Open Radar

 
Summary:
Like shown at WWDC 2015 in Session 408 "Protocol-Oriented Programming in Swift", methods both implemented in protocol extensions and classes have very obscure and prone to error overriding logic.
This article explains the issue to the point: https://medium.com/ios-os-x-development/swift-protocol-extension-method-dispatch-6a6bf270ba94
This should be fixed before Swift 2.0 is released, as it will cause many implementation errors.

Steps to Reproduce:
* Define a struct S implementing a protocol P
* In P define method m
* In S implement m
* Define a protocol extension for P which implements m
* Which of the 2 methods m gets called a runtime is dependent of the inferred type of a variable which holds an instance of S - which is intransparent and prone to error
* For more info see the above linked blog post

Expected Results:
This should not even compile.

Actual Results:
It compiles and has a very confusing behavior. For more info see the above linked blog post.

Notes:
While protocol extensions are a very helpful feature, the above described behavior could lead to code that behaves wrong without an obvious reason. In order to avoid such problems in advance, preventive measures should be taken and a solution be found before Swift 2.0 is released.