Extension to the canImport statement

[tshortli (Allan Shortlidge)]

I remember a couple of past pitches that dealt with this same topic:

[Discussion] `#if hasSymbol(Module.Symbol)`, a compile time check for a module symbol's availability

Description The hasSymbol compiler directive aims to be a viable alternative to availability where it has not been explicitly attributed, or where a symbol is available in a library on one platform, but not available on another. There are many instances where you would want to check the symbol availability at compile time. One good reason to do this during compile time is binary size. Any code inside the #if block wouldn't be compiled if the symbol wasn't available. If the symbol has been mark…

This is not solving quite the same problem, but it's related:

Build-time SDK availability check

Every year around this time, I find myself needing to write code that compiles differently based on the availability of a particular SDK version. Unfortunately, there's no way to do that. My goal is to be able to write code that uses features from the new iOS or Mac SDKs if they are available. I wrote about this two years ago, and everyone seemed to agree that it was necessary then. I think we just need someone to actually implement it. TL;DR: We really need something like #if sdk() or #if mod…

I think that augmenting canImport with this capability makes some sense. Specifying the module that the declaration is expected in sidesteps one of the main problems with other approaches, which was that allowing the queries to match declarations in the module being compiled would create a cycle between parsing and type checking in the compiler. Since canImport only operates on dependencies, by definition that cycle isn't an issue.