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[SR-1024] Type of optionals cannot be inferred correctly in swift 2.2 #43636

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swift-ci opened this issue Mar 22, 2016 · 6 comments
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[SR-1024] Type of optionals cannot be inferred correctly in swift 2.2 #43636

swift-ci opened this issue Mar 22, 2016 · 6 comments
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@jckarter
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself optimized only Flag: An issue whose reproduction requires optimized compilation regression swift 2.2

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@swift-ci
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Previous ID SR-1024
Radar rdar://problem/25451728
Original Reporter sts2055 (JIRA User)
Type Bug
Status Resolved
Resolution Done
Environment

Xcode 7.3
Swift 2.2

Additional Detail from JIRA
Votes 7
Component/s Compiler
Labels Bug, 2.2Regression, OptimizedOnly
Assignee @jckarter
Priority Medium

md5: 2db3fa276c0751b3d29d768627bffb8d

is duplicated by:

  • SR-1113 Incorrect runtime dynamic cast of Optional.none to Optional.none when A&B are statically incompatible.

Issue Description:

Problem:

When running the following code under Xcode 7.3 with swift 2.2, the compiler is unable to correctly infer the type of the optional:

 import Foundation
    
    func whatAmI<T>(inout property:T?)
    {
        switch property {
        case is Int?:
            print("I am an Int?")
        case is String?:
            print("I am a String?")
        default:
            print("I don't know what I am")
        }
    }
    
    var string : String?
    whatAmI(&string)

On my side with Xcode 7.3 this will print `I am an Int?`

However, when I initialize the variable with an empty string before passing it to the function, the switch infers it to be a String?.

This would print `I am a String?` in the previous Xcode version.

Another example:

func genericMethod<T>(property: T?) {
    print(T) // String

    let stringNil = Optional<String>.None

    print(stringNil is String?) // true (warning - always true)    
    print(stringNil is T?) // true

    let intNil = Optional<Int>.None

    print(intNil is String?) // false (warning - always fails)
    print(intNil is T?) // true - BUG
}

genericMethod("")

Stackoverflow question: http://stackoverflow.com/questions/36160934/type-of-optionals-cannot-be-inferred-correctly-in-swift-2-2
@jckarter
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The 7.3 behavior is correct, IMO. An optional containing `nil` can be cast to any other optional type, and the result is a nil of the destination type.

@jckarter
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The 7.3 behavior is correct. A nil in an optional can be cast to a nil in any other optional type.

@swift-ci
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Comment by Ondrej Hanslik (JIRA)

@jckarter Please, look at the second example. The problem is not whether `nil` can be cast to any optional type (which is not actually true), the problem is inconsistency when the type is a generic type.

@jckarter
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I see, you're right, it looks like there's an inconsistency there between the runtime cast for generics and the compiler-generated open-coded cast for Int? to String?. It should always succeed in either case.

@jckarter
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Reopening to fix compiler case.

@slavapestov
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The code example now exhibits consistent behavior between static and dynamic casts.

The bogus warnings are covered by https://bugs.swift.org/browse/SR-3505.

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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@jckarter jckarter

Labels
bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself optimized only Flag: An issue whose reproduction requires optimized compilation regression swift 2.2
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@slavapestov @jckarter @swift-ci @AnthonyLatsis