Type argument with a subset of the parameter constraint's optional keys incorrectly reported as unassignable to constraint

Nobody has claimed this yet.

### 🔎 Search Terms

"does not satisfy the constraint" "keyof" "optional"

### 🕗 Version & Regression Information

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

### ⏯ Playground Link

https://www.typescriptlang.org/play/?#code/C4TwDgpgBAsiAq4IB54BooFUoQB7AgDsATAZygG8oBtAaSgEtCoBrCEAewDMp4BdAPwAuKAFdCLQhwDuzAL4A+KAF4sAbgCwAKG0BjADYBDUuTgBhIydRKK2qFABuh-aIgBCEXESRUGKnUZmNk4eeCgAMihSYAAnJgBzQRFxSRl5BU0tOW0gA

### 💻 Code

```ts
type MyType = U;

class MyClass {
value!: MyType;
}
```

### 🙁 Actual behavior

The following error is reported:

```
Type '{ [K in keyof T & string]?: unknown; }' does not satisfy the constraint '{ [K in keyof T]?: unknown; }'.
Type 'keyof T' is not assignable to type 'keyof T & string'.
Type 'string | number | symbol' is not assignable to type 'keyof T & string'.
Type 'string' is not assignable to type 'keyof T & string'.
Type 'string' is not assignable to type 'keyof T'.
Type 'keyof T' is not assignable to type 'string'.
Type 'string | number | symbol' is not assignable to type 'string'.
Type 'number' is not assignable to type 'string'.
```

### 🙂 Expected behavior

No error should be reported because due to the keys being optional, `{ [K in keyof T & string]?: unknown }` is a subtype of `{ [K in keyof T]?: unknown }` for all `T`.

### Additional information about the issue

_No response_

Reproduce the diagnostic from the linked TypeScript Playground example, then trace generic constraint checking for mapped types with optional properties and intersected keyof keys. The work is done when the example compiles without an error while preserving correct constraint checking for related cases.

Written by the indexing model from the issue text.