[swift-evolution] Availability checking of libraries (weak linking?)

[Swizzlr]

Say I have an awesome type, and I provide some extensions that makes it work really well with a few other packages.

I don't want to force those packages on my users, but if they have them, I want to extend them with my awesome type/function/whatever convenience.

What can be done in code and package manager to weakly link (not literally, I just mean have that behaviour) my awesome type with all these other libraries?

This is a prelude to a pitch, if there isn't a decent alternative.

Tom

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