typealias S = String
typealias T = Any

func pickSomething(_ items: [S]) -> S? {
 return items.first
}

func doSomething<T>() -> T? {
 return pickSomething([]) as? T
}

func doSomething() -> S? {
 return pickSomething([])
}

let result = doSomething() as Any? != nil
print("result: \(result)")

@rudkx, you've been looking at these lately, right?

This is similar to https://bugs.swift.org/browse/SR-8725 – and is due to a change where the compiler is now more conservative with unwrapping an optional value that's being cast to a generic placeholder type.

Now, the results you get are consistent with those that you would get in a non-generic context, for example:

typealias S = String

func pickSomething(_ items: [S]) -> S? {
  return items.first
}

func doSomething<T>() -> T? {
  return pickSomething([]) as? T
}

let result1 = doSomething() as Any?
print(result1 as Any) // Optional(nil)

let result2 = pickSomething([]) as? Any?
print(result2 as Any) // Optional(nil)

However this change wasn't properly gated by Swift version – I'm working on restoring the old behaviour under Swift 4 mode (#19217