microsoft / microsoft/TypeScript

Type alias circularly references itself

Open
#61,443 6 comments 2 reactions 0 assignees View on GitHub
Domain: check: Type Circularity Help Wanted Possible Improvement
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Description

### 🔎 Search Terms

circular reference, generic, error TS2456, recursive

### 🕗 Version & Regression Information

- This is the behavior in every version I tried, and I reviewed the FAQ for entries about circular reference

### ⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.8.2#code/C4TwDgpgBAYg9nAKuaBeKBvAsAKClAMwQH4AuKAIwQBsIBDAOwG5cBfFnXUSKAIToDOEAKK0AthAbAAPIgB8UdPCQooAMky58AYwAWAS2oATAE6TyAYQCuA4HDGiIEqQG0Auh3zcI5RB3a4XKrWtvaOzsCKfIIi4pIyAOS6AIwJchxAA

### 💻 Code

```ts
type FooType = {
foo?: boolean;
};

type BaseElement = FooType & {
children: CustomElement[];
type: T;
};

type CustomElement = BaseElement<'h1'>;

```

### 🙁 Actual behavior

When defining a recursive type structure using a generic type, TypeScript incorrectly reports a `Type alias 'CustomElement' circularly references itself.(2456)` error, even though the type should be resolvable.
It gets weird when you move `type: T;` line above the `children: CustomElement[];` and the error disappears. Also removing `FooType` will resolve the issue.

### 🙂 Expected behavior

In the first place the code should not throw any errors, in the second place the behavior should not change by reordering records of a type.

### Additional information about the issue

_No response_

Contributor guide

Open the contributing guide

Research direction

Start with the linked TypeScript Playground reproduction and compare the two member orderings in the generic intersection type. Trace the circular-reference diagnostic for recursive type aliases and determine why adding FooType changes it; done means the valid recursive structure no longer reports TS2456 and member order does not change the result.

Written by the indexing model from the issue text.

Assessment

Tech stack
typescript
Domain
compilers
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Clearly specified
Newbie friendliness
35/100

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