False positive for unused vars used as decorators of properties

[LucaPeng]

What version of TypeScript are you using?
3.1.4

What version of typescript-eslint-parser are you using?
20.1.0

What code were you trying to parse?

import {
  action,
  observable,
  useStrict,
  runInAction,
  IObservableArray,
} from 'mobx';
import orderReadService from '@client/model/purchase/orderReadService';
import {
  BizPurchaseOrderTO,
  PageTO,
  BizQueryOrderConditionTO,
} from '@client/types/thrift/purchase';
import { PURCHASE_ORDER_TYPE_ENUM } from '@constants/order';

useStrict(true);
export default class ListModalStore {
  // order list
  @observable
  orderList: IObservableArray<BizPurchaseOrderTO> = observable([]);
  @observable
  orderListPageInfo: PageTO = { currentPage: 1, perPage: 10, total: '5' };
}

What did you expect to happen?

no eslint error for the 'observable' which is imported from mobx.

What happened?

[eslint] 'observable' is defined but never used.

This is happened for the 'observable' which is imported from mobx.

[Primajin]

It's not only decorators. I'm having the problem, that since 20.1.0 (including 20.1.1) all my proptypes in react are considered unused.

import { bool, number, string } from 'prop-types';
...
class MyClass extends PureComponent<Props, StateAttributes> {
  public static propTypes = {
    height: number.isRequired,
    leftIcon: bool,
    text: string,
    value: string.isRequired
  };
...

all the above are considered unused in > 20.1+ while fine in <= 20.0