[SR-7217] Protocol composition with conflicting typealiases does not diagnose

[itaiferber]

Previous ID	SR-7217
Radar	None
Original Reporter	@itaiferber
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, TypeChecker
Assignee	None
Priority	Medium

md5: c5f27b0f94d34e042696e3afc9cf3681

relates to:

• SR-7516 Compiler accepts ambiguous and/or invalid use of associated types
• SR-5392 Inconsistent "invalid redeclaration of typealias"
• SR-5440 Typealias in constrained extension misinterprets the where clause

Issue Description:

From discussion in https://forums.swift.org/t/decode-a-json-object-of-unknown-format-into-a-dictionary-with-decodable-in-swift-4/6138/38, the following typealias conflict produces no diagnostics in Swift 4.1:

struct Foo {}
struct Bar {}

protocol P1 { typealias T = Foo }
protocol P2 { typealias T = Bar }

typealias P3 = P1 & P2
print(P3.T.self) // => Bar()

typealias P4 = P2 & P1
print(P4.T.self) // => Bar()

Code completion shows two overloads for P3.T and P4.T but there are no warnings or complaints otherwise. The ordering of the composition makes no difference on which type "wins out".

[belkadan]

cc @rudkx, @DougGregor