this is not a rollback, you need to remove null from the definition as well

otherwise there will be still errors:

this will still give you might be null errors when you're gonna read the state, which is again, idiomatic typescript - properties need to be defined on class if you gonna use them in advance, or to be more precise -> TS is using state definition from base class as it was not overridden in my component

I am too late in making this comment, but I don't see why removing this readonly is at all a fix. According to TypeScript rules, readonly does not affect whether a property is writable in the constructor.

The readonly should be here to guard against assigning to state outside the constructor, which is invalid according to both TypeScript and React rules.

Also, ideally, it would be nice to be able to use conditional types to make state abstract if the type of S is not empty; this would probably fix all the problems with state being left accidentally uninitialized without adding this null to the type. The default also should be null, and not {}, but apparently that breaks some of the mapped types that depend on S.

You are partially correct. Readonly properties are writable in the constructors of the the class that defines the property. Not in subclasses. But it is completely valid and recommended to do so in React. See microsoft/TypeScript#25134 (comment) for more details.

I would like to emphasize that union with null is the correct way to go as react state is null unless initialized. Therefore this typescript construction describes it in the best possible way.

what @magnetnation said.

The readonly should be here to guard against assigning to state outside the constructor, which is invalid according to both TypeScript and React rules.

That wont help, when you'll define state on your class via class property, it will override Base class readonly state and will left you just with state so you need to provide the readonly explicitly ( this might be issue on TS level )

readonly state from base class ( correct errors, but wrong runtime as no state has been defined )

readonly overridden:

The default also should be null, and not {}

yeah that's right, also props should be object not {} etc, which would introduce another breaking changes :)