main.js

// @flow


declare function getElem(name: 'a'): HTMLCollection<HTMLAnchorElement>;
declare function getElem(name: 'button'): HTMLCollection<HTMLButtonElement>;

function getElem(name) {
  if (name === 'a') {
    return new HTMLCollection(/*...*/);
  } else {
    return new HTMLCollection(/*...*/);
  }
};

let a: HTMLCollection<HTMLButtonElement> = getElem('a'); // Error
let b: HTMLCollection<HTMLAnchorElement> = getElem('a'); // No Error

output

> flow version
Flow, a static type checker for JavaScript, version 0.23.0
> flow check
main.js:4
  4: declare function getElem(name: 'a'): HTMLCollection<HTMLAnchorElement>;
                                                         ^^^^^^^^^^^^^^^^^ HTMLAnchorElement. This type is incompatible with
 15: let a: HTMLCollection<HTMLButtonElement> = getElem('a'); // Error
                           ^^^^^^^^^^^^^^^^^ HTMLButtonElement

main.js:15
 15: let a: HTMLCollection<HTMLButtonElement> = getElem('a'); // Error
                           ^^^^^^^^^^^^^^^^^ HTMLButtonElement. This type is incompatible with
  4: declare function getElem(name: 'a'): HTMLCollection<HTMLAnchorElement>;
                                                         ^^^^^^^^^^^^^^^^^ HTMLAnchorElement


Found 2 errors