format-spec should consume the input or end with a '}'", SV("{:#}"), input);
 
   // *** zero-padding ***
   check_exception("A format-spec width field shouldn't have a leading zero", SV("{:0}"), input);
 
   // *** precision ***
   check_exception("The format-spec should consume the input or end with a '}'", SV("{:.}"), input);
 
   // *** locale-specific form ***
   check_exception("The format-spec should consume the input or end with a '}'", SV("{:L}"), input);
 
   // *** n
-  check(SV("__(1, a), (42, *)___"), SV("{:_^20n}"), input);
-  check(SV("__(1, a), (42, *)___"), SV("{:_^20nm}"), input); // m should have no effect
+  check(SV("__(1, 'a'), (42, '*')___"), SV("{:_^24n}"), input);
+  check(SV("__(1, 'a'), (42, '*')___"), SV("{:_^24nm}"), input); // m should have no effect
 
   // *** type ***
-  check(SV("__{(1, a), (42, *)}___"), SV("{:_^22m}"), input);
+  check(SV("__{(1, 'a'), (42, '*')}___"), SV("{:_^26m}"), input);
   check_exception("The range-format-spec type s requires formatting a character type", SV("{:s}"), input);
   check_exception("The range-format-spec type ?s requires formatting a character type", SV("{:?s}"), input);
 
   for (std::basic_string_view<CharT> fmt : fmt_invalid_types<CharT>("s"))
     check_exception("The format-spec should consume the input or end with a '}'", fmt, input);
 
   // ***** Only underlying has a format-spec
   check(SV("{(1, 'a')   , (42, '*')  }"), SV("{::11}"), input);

libcxx/test/std/utilities/format/format.range/format.range.fmtset/format.pass.cpp

 // the format.functions test.
 
 #include <cassert>
 #include <concepts>
 #include <iterator>
 #include <format>
 #include <set>
 
+#include "assert_macros.h"
+#include "format.functions.common.h"
 #include "test_format_context.h"
 #include "test_macros.h"
 #include "make_string.h"
 
 #define SV(S) MAKE_STRING_VIEW(CharT, S)
 
 template <class StringViewT>
 void test_format(StringViewT expected, std::set<int> arg) {
   using CharT      = typename StringViewT::value_type;
   using String     = std::basic_string<CharT>;
   using OutIt      = std::back_insert_iterator<String>;
   using FormatCtxT = std::basic_format_context<OutIt, CharT>;
 
   std::formatter<std::set<int>, CharT> formatter;
 
   String result;
   OutIt out             = std::back_inserter(result);
   FormatCtxT format_ctx = test_format_context_create<OutIt, CharT>(out, std::make_format_args<FormatCtxT>(arg));
   formatter.format(arg, format_ctx);
   assert(result == expected);
 }
 
 template <class CharT>
+void test_assure_parse_is_called(std::basic_string_view<CharT> fmt) {
+  using String     = std::basic_string<CharT>;
+  using OutIt      = std::back_insert_iterator<String>;
+  using FormatCtxT = std::basic_format_context<OutIt, CharT>;
+  std::set<parse_call_validator> arg{parse_call_validator{}};
+
+  String result;
+  OutIt out             = std::back_inserter(result);
+  FormatCtxT format_ctx = test_format_context_create<OutIt, CharT>(out, std::make_format_args<FormatCtxT>(arg));
+
+  std::formatter<decltype(arg), CharT> formatter;
+  std::basic_format_parse_context<CharT> ctx{fmt};
+
+  formatter.parse(ctx);
+  formatter.format(arg, format_ctx);
+}
+
+template <class CharT>
+void test_assure_parse_is_called() {
+  using String     = std::basic_string<CharT>;
+  using OutIt      = std::back_insert_iterator<String>;
+  using FormatCtxT = std::basic_format_context<OutIt, CharT>;
+  std::set<parse_call_validator> arg{parse_call_validator{}};
+
+  String result;
+  OutIt out             = std::back_inserter(result);
+  FormatCtxT format_ctx = test_format_context_create<OutIt, CharT>(out, std::make_format_args<FormatCtxT>(arg));
+
+  { // parse not called
+    [[maybe_unused]] const std::formatter<decltype(arg), CharT> formatter;
+    TEST_THROWS_TYPE(parse_call_validator::parse_function_not_called, formatter.format(arg, format_ctx));
+  }
+
+  test_assure_parse_is_called(SV("5"));
+  test_assure_parse_is_called(SV("n"));
+  test_assure_parse_is_called(SV("5n"));
+}
+
+template <class CharT>
 void test_fmt() {
   test_format(SV("{42}"), std::set<int>{42});
+
+  test_assure_parse_is_called<CharT>();
 }
 
