Missing warning on unreachable code

[SimonGustavsson]

Sorry if this has been reported before, I tried my best searching for similar issues. I guess this is more of a "Is this 'As Designed'?" question as opposed to a bug report. It's caught me off guard a few times and thought I'd mention it at least.

TypeScript Version: 3.1.1

Search Terms: Type check, null, undefined

Code

// Example 1
const myNumber = 32;
if (myNumber === undefined) {
  console.log('This will never run');
}

// Example 2
const test: number | null = null;
if (test === undefined) {
  console.log('Woof');
} else {
  console.log('Meow');
}

Expected behavior:
A warning on line 2 in both examples that myNumber/test will never be undefined. (Or line 3 that the code is unreachable).

Actual behavior:
Code compiles fine without issue.

Playground Link: http://www.typescriptlang.org/play/#src=%2F%2F%20Example%201%0D%0Aconst%20myNumber%20%3A%20number%20%3D%2032%3B%0D%0Aif%20(myNumber%20%3D%3D%3D%20undefined)%20%7B%0D%0A%20%20console.log('This%20will%20never%20run')%3B%0D%0A%7D%0D%0A%0D%0A%2F%2F%20Example%202%0D%0Aconst%20test%20%3A%20number%20%7C%20null%20%3D%20null%3B%0D%0Aif%20(test%20%3D%3D%3D%20undefined)%20%7B%0D%0A%20%20console.log('Woof')%3B%0D%0A%7D%20else%20%7B%0D%0A%20%20console.log('Meow')%3B%0D%0A%7D

Related Issues: No

[RyanCavanaugh]

Comparisons with undefined are always allowed in order to facilitate e.g. defensive parameter checks

[SimonGustavsson]

That's a good point.

I guess there's no way to add a warning in my case where the types and values are well known without it getting messy/confusing for users?

[jack-williams]

Perhaps a related solution might be from: #27634

E.g

function isUndefined<T extends (undefined extends T ? any : never)>(value: T): value is T & undefined {
    return value === undefined;
}

// Example 1
const myNumber = 32;
if (isUndefined(myNumber)) {
  //            ^^^^^^^^ Error: argument of type number is not assignable to type never 
  console.log('This will never run');
}

The error is not in unreachable code, but a useless comparison.

[typescript-bot]

This issue has been marked as 'Question' and has seen no recent activity. It has been automatically closed for house-keeping purposes. If you're still waiting on a response, questions are usually better suited to stackoverflow.