[fs.path.fmtr.funcs]

Formatting of paths uses formatting specifiers of the form

{o p t width {o p t ? {o p t g {o p t}_{o p t}}_{o p t g {o p t}_{o p t}}}_{o p t ? {o p t g {o p t}_{o p t}}_{o p t g {o p t}_{o p t}}}}_{o p t width {o p t ? {o p t g {o p t}_{o p t}}_{o p t g {o p t}_{o p t}}}_{o p t ? {o p t g {o p t}_{o p t}}_{o p t g {o p t}_{o p t}}}}

where the productions fill-and-align and width are described in [format.string].

If the ? option is used then the path is formatted as an escaped string ([format.string.escaped]).

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constexpr void set_debug_format();

Effects: Modifies the state of the formatter to be as if the path-format-spec parsed by the last call to parse contained the ? option.

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constexpr typename basic_format_parse_context<charT>::iterator
  parse(basic_format_parse_context<charT>& ctx);

Effects: Parses the format specifier as a path-format-spec and stores the parsed specifiers in *this.

Returns: An iterator past the end of the path-format-spec.

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template<class FormatContext>
  typename FormatContext::iterator
    format(const filesystem::path& p, FormatContext& ctx) const;

Effects: Let s be p.generic_string<filesystem::path::value_type>() if the g option is used, otherwise p.native().

Writes s into ctx.out(), adjusted according to the path-format-spec.

If charT is char, path::value_type is wchar_t, and the literal encoding is UTF-8, then the escaped path is transcoded from the native encoding for wide character strings to UTF-8 with maximal subparts of ill-formed subsequences substituted with U+fffd replacement character per the Unicode Standard, Chapter 3.9 U+fffd Substitution in Conversion.

If charT and path::value_type are the same then no transcoding is performed.

Otherwise, transcoding is implementation-defined.

Returns: An iterator past the end of the output range.