 void test() {
   test_fmt<char>();
 #ifndef TEST_HAS_NO_WIDE_CHARACTERS
   test_fmt<wchar_t>();
 #endif
 }
 
 int main(int, char**) {
   test();
 
   return 0;
 }

libcxx/test/std/utilities/format/format.range/format.range.formatter/format.functions.format.pass.cpp

 #include "assert_macros.h"
 #include "concat_macros.h"
 
 auto test = []<class CharT, class... Args>(
                 std::basic_string_view<CharT> expected, test_format_string<CharT, Args...> fmt, Args&&... args) {
   std::basic_string<CharT> out = std::format(fmt, std::forward<Args>(args)...);
   TEST_REQUIRE(out == expected,
                TEST_WRITE_CONCATENATED(
-                   "\nFormat string   ", fmt, "\nExpected output ", expected, "\nActual output   ", out, '\n'));
+                   "\nFormat string   ", fmt.get(), "\nExpected output ", expected, "\nActual output   ", out, '\n'));
 };
 
 auto test_exception = []<class CharT, class... Args>(std::string_view, std::basic_string_view<CharT>, Args&&...) {
   // After P2216 most exceptions thrown by std::format become ill-formed.
   // Therefore this tests does nothing.
 };
 
 int main(int, char**) {
   format_tests<char>(test, test_exception);
 
 #ifndef TEST_HAS_NO_WIDE_CHARACTERS
   format_tests<wchar_t>(test, test_exception);
 #endif
 
   return 0;
 }

libcxx/test/std/utilities/format/format.range/format.range.formatter/format.functions.tests.h

void test_pair_tuple(TestFunction check, ExceptionTest check_exception, auto&& input) {
   //   constexpr typename ParseContext::iterator
   //    parse(ParseContext& ctx);
   // [format.tuple]/7
   //   ... if e.set_debug_format() is a valid expression, calls
   //   e.set_debug_format().
   // So when there is no range-underlying-spec, there is no need to call parse
   // thus the char element is not escaped.
   // TODO FMT P2733 addresses this issue.
-  check(SV("[(1, a), (42, *)]"), SV("{}"), input);
+  check(SV("[(1, 'a'), (42, '*')]"), SV("{}"), input);
 
   // ***** underlying has no format-spec
 
   // *** align-fill & width ***
-  check(SV("[(1, a), (42, *)]     "), SV("{:22}"), input);
-  check(SV("[(1, a), (42, *)]*****"), SV("{:*<22}"), input);
-  check(SV("__[(1, a), (42, *)]___"), SV("{:_^22}"), input);
-  check(SV("#####[(1, a), (42, *)]"), SV("{:#>22}"), input);
-
-  check(SV("[(1, a), (42, *)]     "), SV("{:{}}"), input, 22);
-  check(SV("[(1, a), (42, *)]*****"), SV("{:*<{}}"), input, 22);
-  check(SV("__[(1, a), (42, *)]___"), SV("{:_^{}}"), input, 22);
-  check(SV("#####[(1, a), (42, *)]"), SV("{:#>{}}"), input, 22);
+  check(SV("[(1, 'a'), (42, '*')]     "), SV("{:26}"), input);
+  check(SV("[(1, 'a'), (42, '*')]*****"), SV("{:*<26}"), input);
+  check(SV("__[(1, 'a'), (42, '*')]___"), SV("{:_^26}"), input);
+  check(SV("#####[(1, 'a'), (42, '*')]"), SV("{:#>26}"), input);
+
+  check(SV("[(1, 'a'), (42, '*')]     "), SV("{:{}}"), input, 26);
+  check(SV("[(1, 'a'), (42, '*')]*****"), SV("{:*<{}}"), input, 26);
+  check(SV("__[(1, 'a'), (42, '*')]___"), SV("{:_^{}}"), input, 26);
+  check(SV("#####[(1, 'a'), (42, '*')]"), SV("{:#>{}}"), input, 26);
 
