Disallow 'declare' modifier on private named properties

[ajafff]

TypeScript Version: 3.8.0-dev.20200119

Search Terms:

Code

class C {
    declare #foo: number;
    constructor() {
        this.#foo = 1;
    }
}

Expected behavior:

Error on declare modifier as it doesn't make sense for private named properties.

Actual behavior:

No compile error. Instead the emit is invalid. Depending on the target either the property declaration is missing (ESNext) or the whole downleveling is not done at all.

Playground Link: http://www.typescriptlang.org/play/?target=7&ts=3.8.0-dev.20200119#code/MYGwhgzhAEDC0G8BQ1XQCYFNRgE6egGIAzAe1IC5oA7AVwFsAjTXAbhTWFOogBddawXqVwAKAJSIOaNLwAWASwgA6EuWgBeaAEZ2MgL5J9QA

Related Issues:

[DanielRosenwasser]

Same with abstract too.

[ajafff]

abstract is already an error, there's even a test for that. Fix for declare is up at #36381