[swift-evolution] [Pitch] Remove associated type inference

[L. Mihalkovic]

> On May 25, 2016, at 11:43 PM, David Hart via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Here’s a pitch for removing associated type inference as per the Generics Manifesto. If we want to do it, we’d better do it before Swift 3:
> 
> Remove associated type inference
> Proposal: SE-XXXX
> Author: David Hart, Douglas Gregor
> Status: TBD
> Review manager: TBD
> Introduction
> 
> This proposal seeks to remove the inference of associated types in types conforming to protocols.
> 
> Motivation
> 
> Even if associated types inference in a useful feature, it is also a big source of bugs in the compiler. This proposal argues that the usefulness does not outweight its architectural complexity. As per the Generics Manifesto:
> 
> associated type inference is the only place in Swift where we have a global type inference problem: it has historically been a major source of bugs, and implementing it fully and correctly requires a drastically different architecture to the type checker.
> Because this is a breaking change, it would be beneficial to implement it for Swift 3. 
> 
> Detailed Design
> 
> The proposal would remove associated type inference and make code which relied on it invalid:
> 
> protocol IteratorProtocol {
>   associatedtype Element
>   mutating func next() -> Element?
> }
> 
> struct IntIterator : IteratorProtocol {
>   mutating func next() -> Int? { ... }  // used to infer Element = Int
> }
> The compiler would generate an error message stating: error: IntIterator is missing its Element associated type declaration. The code would have to be modified as follows to fix the error:
> 

From a purely english syntax viewpoint, i would prefer the error to be along the lines of

> IntIterator is missing a declaration for the Element associated type
> 
Or 

> IntIterator is missing a declaration for its Element associated type
> 

> struct IntIterator : IteratorProtocol {
>     typealias Element = Int
>     mutating func next() -> Int? { return nil }  // used to infer Element = Int
> }
> Impact on Existing Code
> 
> This is a breaking change that will require conforming types which relied on the inference, including in the Standard Library, to explicitly declare associated types. A Fix-It could be introduced to add the typealias and leave the type to be filled in. That way, all the type inference could be removed from the compiler.
> 
> Alternatives Considered
> 
> The only alternative is to keep the inference with the known consequences on the compiler.
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