Usually, when a function returns an error, its other return values are considered
undefined. The current implementation of strconv.ParseInt, however, makes sure to return
meaningful values even in error cases. For format errors, it always returns 0 instead of
leaking parser state, and for overflow it returns the biggest/smallest possible value
and preserves the sign, as opposed to just wrapping around or behaving in other
unexpected ways.

These attributes can simplify code at the caller site, but only if they are documented
and guaranteed to stay in future versions and other implementations of the standard
library.

If they are in fact intended parts of the API, they should be documented as such.