Bloomberg feedback for 5.9

### Comment

We evaluated the 5.9 RC releases and 5.9 is a low impact release for us.

| # | Change | Affects | Release notes | Packages affected | PR / Issue |
| - | -------------------------------------------------------------------------- | ---------------- | ------------- | ----------------- | ---------- |
| 1 | Class expressions with private fields produce invalid `.d.ts` output | Declaration Emit | No | <1% | [#62153](https://github.com/microsoft/TypeScript/issues/62153) |
| 2 | `Promise.all` over constant tuples no longer infers tuple | Type Checking | No | <1% | [#62071](https://github.com/microsoft/TypeScript/issues/62071) |
| 3 | JavaScript emit removes parentheses around `??` in conditional expressions | Emit | No | <1% | – |
| 4 | Better detection of always-true errors in `??` expressions | Type Checking | No | <1% | – |

## Class expressions with private fields produce invalid `.d.ts` output

With TypeScript 5.9, the following code:

```ts
export const ClassExpression = class {
#context = 0
};
```

Produces invalid declaration output:

```ts
export declare const ClassExpression: {
new (): {
#context: number;
};
};
```

5.8 output was also incorrect but at least syntactically valid:

```ts
export declare const ClassExpression: {
new (): {
"__#1@#context": number;
};
};
```

❗This seems like a major regression, since emitted declarations are now invalid.
Tracked in [#62153](https://github.com/microsoft/TypeScript/issues/62153)

## `Promise.all` over array literal no longer infers a tuple

This issue is easier to reproduce than the linked ticket may suggest. It affects any use of `Promise.all` in the return position of a `.then()` that chains from a promise resolving to an array:

```ts
const p = Promise.resolve([]).then(() => {
return Promise.all([0, ""]);
});
const test: Promise<[number, string]> = p; // error
```

This now fails to infer a tuple and infers `Promise<(string | number)[]>` instead.
While we can work around this issue by assigning the result of `Promise.all` to an intermediate variable, this does seem like an unfortunate regression.

Tracked in [#62071](https://github.com/microsoft/TypeScript/issues/62071)

## JavaScript emit removes parentheses around `??` in conditional expressions

In some cases, parentheses are removed from nullish coalescing expressions in emitted JavaScript. This change seems benign.

```ts
(test ?? true) ? A : B
```

Becomes:

```ts
test ?? true ? A : B
```

## Better detection of always-true errors in `??` expressions

We observed two new diagnostics related to always-true `??` expressions.
The improved detection highlighted logic errors that were previously missed.

Start with the reproduction snippets in the issue and the linked tickets #62153 and #62071, which track two of the reported regressions. Compare the 5.8 and 5.9 declaration or type-checking behavior, then scope any work to one confirmed regression; the report itself does not identify files, tests, or a single definition of done.

Written by the indexing model from the issue text.