DDSXTY15 — floating point range with NaN or other out-of-range values

For numeric types, it is often convenient for a data model to say "value is in range or a designated out-of-range marker".
IEEE-754 defines the "not a number" NaN specifically for this purpose. It fits within the numeric type and is "infectious", so "x op NaN = NaN" for most mathematical operations and all values of x. Other values such as 0 or 1 are sometimes useful defaults for out of range in specific contexts.

While the semantics are similar to optional<float>, a purely numeric data type can be more efficient for communications and processing, especially on embedded platforms.

Feature request: add an annotation for defining such disjoint numeric sets.