[SR-14917] Inconsistent/missing warning about redundant same-type constraint

[jepers]

Previous ID	SR-14917
Radar	rdar://problem/80820294
Original Reporter	@jepers
Type	Bug
Status	Resolved
Resolution	Done

Environment

macOS 11.3, Xcode 12.5 and 12.5.1

Additional Detail from JIRA

Votes	0
Component/s
Labels	Bug
Assignee	None
Priority	Medium

md5: 5107e4b57e6a7f5c8dd5e4d6703b74f3

Issue Description:

This compiles with the expected warning:

struct S<T: Collection> where T.Index == T.Indices.Index {} // Warning: Redundant same-type constraint `T.Index` == `T.Indices.Index`

But this compiles without any warning:

extension Collection where Self.Index == Self.Indices.Index {} // No warning

I would have expected the same warning here.

This inconsistent behaviour is reproduced by eg:

protocol P {
  associatedtype A
  associatedtype AS: Q where AS.B == A
}

protocol Q {
  associatedtype B
}

struct S<T: P> where T.AS.B == T.A {} // Warning as expected

extension P where AS.B == A {} // No warning

[typesanitizer]

cc @slavapestov

[slavapestov]

@swift-ci create

[slavapestov]

We're probably losing track of a source location somewhere.

[slavapestov]

#42113