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They want functionality that is similar to the C preprocessor’s #if, but different in that they want to require both branches to syntax check.

They also want to locally evaluate the equivalent of such #if expressions (in contrast, in C, #if X requires preprocessing the file. That can mean reading dozens of header files, and is dependent on compiler flags)

I think this is a pragmatic way to accomplish that. It restricts you to conditions that Apple has deemed useful, but makes it easier to write good tooling.

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