microsoft / microsoft/TypeScript

Ghost error in a circular situation

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Bug Domain: check: Error Instability Help Wanted
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Description

### 🔎 Search Terms

ghost error circular error mapped type

### 🕗 Version & Regression Information

- This changed in commit or PR https://github.com/microsoft/TypeScript/pull/58337

### ⏯ Playground Link

_No response_

### 💻 Code

```ts
///

// @strict: true
// @target: esnext
// @lib: esnext

//// interface ZodType {
//// optional: "true" | "false";
//// output: T;
//// }
////
//// interface ZodString extends ZodType {
//// optional: "false";
//// }
////
//// type ZodShape = Record;
//// type Prettify = { [K in keyof T]: T[K] } & {};
//// type InferObjectType = Prettify<
//// {
//// [k in keyof Shape as Shape[k] extends { optional: "true" }
//// ? k
//// : never]?: Shape[k]["output"];
//// } & {
//// [k in keyof Shape as Shape[k] extends { optional: "true" }
//// ? never
//// : k]: Shape[k]["output"];
//// }
//// >;
//// interface ZodObject extends ZodType> {
//// optional: "false";
//// }
////
//// interface ZodOptional>
//// extends ZodType {
//// optional: "true";
//// }
////
//// declare function object(shape: T): ZodObject;
//// declare function string(): ZodString;
//// declare function optional>(schema: T): ZodOptional;
////
//// const Category = object({
//// name: string(),
//// get parent/*1*/() {
//// return optional(Category);
//// },
//// });
////
//// export const output = Category.output;

verify.quickInfoAt("1", `(getter) parent: any`);
verify.getSemanticDiagnostics([]);
```

### 🙁 Actual behavior

Depending on the presence of `verify.quickInfoAt("1", `(getter) parent: any`);` the compiler produces 1 or 2 errors (observable in the expected output of the failing `verify.getSemanticDiagnostics([])` assertion).

### 🙂 Expected behavior

I'd expect the error count to be the same.

### Additional information about the issue

Relates to https://github.com/microsoft/TypeScript/issues/62180

An extra test case that currently works OK but that suffered from a similar issue in the past:

an extra test case

```ts
///

// @strict: true
// @target: esnext
// @lib: esnext

//// interface ZodType {
//// optional: "true" | "false";
//// output: any;
//// }
////
//// interface ZodString extends ZodType {
//// optional: "false";
//// output: string;
//// }
////
//// type ZodShape = Record;
//// type Prettify = { [K in keyof T]: T[K] } & {};
//// type InferObjectType = Prettify<
//// {
//// [k in keyof Shape as Shape[k] extends { optional: "true" }
//// ? k
//// : never]?: Shape[k]["output"];
//// } & {
//// [k in keyof Shape as Shape[k] extends { optional: "true" }
//// ? never
//// : k]: Shape[k]["output"];
//// }
//// >;
//// interface ZodObject extends ZodType {
//// optional: "false";
//// output: InferObjectType;
//// }
////
//// interface ZodOptional extends ZodType {
//// optional: "true";
//// output: T["output"] | undefined;
//// }
////
//// declare function object(shape: T): ZodObject;
//// declare function string(): ZodString;
//// declare function optional(schema: T): ZodOptional;
////
//// const Category = object({
//// name: string(),
//// get parent/*1*/() {
//// return optional(Category);
//// },
//// });
////
//// export const output = Category.output

verify.quickInfoAt("1", `(getter) parent: ZodOptional>;
}>>`);
verify.getSemanticDiagnostics([]);
```

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start with the embedded fourslash.ts repro and compare diagnostics with and without verify.quickInfoAt("1", `(getter) parent: any`). Trace the circular mapped-type case introduced by PR 58337, using the failing verify.getSemanticDiagnostics([]) assertion as the check. Done means both variants produce the same error count while the reported quick info remains correct.

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
Mostly clear
Newbie friendliness
35/100

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