Conditional types break with property chaining

TypeScript Version: 3.5.1

Search Terms:

property chaining, generic, object type param, conditional type

Code

Modified version of @fatcerberus ’ attempt to break TS.

type Droste<T extends {x:number|string}> = {
    value: T,
    /**
     * Should alternate between string and number
     */
    droste: Droste<{x:(T["x"] extends number ? string : number)}>
};

declare const droste: Droste<{x:number}>;

//number
const _0 = droste.droste.droste.droste.droste.droste.droste.droste.droste.droste.droste
    .droste.droste.droste.droste.droste.droste.droste.droste.droste.droste
    .droste.droste;
const _1 = _0.droste; //string
const _2 = _1.droste; //number
const _3 = _2.droste; //string

const _4 = _3.droste; //Expected number, actual string, ???
const _5 = _4.droste; //string
const _6 = _5.droste; //string
const _7 = _6.droste; //string
const _8 = _7.droste; //string
const _9 = _8.droste; //string
//string forever and ever. Where is `number`?

Expected behavior:

Each access to .droste should alternate between string and number. Or give a max instantiation depth/count error.

Actual behavior:

It alternates between string and number and then breaks after while. From that point, it just sticks with string.

No errors given.

Playground Link:

Playground

Related Issues:

It is similar to https://github.com/microsoft/TypeScript/issues/32707 Except, it gives up and resolves the type to string, rather than any.

[jack-williams]

If I had to guess (and it really is a guess), there is a caching of instantiations done on type id. With your original example you create a new object literal type (with a new id) for each recursive instantiation that will have no cached value.

In the modified example each the recursion goes through NextDrosteFix which is instantiated with a primitive type (and primitive types are interned). NextDrosteFix will be cached for the instantiations at string and number.

[RyanCavanaugh]