The issue at hand lies with the assignment:
const value: string | null = 0 as any
Since the compiler recognizes that value is a constant, it understands that it can never be null.
Even if you use let, the situation doesn't improve much:
let value: string | null = 'foo';
if (isNotNull(value)) {
// `value` is of type `string` in this block
} else {
// `value` is of type `never` in this block
}
Once again, TypeScript's static analysis knows that the value cannot be null.
However, if you assign a value where TypeScript cannot deduce a constant, then you will find your anticipated string in the if condition and null in the else:
function isNotNull<T> (arg: T): arg is Exclude<T, null> {
return arg !== null
}
let value: string | null = foo();
if (isNotNull(value)) {
// `value` is of type `string` here
} else {
// `value` is of type `null` here
}
function foo(): string | null {
return "foo"
}
Even in this scenario, it only functions when the strictNullChecks compilation setting is enabled. Without strictNullChecks, you cannot eliminate null from the type, and the if branch defaults to being just string which aligns with string | null, hence resulting in never for the else section.
Edit:
The reason behind this not functioning without strictNullChecks is intriguing. In such instances, the types string | null and string are considered identical. Consequently, value fundamentally possesses the type string, leading Exclude<string, null> to essentially represent string (thus still encompassing null), leaving the else portion with type never for value.