microsoft / microsoft/TypeScript
Intersection for same key read-only property maybe wrong
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Description
### 🔎 Search Terms
Intersection readonly property
### 🕗 Version & Regression Information
- This changed between versions 4.4.4 and 4.5.5 (But all versions have errors)
### ⏯ Playground Link
https://www.typescriptlang.org/play/?ts=5.4.5#code/FAFwngDgpgBAgjAvDA3jA1gLhgOwK4C2ARlAE4wC+oksAQkqjKVAIYAmA9jgDZgbb5iZSsGAAKFBRgsAzvBgAyGLQCUAOnQMADAG5xk6XPpK46zcl2jw0GAFkW6KAHVSASxAsi3KAB4AKgB8DGgAtMzsXLwwANoA0jCuOBhQYBwAZjB+ALrYfnFZlHqiVjQwAMIM9o4u7p7ePrQBehJSsuVm2s0GbRVKqhqd+q1Giu0DFkVAA
### 💻 Code
```ts
type A = { k: number }
type B = { readonly k: number }
({} as A & B).k = 0;
({} as B & A).k = 0;
type MakeWritable = { -readonly [K in keyof T]: T[K] };
type C = MakeWritable;
({} as C).k = 0;
({} as C & B).k = 0;
({} as B & C).k = 0;
```
### 🙁 Actual behavior
command:
```
z:\webdl\test>tsc --target esnext --lib esnext test.ts
```
output:
```
test.ts:15:15 - error TS2540: Cannot assign to 'k' because it is a read-only property.
15 ({} as C & B).k = 0;
~
test.ts:16:15 - error TS2540: Cannot assign to 'k' because it is a read-only property.
16 ({} as B & C).k = 0;
~
Found 2 errors in the same file, starting at: test.ts:15
```
### 🙂 Expected behavior
no error.
### Additional information about the issue
_No response_
Contributor guide
Research direction
Start with the Playground link and the `test.ts` reproduction, then run the shown `tsc --target esnext --lib esnext test.ts` command. Trace TypeScript's checker handling of intersections and readonly properties; done means the two `C` intersection assignments compile without TS2540 while the reproduction remains covered.
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