[Cfp-interest 2256] printf and rounding recommendation

[Vincent Lefevre]

In the current C2x draft N2731, 7.21.6.1 The fprintf function:

    Recommended practice
[...]
 13 [...] If the number of significant decimal digits is more than M
    but the source value is exactly representable with M digits, then
    the result should be an exact representation with trailing zeros.
    Otherwise, the source value is bounded by two adjacent decimal
    strings L < U, both having M significant digits; the value of
    the resultant decimal string D should satisfy L ≤ D ≤ U, with the
    extra stipulation that the error should have a correct sign for
    the current rounding direction.

In the case of directed rounding, this means that correct rounding is
recommended (in both cases: exact[*] or not). But this is not the case
for rounding-to-nearest. This is surprising.

[*] In directed rounding, this involves the Table Maker's Dilemma if
one ensures the correct direction (i.e. that the error has a correct
sign).

-- 
Vincent Lefèvre <vincent at vinc17.net> - Web: <https://www.vinc17.net/>
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