Regression: something is wrong with type checking in dev.20180919

[qwertie]

TypeScript Version: Version 3.1.0-dev.20180919

Code

This code compiles in v3.0.3 but fails in 3.1.0-dev.20180919.

/** Represents a React/preact component with a state property and setState method. */
export type StatefulComponent<S> = {
  state: S, setState: (s: any /*Partial<S>*/) => any
};

export function f<This extends StatefulComponent<S>, S=This["state"]>(component: This): void
{
  if (Math.random()>0.5) f(component);
}

Expected behavior:

Compiles without error.

Actual behavior:

hold.ts:8:28 - error TS2345: Argument of type 'This' is not assignable to parameter of type 'StatefulComponent<This["state"]>'.
  Type 'StatefulComponent<S>' is not assignable to type 'StatefulComponent<This["state"]>'.
    Types of property 'state' are incompatible.
      Type 'S' is not assignable to type 'This["state"]'.

8   if (Math.random()>0.5) f(component);

Playground Link: No error in Playground.

You can work around this for now by adding manual type arguments f<This, S>(component).

[ahejlsberg]

This is actually working as intended. The error is caused by fixes in #26698 which improve accuracy of reasoning over constraints of indexed access types (types of the form T[K]).

The following simplified repro previously wasn't an error but now is:

function f1<S, T extends { state: S }>(s: S, t: T["state"]) {
    s = t;
    t = s;  // Error, S not assignable to T["state"]
}

The error definitely is correct--T["state"] constrained to S meaning that it is assignable to S, but not the other way around. It is essentially the same error as we report here:

function f2<S, T extends S>(s: S, t: T) {
    s = t;
    t = s;  // Error, S not assignable to T
}

[qwertie]

Well, this raises the question: I want to say T extends StatefulComponent<S> for any S; is there a better way to do it that wouldn't cause this error? (Edit: I assume T extends StatefulComponent<any> would do the job well enough, even if it weakens type checking in some way...)

Why not just f<S>(component: StatefulComponent<S>): void?

[qwertie]

Good point, @Andy-MS. I should mention that the repro example code above is not the real code, and in real code I may have a good reason to use a type parameter T to preserve a more specific type. By now I have uninstalled the dev build (since I only installed it to verify a different bug which, it turned out, was fixed in 3.1-dev.)