get and set Accessors Are Allowed in Ambient Contexts

In previous versions of TypeScript, the language didn’t allow get and set accessors in ambient contexts (like in declare-d classes, or in .d.ts files in general).The rationale was that accessors weren’t distinct from properties as far as writing and reading to these properties;however, because ECMAScript’s class fields proposal may have differing behavior from in existing versions of TypeScript, we realized we needed a way to communicate this different behavior to provide appropriate errors in subclasses.

As a result, users can write getters and setters in ambient contexts in TypeScript 3.6.

declare class Foo {
    // Allowed in 3.6+.
    get x(): number;
    set x(val: number): void;
}

In TypeScript 3.7, the compiler itself will take advantage of this feature so that generated .d.ts files will also emit get/set accessors.