type `nothing` emitted in generated .d.ts files · Issue #8345 · microsoft/TypeScript · GitHub

function f() {
    var x: string | undefined = undefined;
    y = 0;
    if (x) {
       var y: typeof x;
    }
    return y; // y has type nothing
}

generates invalid .d.ts file

declare function f(): nothing;

this is a regression introduced by #8340

type nothing emitted in generated .d.ts files #8345