   check_exception("The format-spec range-fill field contains an invalid character", SV("{:}<}"), input);
   check_exception("The format-spec range-fill field contains an invalid character", SV("{:{<}"), input);
   check_exception("The format-spec range-fill field contains an invalid character", SV("{::<}"), input);
 
   // *** sign ***
   check_exception("The format-spec should consume the input or end with a '}'", SV("{:-}"), input);
   check_exception("The format-spec should consume the input or end with a '}'", SV("{:+}"), input);
   check_exception("The format-spec should consume the input or end with a '}'", SV("{: }"), input);
 
   // *** alternate form ***
   check_exception("The format-spec should consume the input or end with a '}'", SV("{:#}"), input);
 
   // *** zero-padding ***
   check_exception("A format-spec width field shouldn't have a leading zero", SV("{:0}"), input);
 
   // *** precision ***
   check_exception("The format-spec should consume the input or end with a '}'", SV("{:.}"), input);
 
   // *** locale-specific form ***
   check_exception("The format-spec should consume the input or end with a '}'", SV("{:L}"), input);
 
   // *** n
-  check(SV("__(1, a), (42, *)___"), SV("{:_^20n}"), input);
-  check(SV("__(1, a), (42, *)___"), SV("{:_^20nm}"), input); // m should have no effect
+  check(SV("__(1, 'a'), (42, '*')___"), SV("{:_^24n}"), input);
+  check(SV("__(1, 'a'), (42, '*')___"), SV("{:_^24nm}"), input); // m should have no effect
 
   // *** type ***
-  check(SV("__{(1, a), (42, *)}___"), SV("{:_^22m}"), input);
+  check(SV("__{(1, 'a'), (42, '*')}___"), SV("{:_^26m}"), input);
   check_exception("The range-format-spec type s requires formatting a character type", SV("{:s}"), input);
   check_exception("The range-format-spec type ?s requires formatting a character type", SV("{:?s}"), input);
   for (std::basic_string_view<CharT> fmt : fmt_invalid_types<CharT>("s"))
     check_exception("The format-spec should consume the input or end with a '}'", fmt, input);
 
   // ***** Only underlying has a format-spec
   check(SV("[(1, 'a')   , (42, '*')  ]"), SV("{::11}"), input);
   check(SV("[(1, 'a')***, (42, '*')**]"), SV("{::*<11}"), input);

libcxx/test/std/utilities/format/format.range/format.range.formatter/format.pass.cpp

 // the format functions test.
 
 #include <cassert>
 #include <concepts>
 #include <iterator>
 #include <format>
 #include <vector>
 
+#include "assert_macros.h"
+#include "format.functions.common.h"
+#include "make_string.h"
 #include "test_format_context.h"
 #include "test_macros.h"
-#include "make_string.h"
 
 #define SV(S) MAKE_STRING_VIEW(CharT, S)
 
 template <class StringViewT>
 void test_format(StringViewT expected, std::vector<int> arg) {
   using CharT      = typename StringViewT::value_type;
   using String     = std::basic_string<CharT>;
   using OutIt      = std::back_insert_iterator<String>;
   using FormatCtxT = std::basic_format_context<OutIt, CharT>;
 
   const std::range_formatter<int, CharT> formatter;
 
