[SR-11997] Compiler creates warning 'Redundant conformance constraint' for constraint that is necessary and removes this constraint from code in compilation. This causes wrong compilation errors.

[swift-ci]

Previous ID	SR-11997
Radar	rdar://problem/58455439
Original Reporter	vzakh (JIRA User)
Type	Bug
Status	Resolved
Resolution	Done

Environment

Xcode 11.3 (11C29), Swift 5

Additional Detail from JIRA

Votes	0
Component/s
Labels	Bug
Assignee	None
Priority	Medium

md5: 83907b1c17778ddab16f3d29a9926153

Issue Description:

Pulling the code in from SO:

protocol Autumn {
  associatedtype WarmUp: Spring
    where WarmUp.CoolDown == Self
}

protocol Spring {
  associatedtype CoolDown: Autumn
    where CoolDown.WarmUp == Self
}

protocol Winter: Autumn where WarmUp: Summer { //warning: Redundant conformance constraint 'Self': 'Autumn'
}

protocol Summer: Spring where CoolDown: Winter {
  associatedtype Bike: Harley
    where Bike.Season == Self
}

protocol Harley {
  associatedtype Season: Summer
    where Season.Bike == Self
}

The redundant constraint is implied by itself according to its own note:

> Conformance constraint 'Self': 'Autumn' implied here.

We shouldn't be emitting this diagnostic at all.

[beccadax]

@swift-ci create

[CodaFi]

The actual redundancy is that you can get to Winter: Autumn in a few different ways:

The short path: Winter: Autumn by fiat
The long path: Winter: Autumn -> Winter.WarmUp.CoolDown == Winter
The longer path: Winter: Autumn -> Winter.WarmUp: Summer -> Winter.WarmUp.Bike.Season.CoolDown == Winter

etc.

You can break either the Bike.Season == Self part of the cycle or break the WarmUp.CoolDown == Self part of the cycle.

[slavapestov]

#42113