This is achieved by marking a void function with the void_return
attribute and then calling that function in an @return expression.
@return can be used only inside a void function and only with void
functions marked with the void_return attribute. As this is intended for
Objective-QC message forwarding, it is deliberately "difficult" to use
as returning a larger than expected value is unlikely to end well for
the calling function.
However, as a convenience, "@return nil" is allowed (in a void
function). It always returns an integer (which, of course,can be
interpreted as a pointer). This is safe because if the return value is
ignored, it will go into the progs return buffer, and if it is not
ignored, it is the smallest value that can be returned.
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