Add draft proposal for ownership keyword removal in protocols.

Author: Greg Spiers

proposals/NNNN-remove-ownership-keyword-support-in-protocols.md

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+# Remove ownership keyword support in protocols
+
+* Proposal: [SE-NNNN](NNNN-remove-ownership-keyword-support-in-protocols.md)
+* Authors: [Greg Spiers](https://github.com/gspiers)
+* Review Manager: TBD
+* Status:
+* Bug: [SR-479](https://bugs.swift.org/browse/SR-479)
+
+## Introduction
+
+This proposal removes support for the keywords `weak` and `unowned` for property declarations in a protocol.
+
+Swift-evolution thread: [Ownership on protocol property requirements](https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170501/036495.html) thread.
+
+## Motivation
+
+Currently it's possible to use the weak/unowned keywords for a property requirement in a protocol. This can lead to confusion as specifying one of these keywords does not enforce or raise any warnings in the adopting type of that protocol:
+
+```swift
+
+class A {}
+
+protocol P {
+    weak var weakVar: A? { get set }
+}
+
+class B: P {
+    var weakVar: A? // Not declared weak, no compiler warning/error
+}
+
+```
+
+This can lead to unexpected and surprising behaviour from the point of view of users. The keywords themselves are currently meaningless inside of a protocol but look like they would have an effect when the protocol is adopted.
+
+This change is consistent with removing keywords that do not have any meaning like `final` in protocol extensions: [SE-0164](0164-remove-final-support-in-protocol-extensions.md).
+
+## Proposed solution
+
+Although the case could be made that the keywords should have meaning in a protocol, as they are currently implemented today they don't have an effect. This proposal aims to cleanup the misleading syntax and isn't meant to remove functionality only correct to existing behaviour.
+
+This proposal suggests removing support for `weak` and `unowned` in a protocol.
+
+## Detailed design
+
+The compiler will flag the use of `weak` and `unowned` in a protocol and suggest a fix to remove the keyword.
+
+## Source compatibility
+
+This is a source breaking change but one that would only correct code that already has broken assumptions. For existing use the compiler will raise a compilation error. When running in Swift 3 mode a warning can be generated instead of an error. It could be possible to address source compatibility through source migration as well.
+
+## Effect on ABI stability
+
+This proposal does not affect ABI stability.
+
+## Effect on API resilience
+
+This proposal should not effect API resilience.
+
+## Alternatives considered
+
+There is an argument in making `weak` and `unowned` have meaning in a protocol but this does open up other questions and is probably better as a topic of a separate discussion/proposal. As this would be additive it can be addressed at a later point when we have a clearer understanding.