   String result;
   OutIt out             = std::back_inserter(result);
   FormatCtxT format_ctx = test_format_context_create<OutIt, CharT>(out, std::make_format_args<FormatCtxT>(arg));
   formatter.format(arg, format_ctx);
   assert(result == expected);
 }
 
 template <class CharT>
+void test_assure_parse_is_called(std::basic_string_view<CharT> fmt) {
+  using String     = std::basic_string<CharT>;
+  using OutIt      = std::back_insert_iterator<String>;
+  using FormatCtxT = std::basic_format_context<OutIt, CharT>;
+  std::vector<parse_call_validator> arg{1};
+
+  String result;
+  OutIt out             = std::back_inserter(result);
+  FormatCtxT format_ctx = test_format_context_create<OutIt, CharT>(out, std::make_format_args<FormatCtxT>(arg));
+
+  std::range_formatter<parse_call_validator, CharT> formatter;
+  std::basic_format_parse_context<CharT> ctx{fmt};
+
+  formatter.parse(ctx);
+  formatter.format(arg, format_ctx);
+}
+
+template <class CharT>
+void test_assure_parse_is_called() {
+  using String     = std::basic_string<CharT>;
+  using OutIt      = std::back_insert_iterator<String>;
+  using FormatCtxT = std::basic_format_context<OutIt, CharT>;
+  std::vector<parse_call_validator> arg{1};
+
+  String result;
+  OutIt out             = std::back_inserter(result);
+  FormatCtxT format_ctx = test_format_context_create<OutIt, CharT>(out, std::make_format_args<FormatCtxT>(arg));
+
+  { // parse not called
+    [[maybe_unused]] const std::range_formatter<parse_call_validator, CharT> formatter;
+    TEST_THROWS_TYPE(parse_call_validator::parse_function_not_called, formatter.format(arg, format_ctx));
+  }
+
+  // The range-format-spec has no range-underlying-spec
+  // This uses various variants, which have different code paths in libc++
+  test_assure_parse_is_called(SV("5"));
+  test_assure_parse_is_called(SV("n"));
+  test_assure_parse_is_called(SV(":"));
+  test_assure_parse_is_called(SV("5"));
+  test_assure_parse_is_called(SV("5n"));
+  test_assure_parse_is_called(SV("5n:"));
+}
+
+template <class CharT>
 void test_fmt() {
   test_format(SV("[1]"), std::vector<int>{1});
   test_format(SV("[0]"), std::vector<int>{0});
+
+  test_assure_parse_is_called<CharT>();
 }
 
 void test() {
   test_fmt<char>();
 #ifndef TEST_HAS_NO_WIDE_CHARACTERS
   test_fmt<wchar_t>();
 #endif
 }
 
 int main(int, char**) {
   test();
 
   return 0;
 }

libcxx/test/std/utilities/format/format.tuple/format.functions.format.pass.cpp

 #include "assert_macros.h"
 #include "concat_macros.h"
 
 auto test = []<class CharT, class... Args>(
                 std::basic_string_view<CharT> expected, test_format_string<CharT, Args...> fmt, Args&&... args) {
   std::basic_string<CharT> out = std::format(fmt, std::forward<Args>(args)...);
   TEST_REQUIRE(out == expected,
                TEST_WRITE_CONCATENATED(
-                   "\nFormat string   ", fmt, "\nExpected output ", expected, "\nActual output   ", out, '\n'));
+                   "\nFormat string   ", fmt.get(), "\nExpected output ", expected, "\nActual output   ", out, '\n'));
 };
 
 auto test_exception = []<class CharT, class... Args>(std::string_view, std::basic_string_view<CharT>, Args&&...) {
   // After P2216 most exceptions thrown by std::format become ill-formed.
   // Therefore this tests does nothing.
   // A basic ill-formed test is done in format.verify.cpp
   // The exceptions are tested by other functions that don't use the basic-format-string as fmt argument.
 };

libcxx/test/std/utilities/format/format.tuple/format.functions.tests.h

void test_nested(TestFunction check, ExceptionTest check_exception, Nested&& input) {
   //   public, constexpr, non-static member function set_debug_format()
   //   which modifies the state of the formatter to be as if the type of the
   //   std-format-spec parsed by the last call to parse were ?.
   // pair and tuple are not debug-enabled specializations to the
   // set_debug_format is not propagated. The paper
   //   P2733 Fix handling of empty specifiers in std::format
   // addressed this.
 
