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

From cppreference.com

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