[SR-9761] Swift 5 migrator adds unnecessary type annotations

[keith]

Previous ID	SR-9761
Radar	None
Original Reporter	@keith
Type	Bug

Environment

Xcode 10.2 beta 2
Swift 4.2 -> Swift 5 migrator

Additional Detail from JIRA

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

md5: db350fb3d6f66b39b682595cf6f7a76d

Issue Description:

When running the migrator on this project https://github.com/lyft/mapper/ in Xcode 10.2 beta 1, it tries to migrate this:

public func optionalFrom<T>(_ field: String, transformation: (Any) throws -> T?) -> T? {
    return (try? transformation(try self.JSONFromField(field))).flatMap { $0 }
}

To this:

public func optionalFrom<T>(_ field: String, transformation: (Any) throws -> T?) -> T? {
    return (((try? transformation(try self.JSONFromField(field))) as T??)).flatMap { $0 }
}

The type of `JSONFromField` is:

func JSONFromField(_ field: String) throws -> Any

If I ignore this migration, and still move to Swift 5, it compiles fine. You should be able to reproduce this by cloning that repo, and migrating.

[belkadan]

cc @nkcsgexi