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".