a.js

// @flow
var a = 42;
export default a;

main.js

// @flow

import type a from "./a.js";

output

> flow version
Flow, a static type checker for JavaScript, version 0.23.0
> flow check
main.js:3
  3: import type a from "./a.js";
                 ^ Default import from `./a.js`. The default export is a value, but not a type. `import type` only works on type exports like type aliases, interfaces, and classes. If you inteded to import the type *of* a value, please use `import typeof` instead.


Found 1 error