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[SR-5357] Type-constrained protocol cannot be used as a generic argument for similarly constrained generic parameter #47931

jarrodldavis opened this issue Jul 2, 2017 · 2 comments


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@jarrodldavis jarrodldavis commented Jul 2, 2017

Previous ID SR-5357
Radar rdar://problem/33095936
Original Reporter @jarrodldavis
Type Bug
Status Resolved
Resolution Duplicate

macOS Sierra 10.12.5 (16F73), Xcode 9.0 beta 2 (9M137d)

Additional Detail from JIRA
Votes 0
Component/s Compiler
Labels Bug
Assignee None
Priority Medium

md5: 503457675ee4c46224ec7dac3e0dc725


  • SR-55 non-@objc protocol existentials do not conform to their own protocol type

Issue Description:

Given a generic type X<Y> where Y is constrained to some type Z and a protocol A that constrains Self to Z, a variable or constant declaration of X<A> should be considered valid. Currently, as of Xcode 9 beta 2, the compiler gives a warning similar to "'X' requires that 'A' inherit from 'Z'" even though that is already true.

// Imagine these sheet classes and the bed struct in a separate module somewhere

class Sheet {
    func pullOver() {
        print("sleep time")

class ComfySheet : Sheet {
    public static let veryComfy: ComfySheet = {
        return ComfySheet()

    func comfort() {

class BouncySheet : Sheet {
    public static let veryBouncy: BouncySheet = {
        return BouncySheet()

    func bounce() {

class AwfulSheet : Sheet {
    override func pullOver() {

struct Bed<SheetType> where SheetType : Sheet {
    public let size: Double
    public let sheets: SheetType

// This protocol and the corresponding extensions are editable

protocol EnjoymentMeasurable where Self : Sheet {
    func enjoy()

extension ComfySheet : EnjoymentMeasurable {
    func enjoy() {

extension BouncySheet : EnjoymentMeasurable {
    func enjoy() {

func doSomething() {
    let bed: Bed<EnjoymentMeasurable> // error here: "'Bed' requires that 'EnjoymentMeasurable' inherit from 'Sheet'"
    bed = Bed(size: 4.0, sheets: ComfySheet.veryComfy)
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@belkadan belkadan commented Jul 4, 2017

This is arguably correct because EnjoymentMeasurable could have static requirements, and then Bed wouldn't be able to call those methods because the protocol type doesn't implement them. But it is a little frustrating when the protocol does not have static requirements.

@slavapestov, @jckarter, how are we tracking these?

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@jckarter jckarter commented Jul 5, 2017

SR-55 is the canonical "protocol types should conform to their protocol" issue.

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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