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[SR-6000] Let protocols inherit from class types #48557

swift-ci opened this issue Sep 26, 2017 · 3 comments

[SR-6000] Let protocols inherit from class types #48557

swift-ci opened this issue Sep 26, 2017 · 3 comments


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swift-ci commented Sep 26, 2017

Previous ID SR-6000
Radar rdar://problem/34812895
Original Reporter mdmorris (JIRA User)
Type Bug
Status Resolved
Resolution Done

Swift 4.

Additional Detail from JIRA
Votes 0
Component/s Compiler
Labels Bug
Assignee @slavapestov
Priority Medium

md5: b4794a58f4d0c7b151aaad9efd595e7a

is duplicated by:

  • SR-6001 Protocols inheriting from classes
  • SR-6006 Compiler abort trap
  • SR-6521 Compiler fails to see a custom initializer in a protocol
  • SR-7452 Swift compiler segmentation fault
  • SR-7839 Protocol Restricting Class Inheritance Cannot Consistently Access Valid Instance Properties
  • SR-8031 Compiler crash in swift::irgen::emitImpliedWitnessTableRef
  • SR-8909 Crash accessing self from a extension
  • SR-6390 Let protocols inherit from class types
  • SR-7634 EXC_BAD_ACCESS when using a Protocol which has a Constraint on it's definition

Issue Description:

Swift can't decide whether a variable belonging to a class constrained protocol counts as a subtype of the constrained class and therefore has access to the members of that class. I would expect it to always have such access, but it's possible that it's not supposed to. Either way the compiler is being nutty here.

class Strawman {
    let name: String
    public func bar(_ x: Strawman) {
        print("\(name) bars \( from entering.")
    public init(name: String) { = name

// A type must inherit from Strawman in order to implement
// Fooable, therefore all Fooable are a kind of Strawman.
// Q.E.D.
protocol Fooable where Self: Strawman {
    func foo()

class StrawFooable: Strawman, Fooable {
    public func foo() { print("Also Foo!") }

let sm1 = StrawFooable(name: "Strawman1")
let sm2 = StrawFooable(name: "Strawman2")

// This will not compile if you define doStuff as
// func doStuff(with x: Fooable, and y: Fooable) {
func doStuff<T: Fooable>(with x: T, and y: T) {

// This will not compile if you annotate sm1 and sm2 as Fooable.
doStuff(with: sm1, and: sm2)
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belkadan commented Sep 27, 2017

The last part, "This will not compile if you annotate sm1 and sm2 as Fooable", is a well-known corner of the language: a protocol value doesn't necessarily provide a type for a generic parameter constrained by that protocol. (Note that sm1 and sm2 could have different dynamic types.)

However, actually using protocol-typed arguments probably ought to work. @slavapestov, @huonw?

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slavapestov commented Sep 27, 2017

I think the problem here is the same as SR-6001, the declaration "protocol Fooable where Self: Strawman" is not fully supported.

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slavapestov commented Jul 10, 2018

Done: #17851

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
This issue was closed.
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