Conditional types break with property chaining

[AnyhowStep]

TypeScript Version: 3.5.1

Search Terms:

property chaining, generic, object type param, conditional type

Code

Modified version of Bruce Pascoe (@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 #32707
Except, it gives up and resolves the type to string, rather than any.

[AnyhowStep]

I know, I know, recursive type alias, bad.

Interfaces break, too,

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

Playground

It breaks in a different way, though.

[webstrand]

Looks like it's falling afoul of the instantiation limit

type check<T> = T extends Droste<{ x: string }> ? true : false;
type a = check<typeof _1>; // true
type b = check<typeof _2>; // false 
type c = check<typeof _3>; // Type instantiation is excessively deep and possibly infinite.

[AnyhowStep]

It seems like the workaround outlined in #32707 (comment) works,

/**
 * Uses the workaround discovered here,
 * https://github.com/microsoft/TypeScript/issues/32707#issuecomment-518347966
 * 
 * Now, everything resolves correctly.
 */
type NextDrosteFix<X extends number|string> = (
    Droste<{x:(X extends number ? string : number)}>
);
type NextDroste<T extends {x:number|string}> = (
    NextDrosteFix<T["x"]>
);
interface Droste<T extends {x:number|string}> {
    value: T,
    /**
     * Should alternate between string and number
     */
    droste: NextDroste<T>
};

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; //number
const _5 = _4.droste; //string
const _6 = _5.droste; //number
const _7 = _6.droste; //string
const _8 = _7.droste; //number
const _9 = _8.droste; //string

Playground

And, as always, I know it works but I don't know why it works.

[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.

[AnyhowStep]

Are array types also considered primitive types for interning purposes?

Array types never seem to give problems. Only object literal types.

[Edit]

Object literal types and interfaces and classes (when used as a type param constraint)

[jack-williams]

I think arrays and tuples are cached, but I would wait on confirmation.

[RyanCavanaugh]

[RyanCavanaugh]

Fixed by #45025.

With the added check that _3.droste is assignable to Droste<{ x: number }>, 4.5.0-dev.20210817 reports TS2322 and TS2589 ("Type instantiation is excessively deep and possibly infinite"). 4.5.0-dev.20210818 is clean, and the current nightly (7.1.0-dev.20260902.1) is also clean. The increased instantiation-depth limit allows the chained droste accesses to continue alternating as expected.