Contextual typing doesn't work when mixing up discriminated unions with function variant

### 🔎 Search Terms

contextual typing, discriminated unions

### 🕗 Version & Regression Information

- This is the behavior in every version I tried, and I reviewed the FAQ for entries
- Tested with TS 5.4.5 and TS 5.9.2

### ⏯ Playground Link

https://www.typescriptlang.org/play/?#code/C4TwDgpgBAcg9sA6nATgawJYDsDmBVLDOLKAXgCgoqoAfKACnoDcBDAGwFcIAuKLDgLYAjCCgCUZAHxQmcDABMxlanQDey6tQwBnAMrAU2HLwNcA3Bs1QAZll6NWnHlG0GjE0tNkKxFzQF9LNUsdfUNcAH5ea3ZtCD9qW3tmdi5efmFRDy85RQt-C3IAY2JXKGAIVwBGXngkVExcAiISUgYWbKhVAuLS4HLK4AAmWoRkdCNm4jIukL03XBMULgAaSyT2zu7yHpKsMorXAGZR+ommwmm29U1QheMbWIg1zQ36DqkuwN2+gdcAFlO40a+EurVmrzsm0+2x65FAkCgwMmYLIQQhViod3CD1M8UskOSjjSLnunW8eUsgU0wVu8xxUUebDiCSobxSTnSghE4k+FN8O0KewOgwArLxkRcWjMblp6UYlqt1lD3ltvhZhf1DsAAGwShoo6XXOZhBVMuIvRIqj6eL6C3r7LWDADs+vOoKNGLZ1rVgqAA

### 💻 Code

```ts
type NotWorkingUnion =
| ((value: number) => void)
| {
isString: true;
fn: ((value: string) => void);
}
| {
isString?: false;
fn: ((value: number) => void);
};

const test1: NotWorkingUnion = (a) => {}; // Works
const test2: NotWorkingUnion = { // Works
isString: true,
fn: (a) => {}
};
const test3: NotWorkingUnion = { // Works
isString: false,
fn: (a) => {}
};
const test4: NotWorkingUnion = { // Doesn't work!
fn: (a) => {}
};

type WorkingUnion =
| {
isString: true;
fn: ((value: string) => void);
}
| {
isString?: false;
fn: ((value: number) => void);
};

const test5: WorkingUnion = { // Works
isString: true,
fn: (a) => {}
};
const test6: WorkingUnion = { // Works
isString: false,
fn: (a) => {}
};
const test7: WorkingUnion = { // Works
fn: (a) => {}
};
```

### 🙁 Actual behavior

Contextual typing breaks when an union is both discriminated by a field and one of the union variants is just a function

### 🙂 Expected behavior

Contextual typing should correctly work, as the type system should have enough information to discriminate between a function and an object with or without the field

### Additional information about the issue

`strict` mode is enabled.

Reproduziere die Beispiele im verknüpften TypeScript Playground, indem du test4 mit test7 und den explizit diskriminierten Fällen vergleichst. Verfolge, wie die kontextuelle Typisierung die Union behandelt, die eine Funktionsvariante enthält, füge dann einen Regressionstest hinzu, der den Fall mit fehlendem Diskriminator abdeckt, und überprüfe, dass der Callback-Parameter korrekt inferiert wird.

Vom Indexierungsmodell aus dem Issue-Text verfasst.