[swift-evolution] [Review] SE-0026 Abstract classes and methods

[Brent Royal-Gordon]

> var url : NSURL {
> 
>  assert(false,"Must be overridden”)
>  return NSURL(fileURLWithPath: “/ignore/this”)
> }

For what it's worth, using `fatalError()` or `preconditionFailure()` will allow you to write a version of this with no return statement, because they are annotated with `@noreturn`.

-- 
Brent Royal-Gordon
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