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Remove final support in protocol extensions

Introduction

This proposal disallows the final keyword when declaring functions in protocol extensions.

Discussion took place on the Swift Evolution mailing list in the Remove support for final in protocol extensions thread.

Motivation

In the current version of Swift, the final keyword does not modify dispatch behavior in any way, and it does not generate an error message. This keyword has no use in Swift's current protocol model. Functions in protocol extensions cannot be overridden and will always use direct dispatch.

Jordan Rose described the history behind this behavior:

We originally required `final` to signify that there was no 
dynamic dispatch going on. Once we started allowing protocol extension 
methods to fulfill requirements, it became more confusing than useful.

Detailed design

If adopted, the compiler will flag the use of the final keyword on functions declared within a protocol extension, and emit an error or warning. This behavior is consistent with final use in structures and enumerations.

Source compatibility

This change will impact source compatibility. Existing use of final in protocol extensions will raise a compilation error. The compiler will address this by source migration and fixits. When running in Swift 3 mode, a warning will be generated instead of an error.

Effect on ABI stability

This proposal does not affect ABI stability.

Effect on API resilience

This proposal does not affect API resilience.

Alternatives considered

There are no alternatives considered at this time.