I ran across this while migrating my project to strictNullChecks and it's positively bizarre.

TypeScript Version: master (50f442f)

Search Terms: mapped type narrow narrowing null strictNullChecks never

Code (with strictNullChecks)

let mappedObject: {[K in "foo"]: null | {x: string}} = {foo: {x: "hello"}};
declare function foo<T>(x: T): null | T;

function bar<K extends "foo">(key: K) {
  const element = foo(mappedObject[key]);
  if (element == null)
    return;
  const x = element.x;
}

Expected behavior: Compile without error.

Actual behavior: Error: Property 'x' does not exist on type 'never'.

Playground Link: link (remember to enable strictNullChecks)

Related Issues: Didn't find any.