Regression: Generic function returning union of tuples is not assignable to identical type since v4.2

まだ誰も着手していません。

### 🔎 Search Terms

generic function assignability, union return type, tuple union, TS2322, regression v4.2, generic type alias

### 🕗 Version & Regression Information

- This changed in commit [34f0e32 Bump version to 4.2.3 and LKG](https://github.com/microsoft/TypeScript/commit/34f0e32dc667eb8c7b52b3fc4067316b4ae97774)
- This also reproduces on Nightly version

### ⏯ Playground Link

https://www.typescriptlang.org/play/?#code/C4TwDgpgBAShCGATAglAvFAPDAfACgEp0coBtAM3gBsBnCAGlgF0oAfM4AJwFcHY2o3AHaII5AJZCIiJgG4AsAChQkWAkQAhdFlyFiZSrT4wW7Ul16MYA4aIlSZCxUoD0LqAFFOnAPacAXFAAKuDQAORwSMhhUOI0UEI+wFDwNDTiAOZC8ABGVNDAPlAq4ZGaYQB0SgDGPkI0ycAQDdp4APqBZcgEnepaaCRtTq7uyLRFABbwYJD1UADu4sATUD45AFYQ1Y2hNEolakgA8huoGNj4RANQAN5QiHUQgYZ0slAAbtS8nVAAvgJ3B5SQIWCBvT5Ub78di2MSSaR-JwHMondb9HSXfSAx7PaivD5fJ78f7sbHA4o8MEEyFE6wwkRwhyIpQjTzePyBEKqCLqVHRWLxRLJVLpLK5fLFIoHHnHDYaSo1OotJoNVGtDqHRB8nqa1H9QayIA

### 💻 Code

```ts
type ReadA = () => [false, R] | [true, R | undefined];
type ReadB = () => [false, R] | [true, R | undefined];

// Error: Type 'ReadA' is not assignable to type 'ReadB'.
const test = (_: ReadA): ReadB => _;

// Also happens with object types
type ReadObjA = () => { done: false; value: R } | { done: true; value: R | undefined };
type ReadObjB = () => { done: false; value: R } | { done: true; value: R | undefined };

// Error: Type 'ReadObjA' is not assignable to type 'ReadObjB'.
const testObj = (_: ReadObjA): ReadObjB => _;
```

### 🙁 Actual behavior

The code fails to compile with error TS2322: Type 'ReadA' is not assignable to type 'ReadB'.

It appears that the compiler incorrectly widens the generic type parameter `R` in the source type.
Although `ReadA` is defined as `[false, R] | ...`, the error message reports the source type as having `[false, R | undefined]`.

It seems that during the generic signature assignability check, the undefined from the `[true, R | undefined]` branch is leaking into the inference of `R` for the entire union, causing the `false` branch to become `[false, R | undefined]`. This widened source type is then incompatible with the target's more strict `[false, R]` branch.

```
Type 'ReadA' is not assignable to type 'ReadB'.
Type '[true, R | undefined] | [false, R | undefined]' is not assignable to type '[false, R] | [true, R | undefined]'.
Type '[false, R | undefined]' is not assignable to type '[false, R] | [true, R | undefined]'.
Type '[false, R | undefined]' is not assignable to type '[false, R]'.
Type at position 1 in source is not compatible with type at position 1 in target.
Type 'R | undefined' is not assignable to type 'R'.
'R' could be instantiated with an arbitrary type which could be unrelated to 'R | undefined'.(2322)
```

```
Type 'ReadObjA' is not assignable to type 'ReadObjB'.
Type '{ done: false; value: R | undefined; } | { done: true; value: R | undefined; }' is not assignable to type '{ done: false; value: R; } | { done: true; value: R | undefined; }'.
Type '{ done: false; value: R | undefined; }' is not assignable to type '{ done: false; value: R; } | { done: true; value: R | undefined; }'.
Type '{ done: false; value: R | undefined; }' is not assignable to type '{ done: false; value: R; }'.
Types of property 'value' are incompatible.
Type 'R | undefined' is not assignable to type 'R'.
'R' could be instantiated with an arbitrary type which could be unrelated to 'R | undefined'.(2322)
```

### 🙂 Expected behavior

The code should compile without errors because `ReadA` and `ReadB` are structurally identical types.

### Additional information about the issue

_No response_

リンクされた TypeScript Playground の再現コードから始め、タプルとオブジェクトの例を v4.2 のリグレッション情報と比較します。issue で説明されているジェネリックシグネチャの代入可能性チェックを追跡し、その後、構造的に同一の両方のペアが TS2322 なしでコンパイルできることを検証します。

索引モデルが issue の本文から書いたものです。