"useDefineForClassFields" breaks ambient public fields

[robpalme]

TypeScript Version: 3.8.0-dev.20191105

Search Terms: declare ambient public class field useDefineForClassFields

Code

// Remember to enable compiler flag "useDefineForClassFields"
class C {
    declare field : any;
}

Expected behavior:

Ambient class fields should not be emitted.

Actual behavior:

The class field is included in the emitted JS code.

class C {
    constructor() {
        Object.defineProperty(this, "field", {
            enumerable: true,
            configurable: true,
            writable: true,
            value: void 0
        });
    }
}

When "useDefineForClassFields" is disabled, the declare keyword works correctly and omits the field. When "useDefineForClassFields" is enabled, the declare keyword fails and has no effect on emit. This means the field gets defined on the instance, even though the programmer wanted it to be omitted.

Playground Link:

• Minimal repro
• Amusing example from the 3.7 release announcement

Related Issues: #34942