P1788
I've been writing Level 2 so far assuming that for every supported interval type T, each operation shall have a T-version, for which each interval input & output is a T-interval. But suppose there is a tighter type T' (i.e., such that each T-interval is a T'-interval), and the implementation decides to output T'-intervals instead. Since that gives a better enclosure, shouldn't it be allowed?
E.g., let T be infsup-binary32, and T' be infsup-binary64, and the operation be subtraction. Then suppose, in obvious notation,
xx_32 - yy_32 always gives zz_64.
In fact as these are both 754-conforming types of radix 2 -- call these "nice" types -- any combination is allowed (the "typeOf" feature on the lines of "formatOf" in 754), so the current rules say there shall be an
xx - yy whose inputs may be any combination of nice types, giving result of type T.
Also
xx - yy whose inputs may be any combination of nice types, giving result of type T'.
Suppose the implementation only provides the second of these. (If one wants the first, get it by taking the T-hull explicitly.) Should the standard call this non-conforming?
John Pryce