[swift-evolution] [swift-evolution-announce] [Review] SE-0142: Permit where clauses to constrain associated types

Drew Crawford drew at sealedabstract.com
Sat Sep 24 14:58:55 CDT 2016

This is the #1 Swift feature I have wanted for the past 2 years.

One thing I would like to see (perhaps out of scope for this proposal) is the natural extension for generic parameters:

struct Box<T> {
    var t: T

extension Box where T == Int { //error: Same-type requirement makes generic parameter T non-generic

There is a great deal of overlap between associated types and generic parameters, and it seems to me that it would be favorable to allow `==` with both generic parameter operands and associatedtype operands for consistency.


On September 23, 2016 at 6:51:51 PM, Douglas Gregor (dgregor at apple.com) wrote:

Hello Swift community,

The review of SE-0142: "Permit where clauses to constrain associated types" begins now and runs through September 30, 2016. The proposal is available here:

Reviews are an important part of the Swift evolution process. All reviews should be sent to the swift-evolution mailing list at

or, if you would like to keep your feedback private, directly to the review manager. When replying, please try to keep the proposal link at the top of the message:

Proposal link:

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What goes into a review?

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Thank you,

-Doug Gregor

Review Manager

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