[SR-10897] extensions of @available constrained classes creates warnings

[swift-ci]

Previous ID	SR-10897
Radar	None
Original Reporter	ajo (JIRA User)
Type	Improvement

Additional Detail from JIRA

Votes	2
Component/s	Compiler
Labels	Improvement
Assignee	@xymus
Priority	Medium

md5: aa32c297a13cc9f973b1d1e8e3416ac7

Issue Description:

The following code results in a error on the extension:

'Asdf' is only available in iOS 13.0 or newer

@available(iOS 13.0, *) class Asdf {   }

extension Asdf { }

As there are no cases where the extension can have fewer, or a more relaxed constraint than the object, the constraints on the object itself should be propagated to extensions.

[norio-nomura]

I hope this feature helps to avoid the bugs mentioned in this thread.