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[SR-4248] Runtime exception casting an Any? nil to a generic optional #46831

swift-ci opened this issue Mar 15, 2017 · 5 comments

[SR-4248] Runtime exception casting an Any? nil to a generic optional #46831

swift-ci opened this issue Mar 15, 2017 · 5 comments


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swift-ci commented Mar 15, 2017

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

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:


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: — 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
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belkadan commented Mar 15, 2017

cc @jckarter

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hamishknight commented Jan 12, 2018


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norio-nomura commented Mar 8, 2019

This issue is described in the release note of Swift 5. But was this already changed in 4.2?

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)

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hamishknight commented Mar 8, 2019

@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.

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norio-nomura commented Mar 8, 2019

@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.

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
This issue was closed.
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