Bloomberg feedback for 5.9

[dragomirtitian]

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
2	Promise.all over constant tuples no longer infers tuple	Type Checking	No	<1%	#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:

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

Produces invalid declaration output:

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

5.8 output was also incorrect but at least syntactically valid:

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

❗This seems like a major regression, since emitted declarations are now invalid.
Tracked in #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:

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

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.

(test ?? true) ? A : B

Becomes:

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.

[RyanCavanaugh]

The 5.8 behavior of Promise.all is kind of surprising to me -- normally our inference rules would produce either [0, ""] (like in as const) or (string | number)[] (like in a let initialized with an array).

I would recommend this as a workaround:

type Tuple2 = [unknown, unknown];
const p = Promise.resolve([]).then(() => {
    return Promise.all([0, ""] satisfies Tuple2);
});
const test: Promise<[number, string]> = p; // ok

or

// Make as long as needed
type Tuple = [unknown?,unknown?,unknown?,unknown?,unknown?,unknown?,unknown?,unknown?];
const p = Promise.resolve([]).then(() => {
    return Promise.all([0, ""] satisfies Tuple);
});
const test: Promise<[number, string]> = p; // error