C++ named requirements: Formatter (since C++20)

Formatter is a type that abstracts formatting operations for a given formatting argument type and character type. Specializations of std::formatter provided by the standard library are required to meet the requirements of Formatter except as noted otherwise.

A Formatter is able to format both const and non-const arguments, typically by providing a format member function that takes a const reference.

Requirements

A type satisfies Formatter if it satisfies BasicFormatter and given the following types and values, the expressions shown in the table below are valid and have the indicated semantics:

Type	Definition
CharT	a character type
Arg	a formatting argument type
Formatter	a Formatter type for types Arg and CharT
OutputIt	a LegacyOutputIterator type
ParseCtx	std::basic_format_parse_context<CharT>
FmtCtx	std::basic_format_context<OutputIt,CharT>
Value	Definition
f	a value of type (possibly const-qualified) Formatter
arg	an lvalue of type Arg
t	a value of type convertible to (possibly const-qualified) Arg
parse_ctx	an lvalue of type ParseCtx satisfying all following conditions: parse_ctx.begin() points to the beginning of the format-spec of the replacement field being formatted in the format string. If format-spec is not present or empty, then either parse_ctx.begin() == parse_ctx.end() or *parse_ctx.begin() == '}'.
fmt_ctx	an lvalue of type FmtCtx

Expression	Return type	Semantics
f.format(t, fmt_ctx)	FmtCtx::iterator	Formats t according to the specifiers stored in f, writes the output to fmt_ctx.out() and returns an end iterator of the output range. The output can only depend on t, fmt_ctx.locale(), the range [parse_ctx.begin(), parse_ctx.end()) from the last call to f.parse(parse_ctx), and fmt_ctx.arg(n) for any value n of type std::size_t.
f.format(arg, fmt_ctx)	FmtCtx::iterator	As above, but does not modify arg.

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 3892	C++20	the value of pc.begin() was unclear if format-spec is not present	made clear