Internal error in tsc: "TypeError: Cannot read properties of undefined (reading 'flags')"

@sandersn is already working on this.

Since Apr 16, 2025.

### 🔎 Search Terms

TypeError: Cannot read properties of undefined (reading 'flags')

### 🕗 Version & Regression Information

- This is the behavior in every version I tried, and I reviewed the FAQ for entries about unhandled TypeScript errors

### ⏯ Repository Link

https://github.com/GaurangTandon/tsc-issue-repro

### 💻 Code

```ts
const UPDATE_MARKER_FUNC = 'updateMarkerPosition';
window[UPDATE_MARKER_FUNC] = (coordinates, color = 'red') => {};
```

### 🙁 Actual behavior

```text
/Users/blazegt/test-tsc/node_modules/typescript/lib/_tsc.js:122142
throw e;
^

TypeError: Cannot read properties of undefined (reading 'flags')
at getMembersOfSymbol (/Users/blazegt/test-tsc/node_modules/typescript/lib/_tsc.js:56777:19)
at getLateBoundSymbol (/Users/blazegt/test-tsc/node_modules/typescript/lib/_tsc.js:56787:11)
at getSymbolOfDeclaration (/Users/blazegt/test-tsc/node_modules/typescript/lib/_tsc.js:49588:43)
at getSymbolOfNode (/Users/blazegt/test-tsc/node_modules/typescript/lib/_tsc.js:49591:34)
at getFunctionExpressionParentSymbolOrSymbol (/Users/blazegt/test-tsc/node_modules/typescript/lib/_tsc.js:49598:198)
at getResolvedMembersOrExportsOfSymbol (/Users/blazegt/test-tsc/node_modules/typescript/lib/_tsc.js:56742:27)
at getExportsOfSymbol (/Users/blazegt/test-tsc/node_modules/typescript/lib/_tsc.js:49486:61)
at resolveAnonymousTypeMembers (/Users/blazegt/test-tsc/node_modules/typescript/lib/_tsc.js:57350:19)
at resolveStructuredTypeMembers (/Users/blazegt/test-tsc/node_modules/typescript/lib/_tsc.js:57698:11)
at getSignaturesOfStructuredType (/Users/blazegt/test-tsc/node_modules/typescript/lib/_tsc.js:58389:24)

Node.js v20.18.3
```

### 🙂 Expected behavior

Internal TypeScript errors should not be thrown.

### Additional information about the issue

_No response_