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

### 🔎 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_

Comienza con la reproducción vinculada en TypeScript Playground y compara los ejemplos de tuplas y objetos con la información de regresión de v4.2. Sigue la comprobación de asignabilidad de la firma genérica descrita en la issue y valida después que ambos pares estructuralmente idénticos compilan sin TS2322.

Escrito por el modelo de indexación a partir del texto del issue.