Strange "Object is possibly 'null'" assumption

[simonwep]

Code

I've got the following code:

interface MyGenerator {
    nextItem(): null | string;
    throwError(): never;
}

type GeneratorMapperFn = (gen: MyGenerator) => string | null;
const mapTokenFunction = (fn: GeneratorMapperFn) => {
    return (gen: MyGenerator): string | null => {
        return fn(gen);
    };
};

// The error occurs at this "function", TS seems to know that "gen" implements "MyGenerator"
// but tells me instead that "next" could be null at the very end.
const mappedFunction = mapTokenFunction(gen => {
    const next = gen.nextItem();

    if (!next) {
        gen.throwError();
    }

    // TS2531: Object is possibly 'null'
    return next.toUpperCase();
});

TS Playground

It gets even more strange, if I replace the gen.throwError() with a throw "Foo" it works:

TS Playground

... or if gen got an explicit type-assignment ((gen: )):

TS Playground

Although TS tells me that gen is of type MyGenerator:

Expected behavior:
TS should correctly assume that next cannot be null after the conditional statement.

Actual behavior:
It throws TS2531: Object is possibly 'null'.

Related Issues:
#34795, #9998

[ilogico]

I think this is a current limitation.
I don't know where the documentation for this is, but TS needs explicit type annotations for the assert and never returning functions to contribute to control flow analysis.
For now, you can fix this error by annotating (gen: MyGenerator) => ...