Inconsistent Type Narrowing with never Return Type Between Arrow and Traditional Functions

[akeldamas]

🔎 Search Terms

"Inconsistent", "Arrow", "Narrowing", "Type Guard"

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about version 3.3 till nightly version 5.5.0-dev.20240404. The version I am using is 5.4.3.

⏯ Playground Link

https://www.typescriptlang.org/play?noImplicitAny=false&strictNullChecks=false&strictFunctionTypes=false&strictPropertyInitialization=false&strictBindCallApply=false&noImplicitThis=false&noImplicitReturns=false&alwaysStrict=false&experimentalDecorators=true&target=7&jsx=0&module=1&allowSyntheticDefaultImports=true&ts=5.4.3#code/MYewdgzgLgBFAWAnEB3Ago5KYF4YAoBKALhjAFMA3cxXAPhgG8BYAKBjiVTPOwFFMIRPgBEAZRABbcjAHJEIwgG42AXzZsAZgFcwwKAEtwnLADFd+o2AIke1Wi3YnuFfoOEiA8ghqz3ilVZ1VjZgABsAQwgIGFMQECY2DgAHRANKCKgZACMI4QAPUmg0sABzGAAfMm0wsNtigzLEpw4DTQJ83Bw8MBq65o5B53RBFCJAoeCkwcRyKG1Ea3ylGAB6VZgAFXgDGKgAT2SZGnkIADpp4KnWcKiYgCE8gZhU9MyZTXj8faKoEvKqr1avU-o1yo4hm0CPsuj0+oRnkMEGYLIZwONphxrkNZvNFjB9it1lsdjEUEIANbnS5qIA

💻 Code

const arrow = (): never => {
  throw new Error("Error");
};

function traditional(): never {
  throw new Error("Error");
}

class Foo {
  bar(x: string | null): string {
    if (x === null) {
      arrow();
    }
    return x; //<- Doesn't work.
  }

  private barPrivate(x: string | null): string {
    if (x === null) {
      arrow();
    }
    return x; //<- Doesn't work.
  }

  barThrow(x: string | null): string {
    if (x === null) {
      throw arrow();
    }
    return x; //<- Works.
  }

  barReturn(x: string | null): string {
    if (x === null) {
      return arrow();
    }
    return x; //<- Works.
  }

  baz(x: string | null): string {
    if (x === null) {
      traditional();
    }
    return x; //<- Works.
  }

  private bazPrivate(x: string | null): string {
    if (x === null) {
      traditional();
    }
    return x; //<- Works.
  }

  bazThrow(x: string | null): string {
    if (x === null) {
      throw traditional();
    }
    return x; //<- Works.
  }

  bazReturn(x: string | null): string {
    if (x === null) {
      return traditional();
    }
    return x; //<- Works.
  }
}

🙁 Actual behavior

I created a separate function to handle errors, which also involves throwing them. I expected that explicitly stating the return type as never would lead Typescript to infer this and accordingly narrow the types on the variable. However, contrary to my expectations, I received a type error, which seemed to disregard the type guard.

🙂 Expected behavior

I expect that type narrowing behaves identically for arrow functions and traditional functions.

Additional information about the issue

I'm not entirely sure if this is a bug or merely expected behaviour. However, I am confident that type narrowing for arrow and traditional functions should behave similarly. If I'm mistaken, please let me know.

I attempted to replicate this in the playground, but it seems not all configuration settings are available there, so I am including my TypeScript configuration. Additionally, here is some extra environment information.

VSCODE:

Version: 1.87.2 (user setup)
Commit: 863d2581ecda6849923a2118d93a088b0745d9d6
Date: 2024-03-08T15:20:17.278Z
Electron: 27.3.2
ElectronBuildId: 26836302
Chromium: 118.0.5993.159
Node.js: 18.17.1
V8: 11.8.172.18-electron.0
OS: Windows_NT x64 10.0.22631

Package.json

  "devDependencies": {
    "@types/node": "^20.11.28",
    "@types/pg": "^8.11.2",
    "@types/progress": "^2.0.7",
    "ts-node": "^10.9.2",
    "tslib": "^2.6.2",
    "typescript": "^5.4.2"
  },
  "dependencies": {
    "csvtojson": "^2.0.10",
    "dotenv": "^16.4.5",
    "es7": "npm:@elastic/elasticsearch@^7.17.0",
    "pg": "^8.11.3",
    "progress": "^2.0.3"
  }

Ts Config:

{
  "compilerOptions": {
    "target": "ES2020",
    "experimentalDecorators": true,
    "module": "commonjs",
    "baseUrl": "./src",
    "types": ["node"],
    "allowSyntheticDefaultImports": true,
    "esModuleInterop": true,
    "forceConsistentCasingInFileNames": true,
    "strict": true,
    "skipLibCheck": true
  },
  "exclude": ["node_modules"]
}

[nmain]

🔎 Search Terms
"Inconsistent", "Arrow", "Narrowing", "Type Guard"

The second result when seearching for those is #51075, which this is a duplicate of.

[akeldamas]

Thanks my bad.