[SR-1941] Dispatch object creation can return nil despite non-nil type annotations about swift-corelibs-libdispatch

[belkadan]

cc das (JIRA User)

Workaround: immediately assign the result of the initializer into an optional.

[swift-ci]

Comment by Daniel A. Steffen (JIRA)

This was intentional and follows the rules established by Foundation for nullability annotation of return values.

The only time source object creation can return nil is on programmer error by passing bad input arguments that are non-sensical or contradictory (like in your example with a nonsensical signal number). It never returns nil for conditions that cannot be controlled by the programmer locally at the call site.

Similarly for dispatch_io, path-based creation requires an absolute path and so when that is not the case we treat that input as programmer error. The only other case treated this way is a bad channel type (c.f. source code, all these cases are expressed as DISPATCH_BAD_INPUT)