OK good, I'd like that to work if possible, it seems like something users would want and/or expect?
What I can't figure out, though, is if a floating-point type actually *should* round-trip back from a string in scientific format with (max_digits10 + 1) precision. Remember, that kind of string has (max_digits10 + 1) after the decimal point plus one digit in the mantissa. That would be like asking 8-byte double to be exact when round-tripping with 17 decimal digits of precision since the scientific notation would have 1+16 decimal digits.
I'm not sure I follow you - what the std says is: static constexpr int digits10; 11 Number of base 10 digits that can be represented without change. 12 Meaningful for all specializations in which is_bounded != false. static constexpr int max_digits10; 13 Number of base 10 digits required to ensure that values which differ are always differentiated. Which I take as meaning that if you print at least max_digits10 digits then you should always be able to get back to the same unique value. In other words we get to choose the value of max_digits10 so that we can round trip when exactly that many digits are printed out. In the case of cpp_dec_float, given that there is no binary to decimal conversion going on, I don't see the problem (but of course that doesn't mean there isn't one)? Oh wait.... just looked at the code and I see the issue - you set max_digits10 to digits10+1 which isn't enough, should be set to cpp_dec_float_total_digits10 which is the largest number of digits possible in the number right? Making that change causes the test to pass, so committing, but please advise if that's upping max_digits10 too far ;-) Cheers, John.