https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66464
--- Comment #2 from Leo Carreon <lcarreon at bigpond dot net.au> --- Just clarifying that my comments are to do with codecvt_utf16<char32_t> and codecvt_utf8<char32_t>. The way I understand it, codecvt_utf16<char32_t> should be converting between UTF-16 and UCS-4. UTF-16 uses 2 bytes for characters in the BMP (characters in the range 0x0000 to 0xFFFF) and 4 bytes (surrogate pairs) for characters above the BMP (0x010000 to 0x10FFFF). UCS-4 uses 4 byte values. Therefore, codecvt_utf16<char32_t>::max_length() should be returning 4 if the BOM is not taken into account. codecvt_utf8<char32_t> converts between UTF-8 and UCS-4. UTF-8 can use up to 4 bytes for characters up to the range 0x10FFFF. Therefore, codecvt_utf8<char32_t>::max_length() should be returning 4 if the BOM is not taken into account. As I said in my previous post, I'm not sure if the BOM should be accounted for in max_length(). If I'm not mistaken, the purpose of this function is to allow a user to estimate how many bytes are required to fit a UCS-4 string when converted to either UTF-16 or UTF-8. And my guess, the BOM can be taken into account separately when doing the estimation. For example, when wstring_convert estimates the length of the std::string to be generated by wstring_convert::to_bytes(). It should be the number of UCS-4 characters multiplied by max_length() and then add the size of the BOM if required. The resulting std::string can be resized after the conversion to eliminate the unused bytes. Note that the comment you mentioned in your reply probably only applies to codecvt_utf8_utf16 which converts between UTF-8 and UTF-16 directly without going thru the UCS-4 conversion.
