[swift-evolution] two protocols with the same method name

[Brent Royal-Gordon]

> By giving warning simply for same name, it will be quite annoying when the project run into this situation without any wrong. For example:
> 
> protocol ForwardIndexType : _Incrementable {
>     @warn_unused_result
>     public func advancedBy(n: Self.Distance) -> Self
> }
> 
> extension ForwardIndexType {
>     @warn_unused_result
>     public func advancedBy(n: Self.Distance) -> Self
>     @warn_unused_result
>     public func advancedBy(n: Self.Distance, limit: Self) -> Self
>     @warn_unused_result
>     public func distanceTo(end: Self) -> Self.Distance
> }
> 
> protocol BidirectionalIndexType : ForwardIndexType
> extension BidirectionalIndexType {
>     @warn_unused_result
>     public func advancedBy(n: Self.Distance) -> Self
>     @warn_unused_result
>     public func advancedBy(n: Self.Distance, limit: Self) -> Self
> }

Firstly, for methods and subscriptors the "name" would actually encompass the entire signature, so `advancedBy(_:)` and `advancedBy(_:limit:)` would not conflict because they have different signatures.

Secondly, `ForwardIndexType` and `BidirectionalIndexType` are *not* unrelated protocols—one of them conforms to the other. Thus, we can assume that `BidirectionalIndexType` knows about `ForwardIndexType`'s `advancedBy` methods and intends for its versions to have compatible semantics.

If instead `BidirectionalIndexType` did *not* conform to `ForwardIndexType`, and `RandomAccessIndexType` tried to conform to both `ForwardIndexType` and `BidirectionalIndexType`, *then* we would get an error, because two independent protocols would have declared `advancedBy(_: Self.Distance) -> Self` methods and it's possible they meant for them to have different semantics.

-- 
Brent Royal-Gordon
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