[SR-4248] Runtime exception casting an Any? nil to a generic optional

[swift-ci]

Previous ID	SR-4248
Radar	None
Original Reporter	burgestrand (JIRA User)
Type	Bug
Status	Resolved
Resolution	Done

Environment

Apple Swift version 3.0.2 (swiftlang-800.0.63 clang-800.0.42.1)
Target: x86_64-apple-macosx10.9

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, RunTimeCrash, Runtime
Assignee	@hamishknight
Priority	Medium

md5: adedc48208df41d3ff061c642c889341

is duplicated by:

• SR-14356 Casting from generic placeholder type T to Any? fails, when T resolves to Any?, when build with -swift-version 4.2

relates to:

• SR-8704 Cast from Any? to as? Value where Value == Any produces unexpected result in Xcode 10

Issue Description:

Summary

An unexpected fatal error in a forced downcast from a nil to an optional generic type T, e.g. T = String?.

• NOTE: Casting nil as Any before the forced downcast is a workaround to avoid the runtime fatal error. Perhaps this in itself is its own bug?

• NOTE: This is similar to a previously reported and resolved bug: https://bugs.swift.org/browse/SR-912 — in this case, the input type is Any and not Any?.

Code Example

struct Cast<T> {
  // Crashes if `value` is `nil`
  func optional(_ value: Any?) {
    output(value as! T)
  }

  // Should be identical to `optional`, but it's not. Perhaps a separate bug here?
  func workaround(_ value: Any?) {
    output((value as Any) as! T)
  }

  private func output(_ value: T) {
    print(value as Any)
  }
}

let cast = Cast<String?>()

let full: String? = "Hello"
cast.optional(full as Any)   // Optional("Hello")
cast.workaround(full as Any) // Optional("Hello")

let empty: String? = nil
cast.workaround(empty)      // nil
cast.optional(empty)        // FATAL ERROR: unexpectedly found nil while unwrapping an Optional value

[belkadan]

cc @jckarter

[hamishknight]

#13910

[norio-nomura]

This issue is described in the release note of Swift 5. But was this already changed in 4.2?
https://developer.apple.com/documentation/xcode_release_notes/xcode_10_2_beta_4_release_notes/swift_5_release_notes_for_xcode_10_2_beta_4

In Swift 5 mode, when casting an optional value to a generic placeholder type, the compiler will be more conservative with the unwrapping of the value. The result of such a cast now more closely matches the result you would get in a nongeneric context. (SR-4248) (47326318)

[hamishknight]

@norio-nomura That is correct – the change was initially made to Swift 4.2, however that accidentally caused a compatibility regression ([SR-8704]) so in Swift 5 the behaviour was limited to Swift 5 mode (with Swift 4.x modes using the old behaviour), which is the change described by the changelog.

[norio-nomura]

@hamishknight Thanks for answer.
Hm, the release note seems a little unclear description.
Until I received your answer here, I did not understand that the behavior of Swift 5 with -swift-version 4.2 is not compatible with Swift 4.2.x.