-  check(SV("(42, (hello, red))"), SV("{}"), input);
+  check(SV("(42, (\"hello\", \"red\"))"), SV("{}"), input);
 
   // *** align-fill & width ***
-  check(SV("(42, (hello, red))     "), SV("{:23}"), input);
-  check(SV("(42, (hello, red))*****"), SV("{:*<23}"), input);
-  check(SV("__(42, (hello, red))___"), SV("{:_^23}"), input);
-  check(SV("#####(42, (hello, red))"), SV("{:#>23}"), input);
-
-  check(SV("(42, (hello, red))     "), SV("{:{}}"), input, 23);
-  check(SV("(42, (hello, red))*****"), SV("{:*<{}}"), input, 23);
-  check(SV("__(42, (hello, red))___"), SV("{:_^{}}"), input, 23);
-  check(SV("#####(42, (hello, red))"), SV("{:#>{}}"), input, 23);
+  check(SV("(42, (\"hello\", \"red\"))     "), SV("{:27}"), input);
+  check(SV("(42, (\"hello\", \"red\"))*****"), SV("{:*<27}"), input);
+  check(SV("__(42, (\"hello\", \"red\"))___"), SV("{:_^27}"), input);
+  check(SV("#####(42, (\"hello\", \"red\"))"), SV("{:#>27}"), input);
+
+  check(SV("(42, (\"hello\", \"red\"))     "), SV("{:{}}"), input, 27);
+  check(SV("(42, (\"hello\", \"red\"))*****"), SV("{:*<{}}"), input, 27);
+  check(SV("__(42, (\"hello\", \"red\"))___"), SV("{:_^{}}"), input, 27);
+  check(SV("#####(42, (\"hello\", \"red\"))"), SV("{:#>{}}"), input, 27);
 
   check_exception("The format-spec range-fill field contains an invalid character", SV("{:}<}"), input);
   check_exception("The format-spec range-fill field contains an invalid character", SV("{:{<}"), input);
   check_exception("The format-spec range-fill field contains an invalid character", SV("{::<}"), input);
 
   // *** sign ***
   check_exception("The format-spec should consume the input or end with a '}'", SV("{:-}"), input);
   check_exception("The format-spec should consume the input or end with a '}'", SV("{:+}"), input);
   check_exception("The format-spec should consume the input or end with a '}'", SV("{: }"), input);
 
   // *** alternate form ***
   check_exception("The format-spec should consume the input or end with a '}'", SV("{:#}"), input);
 
   // *** zero-padding ***
   check_exception("A format-spec width field shouldn't have a leading zero", SV("{:0}"), input);
 
   // *** precision ***
   check_exception("The format-spec should consume the input or end with a '}'", SV("{:.}"), input);
 
   // *** locale-specific form ***
   check_exception("The format-spec should consume the input or end with a '}'", SV("{:L}"), input);
 
   // *** type ***
-  check(SV("__42: (hello, red)___"), SV("{:_^21m}"), input);
-  check(SV("__42, (hello, red)___"), SV("{:_^21n}"), input);
+  check(SV("__42: (\"hello\", \"red\")___"), SV("{:_^25m}"), input);
+  check(SV("__42, (\"hello\", \"red\")___"), SV("{:_^25n}"), input);
 
   for (CharT c : SV("aAbBcdeEfFgGopsxX?")) {
     check_exception("The format-spec should consume the input or end with a '}'",
                     std::basic_string_view{STR("{:") + c + STR("}")},
                     input);
   }
 }
 

libcxx/test/std/utilities/format/format.tuple/format.pass.cpp

 
 #include <cassert>
 #include <concepts>
 #include <format>
 #include <iterator>
 #include <tuple>
 #include <utility>
 
