Adding keyword-named properties to functions generates invalid declarations

[ark120202]

TypeScript Version: 4.0.0-dev.20200522

Search Terms:

Code

function foo() {}
foo.null = true;

Expected behavior:

Any syntactically valid output. Since namespaces can't hold keyword names one option is generating an object type with call signature:

declare var foo: {
    (): void;
    null: boolean;
}

This change would also allow to generate declarations for statically-analyzable dynamic property access expressions (i.e. foo["foo bar"] = true), which are currently omitted from declarations, resulting different behavior between in-project and out-of-project values.

Actual behavior:

declare function foo(): void;
declare namespace foo {
    var null: boolean;
    //        ^^^^^^^ TS2693: 'boolean' only refers to a type, but is being used as a value here.
    //  ^^^^^ TS1134: Variable declaration expected.
}

Playground Link: https://www.typescriptlang.org/play?ts=4.0.0-dev.20200522#code/GYVwdgxgLglg9mABMOcAUBKRBvAvgKBTgDowQAbcxAXkSgCcQBTAbiA

Related Issues:

[DanielRosenwasser]

We can generate a temp name and re-export with the right name.

declare function foo(): void;
declare namespace foo {
    var _null: boolean;
    export { _null as null };
}

Just make sure that everything else is still exported (i.e. yadda is accessible in the generated .d.ts file for the following).

function foo() {}
foo.yadda = 100;
foo.null = true;