Method parse_hex_float

src/toml.hpp:14077–14249

		TOML_NODISCARD
		TOML_NEVER_INLINE
		double parse_hex_float()
		{
			return_if_error({});
			assert_not_eof();
			TOML_ASSERT_ASSUME(is_match(*cp, U'0', U'+', U'-'));
			push_parse_scope("hexadecimal floating-point"sv);

#if TOML_LANG_UNRELEASED // toml/issues/562 (hexfloats)

			// sign
			const int sign = *cp == U'-' ? -1 : 1;
			if (is_match(*cp, U'+', U'-'))
				advance_and_return_if_error_or_eof({});

			// '0'
			if (*cp != U'0')
				set_error_and_return_default(" expected '0', saw '"sv, to_sv(*cp), "'"sv);
			advance_and_return_if_error_or_eof({});

			// 'x' or 'X'
			if (!is_match(*cp, U'x', U'X'))
				set_error_and_return_default("expected 'x' or 'X', saw '"sv, to_sv(*cp), "'"sv);
			advance_and_return_if_error_or_eof({});

			// <HEX DIGITS> ([.]<HEX DIGITS>)? [pP] [+-]? <DEC DIGITS>

			// consume value fragments
			struct fragment
			{
				char chars[24];
				size_t length;
				double value;
			};
			fragment fragments[] = {
				{}, // mantissa, whole part
				{}, // mantissa, fractional part
				{}	// exponent
			};
			fragment* current_fragment = fragments;
			const utf8_codepoint* prev = {};
			int exponent_sign		   = 1;
			while (!is_eof() && !is_value_terminator(*cp))
			{
				if (*cp == U'_')
				{
					if (!prev || !is_hexadecimal_digit(*prev))
						set_error_and_return_default("underscores may only follow digits"sv);

					prev = cp;
					advance_and_return_if_error_or_eof({});
					continue;
				}
				else if (prev && *prev == U'_' && !is_hexadecimal_digit(*cp))
					set_error_and_return_default("underscores must be followed by digits"sv);
				else if (*cp == U'.')
				{
					// 0x10.0p-.0 (exponent cannot have '.')
					if (current_fragment == fragments + 2)