+#include "assert_macros.h"
+#include "format.functions.common.h"
+#include "make_string.h"
 #include "test_format_context.h"
 #include "test_macros.h"
-#include "make_string.h"
 
 #define SV(S) MAKE_STRING_VIEW(CharT, S)
 
 template <class StringViewT, class Arg>
 void test(StringViewT expected, Arg arg) {
   using CharT      = typename StringViewT::value_type;
   using String     = std::basic_string<CharT>;
   using OutIt      = std::back_insert_iterator<String>;
   using FormatCtxT = std::basic_format_context<OutIt, CharT>;
 
   const std::formatter<Arg, CharT> formatter;
 
   String result;
   OutIt out             = std::back_inserter(result);
   FormatCtxT format_ctx = test_format_context_create<OutIt, CharT>(out, std::make_format_args<FormatCtxT>(arg));
   formatter.format(arg, format_ctx);
   assert(result == expected);
 }
 
 template <class CharT>
+void test_assure_parse_is_called(std::basic_string_view<CharT> fmt) {
+  using String     = std::basic_string<CharT>;
+  using OutIt      = std::back_insert_iterator<String>;
+  using FormatCtxT = std::basic_format_context<OutIt, CharT>;
+  std::pair<parse_call_validator, parse_call_validator> arg;
+
+  String result;
+  OutIt out             = std::back_inserter(result);
+  FormatCtxT format_ctx = test_format_context_create<OutIt, CharT>(out, std::make_format_args<FormatCtxT>(arg));
+
+  std::formatter<decltype(arg), CharT> formatter;
+  std::basic_format_parse_context<CharT> ctx{fmt};
+
+  formatter.parse(ctx);
+  formatter.format(arg, format_ctx);
+}
+
+template <class CharT>
+void test_assure_parse_is_called() {
+  using String     = std::basic_string<CharT>;
+  using OutIt      = std::back_insert_iterator<String>;
+  using FormatCtxT = std::basic_format_context<OutIt, CharT>;
+  std::pair<parse_call_validator, parse_call_validator> arg;
+
+  String result;
+  OutIt out             = std::back_inserter(result);
+  FormatCtxT format_ctx = test_format_context_create<OutIt, CharT>(out, std::make_format_args<FormatCtxT>(arg));
+
+  { // parse not called
+    [[maybe_unused]] const std::formatter<decltype(arg), CharT> formatter;
+    TEST_THROWS_TYPE(parse_call_validator::parse_function_not_called, formatter.format(arg, format_ctx));
+  }
+
+  test_assure_parse_is_called(SV("5"));
+  test_assure_parse_is_called(SV("n"));
+  test_assure_parse_is_called(SV("m"));
+  test_assure_parse_is_called(SV("5n"));
+  test_assure_parse_is_called(SV("5m"));
+}
+
+template <class CharT>
 void test() {
   test(SV("(1)"), std::tuple<int>{1});
   test(SV("(1, 1)"), std::tuple<int, CharT>{1, CharT('1')});
   test(SV("(1, 1)"), std::pair<int, CharT>{1, CharT('1')});
   test(SV("(1, 1, 1)"), std::tuple<int, CharT, double>{1, CharT('1'), 1.0});
+
+  test_assure_parse_is_called<CharT>();
 }
 
 void test() {
   test<char>();
 #ifndef TEST_HAS_NO_WIDE_CHARACTERS
   test<wchar_t>();
 #endif
 }
 
 int main(int, char**) {
   test();
 
   return 0;
 }

libcxx/test/support/format.functions.common.h

 using context_t = typename context<T>::type;
 
 // A user-defined type used to test the handle formatter.
 enum class status : uint16_t { foo = 0xAAAA, bar = 0x5555, foobar = 0xAA55 };
 
