Issue Description:

(Reporting this as requested by Jordan Rose in https://forums.swift.org/t/ownership-keyword-removal-in-protocols-source-breaking-for-objc-implementations/10743)

“SE-0176-0186: Remove ownership keyword support in protocols” (Pull Request, Proposal2, Discussion) made it a warning to specify weak on Swift properties (vars) in protocols.

In other words this now generates a warning:

protocol MyProtocol {  
    // 'weak' should not be applied to a property declaration in a protocol
    // and will be disallowed in future versions  
    weak var delegate: MyDelegate?  
} 

Instead, weak can be omitted from the protocol declaration, and Swift implementions of this protocol can redeclare the property as weak themselves:

class Impl: MyProtocol {  
    weak var delegate: MyDelegate?  
}

However, this is a problem for ObjC implementations (as of the Xcode 9.3 betas which brought in this warning) since the compiler complains if you redeclare the property and change the strong/weakness:

#import <MyFramework-Swift.h>  
      
@interface ImplObjC : NSObject <MyProtocol>  
      
// warning: 'retain (or strong)' attribute on property 'delegate' does
// not match the property inherited from 'MyProtocol'
// [-Wproperty-attribute-mismatch]  
@property (nonatomic, weak, nullable) id<MyDelegate> delegate;  
      
@end  

Alternatively in a private category:

@interface ImplObjC ()        

// error: illegal redeclaration of property in class extension 'ImplObjC'
// (attribute must be 'readwrite', while its primary must be 'readonly')  
@property (nonatomic, weak, nullable) id<MyDelegate> delegate;  
      
@end  

Alternatively synthesizing into a different ivar with __weak:

@interface ImplObjC ()  
{  
    // error: existing instance variable 'm_delegate' for strong property
    // 'delegate' may not be __weak  
    __weak id<MyDelegate> m_delegate;  
}  
@end  
      
@implementation ImplObjC  
      
@synthesize delegate = m_delegate;  
      
@end  

The only alternative that works is to override the setter and getter for the property and back it with a weak ivar/property. This is ugly and tedious.

Note that even when leaving weak on the Swift protocol property and ignoring the warning, the generated "-Swift.h" header still declares the property as strong:

@property (nonatomic, strong) id <MyDelegate> _Nullable delegate;

We build with “warnings as errors” for ObjC, so this is a source breaking change for our existing code. It doesn’t seem like it would be a good idea to ignore the Wproperty-attribute-mismatch warning. This is a problem for any existing ObjC code that implements Swift protocols like this.

Now, there are legitimate design discussions to be had over whether a delegate property belongs on an abstract protocol, but given how common the (weak) delegate pattern is in ObjC I believe there will be a significant amount of this kind of code in the wild (we ran into this at least).