Repeated mapped type inference causes incorrect typeToString results

[RyanCavanaugh]

TypeScript Version: 2.8.0-insiders.20180320

Search Terms: mapped type inference display typetostring

Code

Via https://stackoverflow.com/questions/50181650/type-inference-lost-inner-level-types-in-typescript

interface IAction {
    type: string;
}

type Reducer<S> = (state: S, action: IAction) => S

function combineReducers<S>(reducers: { [K in keyof S]: Reducer<S[K]> }): Reducer<S> {
    const dummy = {} as S;
    return () => dummy;
}

const test_inner = (test: string, action: IAction) => {
    return 'dummy';
}
const test = combineReducers({
    test_inner
});

const test_outer = combineReducers({
    test
});

// '{test: { test_inner: any } }'
type FinalType = ReturnType<typeof test_outer>;

var k: FinalType;
k.test.test_inner // 'string'

Expected behavior: FinalType's hover type should be { test: { test_inner: string } }

Actual behavior: Shows as { test: { test_inner: any } }

Playground Link: Link

[RyanCavanaugh]

Better repro from #25295

class Foo<A> {
  a: A;
}

class ObjectFoo<A> extends Foo<A> {
  constructor(private b: { [k in keyof A]: Foo<A[k]> }) {
    super();
  }
}

const a = new ObjectFoo({ b: new Foo<string>() });
const obj = new ObjectFoo({ a });

[weswigham]

@sandersn didn't we explcitly limit reverse mapped type output to one level deep for lack of a better method of stopping infinite recursion? Is this potentially related?

[Jomik]

This is a pretty big deal breaker for my validator library 😢 https://github.com/Jomik/cerberus
Is it not possible to either display more layers, 9 or so. Or detect recursion and display that rather than "any"?

[osdiab]

In my case of a recursive type exhibiting this error (#25860), sounds like displaying it with some kind of ellipsis or placeholder type would be nicer than just dropping down to any—that's a misleading type, given that it actually removes information from the display of the type. Not sure what concerns there would be though since I don't know how these serialized types ultimately get used, but in the case of VSCode type display the current behavior isn't great.