Are you submitting a bug report or a feature request?
Both :)
What is the current behavior?
In fact the typescript definition file is not tested and I tkink it leads easily to regressions.
Since last release of both final-form and react-final-form, I get this...

What is the expected behavior?
Sandbox Link
What's your environment?
Other information
On definitively repository, ts definitions files are tested against an example called test.ts, covering every use cases (only syntax), I think it is the way to solve it.
I know that you don't write TS yourself but it could be really nice.