[swift-evolution] Proposal: newtype feature for creating brand new types from existing types

[tuuranton at tutanota.de]

//Existing typealias feature.//Foo acts exactly the same way as Double. It's 
the same thing.//Double can be used instead of Foo anywhere.typealias Foo = 
Double
//Proposed newtype feature.//Bar acts exactly the same way as Double and  is 
the same//thing behind the scenes (until we extend it; see below).//Trying to 
use Double instead of Bar will result in a compile-time error.newtype Bar = 
Double
Of course, after creating Bar - a "copy" of the type Double, so to speak - I 
can extend Bar independenly of Double as one would expect.
This would be highly useful.
Your thoughts?
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