[SR-8719] Associated type 1 == Associated type 2 requires explicitly stating both types

[swift-ci]

Previous ID	SR-8719
Radar	None
Original Reporter	Nasamura (JIRA User)
Type	New Feature

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	New Feature, AssociatedTypeInference
Assignee	@DougGregor
Priority	Medium

md5: 583798a6a88eb7fa7fc4d602ce5716b8

relates to:

• [SR-8705] Attempt to use same type requirement in the method description causes compiler error #51217

Issue Description:

Xcode: Version 10.0 beta 6 (10L232m)

public protocol Prot1 {
    associatedtype AType: StringProtocol
}

public protocol Prot2 {
    associatedtype BType: StringProtocol
}

public protocol Prot3: Prot1, Prot2 where AType == BType {
}

struct Struct1: Prot3 {
    typealias AType = String
}

// Type 'Struct1' does not conform to protocol 'Prot2'

This is a simplified example of where I'm combining two protocols and AType will always equal BType but the compiler is not able to infer one type from the other and has to be explicitly stated. This leads to unnecessary code duplication.

[slavapestov]

Fixed by #70375.