Inconsistent `typeof this` when the `this` parameter declared in the signature of a function parameter

まだ誰も着手していません。

### 🔎 Search Terms

"this parameter", "typeof this"

### 🕗 Version & Regression Information

- This changed between versions 5.9.3 and 6.0.x

### ⏯ Playground Link

https://www.typescriptlang.org/play/?ts=6.0.3#code/JYOwLgpgTgZghgYwgAgILIN7LgLmQZzClAHNkBfAKEo0uWQHoHlRJZEUAhTZEgCjAALYPjyoAlHgBuAe2AATCnUbMAesgDKqALIBRZAAUASgHlOAGV3blraPCTJuWEngHDRaccgC8APmSyCkrKMACuIAhgwDIgyDB8APp4nJIBcooYVCF8tPT0-F65efRgAJ4ADigAGj7IZZUyMHXuysVMeercfPiCMqEANooARigSrUr05OLU9PFF+XyF4yUtxSUV1bX1EI3NIssqHWjjVJPT2fO8i5gH28g13nUbu0L7a4f06qi3q3mnFNMskUwhEojE4olXK8PBIfP5AvJUgjMFlZnwQZForECjc2swACobZDQKAyKCuABMADYABwAZi8AHJoYyWABbcr9YAIYBgfqlZCCOD4J6VZCMuAgUqskYIOChfAoXnIeQyCAikAyMCCuBSFBwUX6kCasBwMEgAB043aAAEwPgALQQAAelUiTqgpKg4zuD0NL1+UxmEIx5quS2K0J9RL92wDbzy7U+xzOwfioax4dxeV9W2eTSjeKO30jgcBlCAA

### 💻 Code

```ts
interface A { a: string }

{
// interface B { g(this: A): void }
// ^ SAME PROBLEM
interface B { g: (this: A) => void }

function f(_: B): void {}

f({
g() {
type X = typeof this
// ^ B (should be A)
}
})

f({
g() {
this
type X = typeof this
// ^ A
}
})

f({
g() {
type X = typeof this
// ^ A
this
}
})
}

{
function f(_: (this: A) => void): void {}

f(function g() {
// Type error: (2683) 'this' implicitly has type 'any' because it does not have a type annotation.
// @ts-expect-error
type X = typeof this
})

f(function g() {
this
type X = typeof this
// ^ A
})

f(function g() {
type X = typeof this
// ^ A
this
})
}
```

### 🙁 Actual behavior

When declaring the `this` parameter in the signature of a function parameter (e.g., a callback function like `function f(callback: (this: A) => void): void {}`), the inferred `typeof this` in the function body can be inconsistent. If `this` is used in the function body, `typeof this` behaves as expected. If `this` is not used, `typeof this` is inferred as if the `this` parameter was never declared in the signature.

### 🙂 Expected behavior

The inferred `typeof this` should be consistent regardless of the usage of `this` in the function body.

### Additional information about the issue

I can reproduce this problem also in `@typescript/native-preview: 7.0.0-dev.20260705.1`.

Locally I used the "default" `tsconfig.json` (`tsc --init`/`tsgo --init`):

```jsonc
{
// Visit https://aka.ms/tsconfig to read more about this file
"compilerOptions": {
// Environment Settings
// See also https://aka.ms/tsconfig/module
"module": "nodenext",
"target": "esnext",
"types": [],

// Other Outputs
"sourceMap": true,
"declaration": true,
"declarationMap": true,

// Stricter Typechecking Options
"noUncheckedIndexedAccess": true,
"exactOptionalPropertyTypes": true,

// Recommended Options
"strict": true,
"jsx": "react-jsx",
"verbatimModuleSyntax": true,
"isolatedModules": true,
"noUncheckedSideEffectImports": true,
"moduleDetection": "force",
"skipLibCheck": true,
}
}
```

リンクされた TypeScript Playground の再現コードから始め、5.9.3 と 6.0.3 の動作を比較し、コールバックの `this` パラメーターの文脈依存の型付けに注目してください。this 型の推論に関するコンパイラーテストを特定し、3 つのオブジェクトメソッドの例と単独のコールバックをカバーする回帰ケースを追加してください。`typeof this` が、実行時の `this` の使用を必要とせずに一貫して A に解決されれば完了です。

索引モデルが issue の本文から書いたものです。