[SR-2843] P1 & P2.Type should be warned

[rintaro]

Previous ID	SR-2843
Radar	None
Original Reporter	@rintaro
Type	Improvement
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Improvement, DiagnosticsQoI, Parser
Assignee	@rintaro
Priority	Medium

md5: fbd582e6c946f2b68fd9f11d57aa8e9b

Issue Description:

These constructs are very confusing for me:

typealias A = P1 & P2? // protocol<P1, P2>?
typealias B = P1 & P2.Type // protocol<P1, P2>.Type
typealias C = P1 & P2.Protocol // protocol<P1, P2>.Protocol

We should warn with fix-it to enclose protocol composition with parens. (e.g. (P1 & P2).Type)
Or reject them with error message such as:

error: non-protocol type 'P2?' cannot form a protocol composition
typealias D = P1 & P2? & P3
                   ^~~

[belkadan]

Warning in Swift 3, error in Swift 4, perhaps?

[rintaro]

PR: #5290

[rintaro]

Merged