[isolatedDeclarations] Optional parameter properties are unnecessarily disallowed in isolatedDeclarations mode

🔎 Search Terms

optional parameter properties, isolated declarations, TS9025: Declaration emit for this parameter requires implicitly adding undefined to its type. This is not supported with --isolatedDeclarations

🕗 Version & Regression Information

• This was present since the introduction of isolatedDeclarations in TS5.5

⏯ Playground Link

https://www.typescriptlang.org/play/?downlevelIteration=true&importHelpers=true&target=99&module=1&isolatedDeclarations=true&ts=5.8.2&ssl=6&ssc=2&pln=1&pc=1#code/C4TwDgpgBAsghiARhAqgOwCYQGYEs0QZQC8UaArgLbIBOUAPlOZjvoQNwBQokUAchABuEGuix4CRUhWoiunCAA8wAexrAoAYwA2cAM56oAMRUqoAb05RrWlWj3Aa5TcDUAKGhDgY72kFEUAfgAuWARkMVZJABooT29ffxAQ-iERSIlCAEoLAF9OXKA

💻 Code

type MaybeUndefined = number | undefined;
type NeverUndefined = number;

export class Foo {
    constructor(readonly x?: MaybeUndefined, readonly y?: NeverUndefined) {}
}

🙁 Actual behavior

Gives an error:

Declaration emit for this parameter requires implicitly adding undefined to its type. This is not supported with --isolatedDeclarations.

🙂 Expected behavior

It seems like this could be safely supported by just always emitting the class fields with a type of |undefined whether that was redundant or not, something like:

export declare class Foo {
    readonly x?: MaybeUndefined | undefined;
    readonly y?: NeverUndefined | undefined;
    constructor(x?: MaybeUndefined, y?: NeverUndefined);
}

Additional information about the issue

The workaround of explicitly adding |undefined to the parameter property types is not too onerous, but still seems like an unnecessary restriction for isolatedDeclarations mode.