Date.getMonth could return a union of all possible values

[nikeee]

According to the MDN docs, Date.getMonth() always returns a number between 0 and 11 (inclusive):

An integer number, between 0 and 11, representing the month in the given date according to local time. 0 corresponds to January, 1 to February, and so on.

Currently, the lib.d.ts definition states that this function returns number.

We could change it to return 0 | 1 | ... | 11 to encode that return semantic.

This would be beneficial if user code contains something like if (date.getMonth() == 12). The compiler can then infer that the entire if branch is dead code.
Also, it serves as a quick documentation to let the user know that the return value is in fact a 0-based (not 1-based) month.

I'm not sure if the narrowing of the return type would cause a breaking change.

If this is up for grabs, I'd take this.

[nikeee]

I think this issue actually belongs to the typescript main repository: microsoft/TypeScript#36401.