Union types behave differently depending whether the variable is direct argument of the function

[zuzusik]

TypeScript Version:

1.8.5

Code

type Result = boolean | string;
type GetResult = () => Result;
type ResultParam = GetResult | Result;

function getResult(result : ResultParam) {
  return result instanceof Function 
    ? result() 
    : result;
}

function getResultFromParams(params : {result : ResultParam}) {
  return params.result instanceof Function 
    ? params.result() 
    : params.result;
}

Code in Playground

Expected behavior:
script is compiled

Actual behavior:
compilation error is thrown on function getResultFromParams:
(13,7): error TS2349: Cannot invoke an expression whose type lacks a call signature.

[Arnavion]

Type guards don't narrow types of property expressions. See #3812

Edit: You can destructure params as a workaround:

function getResultFromParams({ result }: { result: ResultParam}) {
  return result instanceof Function 
    ? result() 
    : result;
}