[swift-evolution] [Pitch] Make the formal type of 'self' consistent in class methods

Andrew Trick atrick at apple.com
Thu Jun 23 16:02:51 CDT 2016

> On Jun 23, 2016, at 1:48 PM, Slava Pestov <spestov at apple.com> wrote:
>> On Jun 23, 2016, at 1:46 PM, Andrew Trick <atrick at apple.com <mailto:atrick at apple.com>> wrote:
>>> On Jun 23, 2016, at 12:53 PM, Slava Pestov via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> The proposal is to change the type of self to always be Self, which can be thought of as a special generic type parameter bound to the dynamic type of the instance.
>> We’re currently specializing functions that take `self` as an argument. I don’t think that will be possible after your proposed change.
>> - Andy
> I’m not sure what that means. Do you currently punt on certain optimizations if a method returns ‘Self’?
> It should be possible to keep the reified type information around, by passing in a metatype or something for example. Can you give a concrete code snippet demonstrating the optimization and how this change would inhibit it?

We bail out of generic specialization, inlining, and function signature specialization when a type substitution contains dynamic self. (hasDynamicSelfTypes). So, yes we currently almost entirely punt on optimization for methods that return Self.

I don’t have an interesting case to point out. You can look into any trivial example:

func foo<T>(_: T) {}

func method() {


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