SE-290 (second review): Unavailability Condition

[xwu (Xiaodi Wu)]

@bjhomer

The compiler implementation may substitute in the minimum deployment target of the current platform, but that's an implementation detail that is not necessary for a user to understand. Critically, that implementation could change so long as the overall behavior does not change; the simple guarantee is that on any unlisted platform, the code runs. We are not required to preserve the idea that * means "minimum deployment target" in other usages of the * symbol.

This is not compiler implementation. It is the publicly stated meaning of the * symbol by the core team:

@tkremenek

The wildcard * for #available has precise semantics, where * indicates the minimum deployment target for the unspecified platforms.

If The Swift Programming Language does not adequately explain those semantics, then it is a defect in the book for which a bug should be filed. @rockbruno has described the meaning of * absolutely accurately.