Protocol composition broken in Swift 2.3?

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Created Aug ’16

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I've been upgrading code to support iOS 10 and migrating it to Swift 2.3 and ran into an issue with this line of code:

typealias CompositeType = protocol<Interface1, Interface2, Interface3>

Xcode 8.0 beta 4 complains with a warning, saying 'protocol<…>' composition syntax is deprecated; join the protocols using '&'. It offers to make the change for me, converting the line to this:

typealias CompositeType = Interface1 & Interface2 & Interface3

But this new code generates a series of compiler errors:

Expressions are not allowed at the top level
'&' is not a prefix unary operator
Consecutive statements on a line must be separated by ';'
Unary operator cannot be separated from its operand

I see from the Swift 3.0 CHANGELOG that SE-0095[1] replaces the protocol<> syntax. However, with Gmane offline it's difficult to find the discussion and see what alternative syntax I should be using, or if this use case was missed and protocol composition is broken in 2.3.

Has anyone else run into this, and know of a solution short of refactoring my types to avoid composition?

[1] https://github.com/apple/swift-evolution/blob/master/proposals/0095-any-as-existential.md

rdar://27661507

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Answer 1

OOPer OP

Aug ’16

What is broken is diagnostics and code suggestions of Xcode.

Other than the protocol composition, you may get some other "Swift 3 only" code suggestions from Xcode when working with Swift 2.3.

The shortest solution is "keep using `protocol<Interface1, Interface2, Interface3>` and ignore the warnings".

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