- Proposal: SE-0164
- Author: Brian King
- Review Manager: Doug Gregor
- Status: Implemented (Swift 4.0)
- Decision Notes: Rationale
- Bug: SR-1762
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.
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.
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.
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.
This proposal does not affect ABI stability.
This proposal does not affect API resilience.
There are no alternatives considered at this time.