Rule request: overriden declaration in extension

[marcelofabri]

New Issue Checklist

• Updated SwiftLint to the latest version
• I searched for existing GitHub issues

Rule Request

1. Why should this rule be added? Share links to existing discussion about what
   the community thinks about this.

Adding override for Swift-only declarations is already not possible. It's only possible for @objc declarations.

From Apple's Programming with Objective-C

Because the methods declared in a category are added to an existing class, you need to be very careful about method names.

If the name of a method declared in a category is the same as a method in the original class, or a method in another category on the same class (or even a superclass), the behavior is undefined as to which method implementation is used at runtime.

2. Provide several examples of what would and wouldn't trigger violations.

// should trigger
extension Foo {
  override var bar: String { return "" }

  override func baz() {}
}

3. Should the rule be configurable, if so what parameters should be configurable?

Just severity.

4. Should the rule be opt-in or enabled by default? Why?
   See README.md for guidelines on when to mark a rule as opt-in.

I think it should be enabled by default, but I'm afraid that this might cause a lot of violations that are not easily fixed.

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I have an initial implementation for this already, I just need some time to clean it up.

[marcelofabri]

cc @AliSoftware who originally pitched this idea

[AliSoftware]

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