'Never': different behavior with and without function type declaration  #57084

🔎 Search Terms

Never, return type never

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play?#code/DYUwLgBAhgXBDOYBOBLAdgcwgHwgVzQBMQAzdEQgKFEgCM5FVMd8jTyqaIBjB5dLLgLEyaCpUrcA9mkTQSEALwQAFAEolAPggBvShAMQwACyRSA7hDGWAokjNJ1lAL6SZc2guXq4YgG4gSFq6+oYmZpbWEHYOTq7SspDcJHDqwf6BSqoaitp6hkamFlYgtvZSjmouEmAAngAOIEYgiACCAGJZAErgeEhoACoNIAA8dY1SClAkmpQA9HOGAHoA-JTjTWAtYABCnco9YH2Dw2PDkxCeswvLaxvNiADC+xCHx0ONZxMKydeLBqsJJQUAoVABCKAaabqADclCgADpQJgTBIQaowbQNJ5YZRaEiQCjjBA0aCwdwNMlcdwCUSIEA

💻 Code

let a: string | undefined
let b: string | undefined
let c: string | undefined

const af = () => {
    throw new Error()
}
const bf = (): never => {
    throw new Error()
}
const cf: () => never = () => {
    throw new Error()
}

if (!a) af();
a.length // Error

if (!b) bf();
b.length // Error

if (!c) cf();
c.length // Fine

🙁 Actual behavior

Typescript does not check 'never' in condition statements but does if the whole function is declared

🙂 Expected behavior

The 'never' return type should always behave in the same way

Additional information about the issue

No response