Mysterious "will never be executed" warning

Using Swift
,
1

Can anyone tell me why I'd get a "will never be executed" warning for workfunc() in this minimal case? If I comment out the if let data {} part then the warning's gone.

The program compiles fine, and triggers no complaints from concurrency even with -strict-concurrency=complete. It certainly runs workfunc() even though the analyzer thinks otherwise.

import Dispatch
import Foundation.NSData

let workItem = DispatchWorkItem(qos: .userInitiated) {
    let result = workfunc()
    print(result)
}

let data : Data? = {
    let baseURL = URL.currentDirectory()
    guard let matfileURL = URL(string: "filename.bin", relativeTo: baseURL)
        else { print("URL error"); return nil }
    guard let data = try? Data(contentsOf: matfileURL, options: .mappedIfSafe)
        else { /* File not found */ return nil };
    return data
}()

DispatchQueue.global().asyncAndWait(execute: workItem)
exit(EXIT_SUCCESS)

/*
    End of top-level code
*/

func workfunc() -> Int {
    /* Do something */
    if let data = data {
        print("File found. \(data)")
        /* Additionally do something else */
    }
    return 0
}
2

this is a misleading diagnostic due to top level code generally behaving oddly in some cases. if you wrap everything in a do {} block or function, you'll see that the problem is that the closure captures a variable before it's declared:

<source>:53:54: error: closure captures 'data' before it is declared [capture_before_declaration]
51 | 
52 | do {
53 | let workItem = DispatchWorkItem(qos: .userInitiated) {
   |                                                      `- error: closure captures 'data' before it is declared [capture_before_declaration]
54 |     let result = workfunc()
55 |     print(result)
56 | }
57 | 
58 | let data : Data? = {
   |     `- note: captured value declared here [captured_value_declared_here]
59 |     let baseURL = URL.currentDirectory()
60 |     guard let matfileURL = URL(string: "filename.bin", relativeTo: baseURL)
   :
74 | func workfunc() -> Int {
75 |     /* Do something */
76 |     if let data = data {
   |                   `- note: captured here [value_captured_here]
77 |         print("File found. \(data)")
78 |         /* Additionally do something else */
Compiler returned: 1

i'm fairly sure there are existing issues and threads on this matter, though they elude me at the moment...

edit: here's a thread and corresponding issue i recall from not too long ago. and another related thread comment from further in the past.

4 Likes
3

Thank you! Wrapping everything in a do{} block is a nice trick to have!

4

You could even go further by not using top level code and by resisting the temptation to capture variable names in your functions. :slight_smile:

// Driver.swift - Main entry point into the program

import Dispatch
import Foundation.NSData

@main
enum Driver {
    static func main () {
        let data : Data? = {
            let baseURL = URL.currentDirectory()
            guard let matfileURL = URL(string: "filename.bin", relativeTo: baseURL)
                else { print("URL error"); return nil }
            guard let data = try? Data(contentsOf: matfileURL, options: .mappedIfSafe)
                else { /* File not found */ return nil };
            return data
        }()

        let workItem = DispatchWorkItem(qos: .userInitiated) {
            let result = workfunc(data)
            print(result)
        }
        
        func workfunc(_ data: Data?) -> Int {
            /* Do something */
            if let data = data {
                print("File found. \(data)")
                /* Additionally do something else */
            }
            return 0
        }

        DispatchQueue.global().asyncAndWait(execute: workItem)
        exit(EXIT_SUCCESS)
    }
}