// A *self-contained* demonstration of the problem follows...
// Test this by running `tsc` on the command-line, rather than through another build tool such as Gulp, Webpack, etc.
type EventType = 'click' | 'dblclick'

const handlerMap: { [P in EventType]?: any[] } = {}

function addHandler<P extends EventType>(evType: P) {
  const handlerList = handlerMap[evType] || []
  handlerList.push({}) // Error here: Object is possibly 'undefined'
  handlerMap[evType] = handlerList
}