False positive error when accessing property method that returns never using --strict

[olafurpg]

Bug Report

I get a compile error when calling a property method that returns never using --strict, when I expect to get no compile error. I am unable to reproduce the error with toplevel functions.

🔎 Search Terms

• never strict label:bug
• never property
• false positive never label:bug

🕗 Version & Regression Information

• I am able to reproduce this behavior with versions 4.1.3 (my application), 4.4.3 (the minimized reproduction above) and 4.5.0-beta (playground link)
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about never

⏯ Playground Link

Playground link https://www.typescriptlang.org/play?ts=4.5.0-beta#code/EQVwzgpgBGAuBOBLAxrYBuAUAMxAO1UQHs8oAjIogWwAoBKALijwgDcJ4oBvTKKWABbwiAd2YQxAUXjD4NAOQVq8upgC+mTMhJwoAeRCwOUALzdefcpVqNx7Tj0tP+Q0eKkyicxdZUW+GhpaADYAhmBgUABCoQ7+UAAOIGTBKFbUAGL4hCT0THggVGTGjs58SjbxGk5JKWkVAArCCRywAJ55zIXFcWV8BkbwAHQV9FXqmEA

💻 Code

Minimized reproduction

❯ cat boom.ts
function boom(): never {
  throw new Error('boom')
}

const Outer = {
    boom(): never {
        throw new Error('boom')
    }
}

class Bar {
    public boomFunction(): number {
        boom()
    }
    public boomProperty(): number {
        Outer.boom()
    }
}

🙁 Actual behavior

❯ npx -p [email protected] tsc --strict boom.ts
boom.ts:18:28 - error TS2366: Function lacks ending return statement and return type does not include 'undefined'.

18     public boomProperty(): number {
                              ~~~~~~


Found 1 error.

🙂 Expected behavior

I expected there to be 0 errors because there is no error for boomFunction(), only boomProperty().

Workarounds

• add return Outer.boom().
• remove the --strict flag.

[MartinJohns]

Duplicate of #12825.

Although I'm surprised only one of those two errors. This changed with TypeScript 3.7, so it's most likely related to the changes of #32695. I would guess the same rules for assertion-functions apply here.

The --strict flag is only loosely related. Without it you most likely have strictNullChecks and noImplicitReturns disabled. The former allows undefined to be a valid value for number, and the latter allows to omit a return statement (resulting in an implicit undefined return value). Having either of these two flags enabled will result in an error.

[olafurpg]

@MartinJohns Nice find! That issue is indeed related but I don't think it's a duplicate. I removed the if condition and I'm still able to reproduce this issue, indicating it's unrelated to control flow. I updated the reproduction above to remove the if condition.

[MartinJohns]

It's still related to control flow. It doesn't consider the Outer.boom() method to be "terminating" (aka never returning). The initial example of #12825 already works, but there have been several comments regarding this behaviour in that issue.

[olafurpg]

@MartinJohns You're right, this issue is about control flow. This is indeed a duplicate. Thank you!