The types passed to the ThunkAction generic type here aren't important. What is important is that I was using the ActionCreator type from the redux library. The reason this is important is that when, during refactoring, I changed the action creator so that instead of a single argument (x) it now requires two (x and y):
I fully expected typescript to complain in the places where someActionCreator was invoked with a single argument. When it didn't, I cussed, went to investigate, and found that the ActionCreator type is defined in the redux library as follows:
Bloody any! Now, I don't know whether there is a proper way to define the type of ActionCreator in typescript at all; but by using any we have just lost the information about how many and what kind of arguments someActionCreator will require.
P.S.: found the discussion of this problem in redux's github issues.