Comparing numeric enum to number narrows variable to 'never'

[ajafff]

Bug Report

🔎 Search Terms

enum, number, never

🕗 Version & Regression Information

• This changed between versions 3.8.3 and 3.9.7

⏯ Playground Link

Playground link with relevant code

💻 Code

enum E {
    a = 1,
    b = 2,
}

declare let v: E;
if (v !== 0) {
    v &= E.a;
}

🙁 Actual behavior

v is narrowed to never inside the IfStatement. The compound assignment gives a compile error: Type 'number' is not assignable to type 'never'.(2322)

🙂 Expected behavior

v is still of type E and there's no error.

[ahejlsberg]

The issue here is the oddball rule we have (for backwards compatibility reasons) that values of type number are assignable to any enum literal type. This rule also extends to the comparable relationship, meaning that any value of type number is comparable to any enum literal type. Where this gets us into trouble is when we use comparability to filter types in control flow analysis. In this specific example, we filter the type of v by removing everything that isn't comparable to 0--but because of the rule, every value in the enum is comparable to 0, and thus we're left with never. Nonsense!

I think the solution is to remove the strange rule from the comparable relationship. That will cause the example to error on the if conditional with the diagnostic "This condition will always return 'true' since the types 'E' and '0' have no overlap." Which is the behavior we want (as it is indeed true).