 // The formatter for a user-defined type used to test the handle formatter.
 template <class CharT>
 struct std::formatter<status, CharT> {
-  int type = 0;
+  // During the 2023 Issaquah meeting LWEG made it clear a formatter is
+  // required to call its parse function. LWG3892 Adds the wording for that
+  // requirement. Therefore this formatter is initialized in an invalid state.
+  // A call to parse sets it in a valid state and a call to format validates
+  // the state.
+  int type = -1;
 
   constexpr auto parse(basic_format_parse_context<CharT>& parse_ctx) -> decltype(parse_ctx.begin()) {
     auto begin = parse_ctx.begin();
-    auto end = parse_ctx.end();
+    auto end   = parse_ctx.end();
+    type       = 0;
     if (begin == end)
       return begin;
 
     switch (*begin) {
     case CharT('x'):
       break;
     case CharT('X'):
       type = 1;
struct std::formatter<status, CharT> {
 
   template <class Out>
   auto format(status s, basic_format_context<Out, CharT>& ctx) const -> decltype(ctx.out()) {
     const char* names[] = {"foo", "bar", "foobar"};
     char buffer[7];
     const char* begin = names[0];
     const char* end = names[0];
     switch (type) {
+    case -1:
+      throw_format_error("The formatter's parse function has not been called.");
+
     case 0:
       begin = buffer;
       buffer[0] = '0';
       buffer[1] = 'x';
       end = std::to_chars(&buffer[2], std::end(buffer), static_cast<uint16_t>(s), 16).ptr;
       buffer[6] = '\0';
       break;
 
case 2:
       end = begin + strlen(begin);
       break;
     }
 
     return std::copy(begin, end, ctx.out());
   }
 
 private:
-  void throw_format_error(const char* s) {
+  void throw_format_error(const char* s) const {
 #ifndef TEST_HAS_NO_EXCEPTIONS
     throw std::format_error(s);
 #else
     (void)s;
     std::abort();
 #endif
   }
 };
 
+struct parse_call_validator {
+  struct parse_function_not_called {};
+
+  friend constexpr auto operator<=>(const parse_call_validator& lhs, const parse_call_validator& rhs) {
+    return &lhs <=> &rhs;
+  }
+};
+
+// The formatter for a user-defined type used to test the handle formatter.
+//
+// Like std::formatter<status, CharT> this formatter validates that parse is
+// called. This formatter is intended to be used when the formatter's parse is
+// called directly and not with format. In that case the format-spec does not
+// require a terminating }. The tests must be written in a fashion where this
+// formatter is always called with an empty format-spec. This requirement
+// allows testing of certain code paths that are never reached by using a
+// well-formed format-string in the format functions.
+template <class CharT>
+struct std::formatter<parse_call_validator, CharT> {
+  bool parse_called{false};
+
+  constexpr auto parse(basic_format_parse_context<CharT>& parse_ctx) -> decltype(parse_ctx.begin()) {
+    auto begin = parse_ctx.begin();
+    auto end   = parse_ctx.end();
+    assert(begin == end);
+    parse_called = true;
+    return begin;
+  }
+
+  auto format(parse_call_validator, auto& ctx) const -> decltype(ctx.out()) {
+    if (!parse_called)
+      throw_error<parse_call_validator::parse_function_not_called>();
+    return ctx.out();
+  }
+
+private:
+  template <class T>
+  [[noreturn]] void throw_error() const {
+#ifndef TEST_HAS_NO_EXCEPTIONS
+    throw T{};
+#else
+    std::abort();
+#endif
+  }
+};
+
 // Creates format string for the invalid types.
 //
 // valid contains a list of types that are valid.
 // - The type ?s is the only type requiring 2 characters, use S for that type.
 // - Whether n is a type or not depends on the context, is is always used.
 //
 // The return value is a collection of basic_strings, instead of
 // basic_string_views since the values are temporaries.