Type Guarding breaks with functions that return never

[kwasimensah]

TypeScript Version: 3.7.x-dev.201xxxxx

Search Terms:

never type guard

Code

function fooWorking(x: number | undefined) : number {
  if (x === undefined) {
    throw new Error("Is undefined");
  }
  return x;
}


function fooBroken(x: number | undefined): number {
  const isAnError = () => {
    throw new Error("Is undefined");
  }
  if (x === undefined) {
    isAnError();
  }
  return x; // errors because it can't tell that x can only be a number
}

Expected behavior:

fooBroken compiles without issue. Typescript can tell that x must be a number because the if branch calls a function that never returns.

Actual behavior:

Typescript complains on the return statement in fooBroken because it doesn't realize x can only be a number.

Playground Link:

https://www.typescriptlang.org/play/?ssl=17&ssc=2&pln=1&pc=1#code/GYVwdgxgLglg9mABMOcDqcBOBrGYDmAFAB4BciYIAtgEYCmmiAPouACZ3B51sCUi5SrQaIA3gChEiGMEQlEAXiWswHLmB78JUqVAAWmOAHcKdEwFFMhzIQBEASQDOKtdza3eAbkmIAvj8w6KBBMJGJvf3FxUEhYBGRUACFDbDowEkFqekYWdk43XkzhRm1ECARHKGlHAEEwS2tFOX4FAD4xH10DY1MLKyw7Jxd8jXcvH38pGTliRWU89U0OnWq6hoHxqUnEQODQxHDEAHojxAZrZ3oIAEMQRzppKpuwAHIqqDoAG0-EfWuq2bPRAIT4AT0Q9EQ1woWQY4l8QA

Related Issues:

[MartinJohns]

Duplicate of #12825.

You can use assertions instead: #32695

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.