exported const arrow function returning never: no CFA

[ryanrhee]

Bug Report

Unable to type an exported const arrow function as returning never and have it participate in control flow analysis (CFA)

This is similar to #37998 but notably different in 2 ways:

1. it deals with exporting a const function for consumption by another module
2. the workaround of adding type information to the const isn't possible, since it's exported

🔎 Search Terms

unreachable code, return never, arrow function, CFA

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about returning never

⏯ Playground Link

Playground link with relevant code

💻 Code

// file1.ts

import process from "process"

interface ExportedObject {
    showHelp: () => never
}

const exportedObject: ExportedObject = {
    showHelp: () => { console.log("help text"); process.exit() }
}

export default exportedObject

// -------------------------------

// file2.ts

// import exportedObject from "./file1"

const { showHelp } = exportedObject
showHelp()

console.log('this should be unreachable but is not')

// workaround
const showHelp2: typeof showHelp = showHelp
showHelp2()
console.log('this line, though, does get marked as unreachable')

🙁 Actual behavior

Unreachable code detected. (7027) warning

🙂 Expected behavior

No warning

[fatcerberus]

const { showHelp } = exportedObject

This line is the problem: showHelp has no type annotation, which violates

each identifier in the function name references an entity with an explicit type

per #32695.

In line with the above, this works:

exportedObject.showHelp();

P.S. The actual and expected behavior seem to be switched in the OP.

[ryanrhee]

but how would you type showHelp in that line? the way to type a spread member is by typing the right side of the assignment, but that doesn't actually help.

also, i tried the workaround you suggested with exportedObject.showHelp but it doesn't seem to work across export boundaries. is there a way to do a typescript playground with multiple modules?

[MartinJohns]

but how would you type showHelp in that line?

Just as you would type any variable declaration.

const { showhelp }: { showhelp: TypeBeHere} = ...

[ryanrhee]

I am still unable to get this to work. I think I need to show a fuller example that more closely mirrors what I have.

FYI the real-world example I opened this issue from is shown here https://github.com/sindresorhus/meow/blob/main/index.d.ts#L271 : using this library, calling showHelp doesn't tell typescript that execution ends.