TS2366: Function lacks ending return statement in function with `unknown` return type with implicit return on some code paths, despite `noImplicitReturns` turned off

[AlCalzone]

Bug Report

🔎 Search Terms

lacks ending

🕗 Version & Regression Information

• This changed between versions 5.0.2 and 5.1.0-dev.20230323

I believe this caused it.

⏯ Playground Link

https://www.typescriptlang.org/play?noImplicitReturns=false&ts=5.1.0-dev.20230323#code/GYVwdgxgLglg9mABFApgZygCgJQC5HgDWYcA7kgN4CwAUAJAzCKYCyAhlABYB0ATm2AAmcALY5EAPkQAGbgFZsiXiigheSKLxAoA3LQC+QA

💻 Code

function test(): unknown {
	if (Math.random() > 0.5) return true;
}

🙁 Actual behavior

> tsc --noEmit

index.ts:1:18 - error TS2366: Function lacks ending return statement and return type does not include 'undefined'.

1 function test(): unknown {
                   ~~~~~~~

🙂 Expected behavior

No error, if the function return type is unknown I'd expect that to allow implicit return of undefined.

[fatcerberus]

This changed between versions 5.0.2 and 5.1.0-dev.20230323

The error message changed, but this is an error in 5.0.2 and earlier too:

[AlCalzone]

Uh, the playground link had noImplicitReturns turned on. If you turn it off, it only becomes an error after 5.0.2. I'll update the link

[fatcerberus]

What do you know, this actually broke some real-world code already. 😄
#53474 (comment)

[AlCalzone]

Believe it or not, I found this in real-world code :D

[DanielRosenwasser]

I think the simplest fix would just be to check for TypeFlags.Unknown - alternatively, we restore the isTypeAssignableTo(undefinedType, type) check.

Let's make sure that we test strictNullChecks on/off with other top-y types.

// @strictNullChecks: true,false

function foo(): unknown {
    if (Math.random()) return true;

    // implicit return
}

function bar(): {} {
    if (Math.random()) return true;

    // implicit return
}

function baz(): Record<string, any> {
    // implicit return
}

function blah(): any {
    // implicit return
}