in allowJs, hovering on function with nested generic parameter causes error: set property on undefined

[HerringtonDarkholme]

TypeScript Version: 2.8.1, next

Search Terms: allowJs undefined

Code
func.ts

interface ComponentOptions<V> {
  watch: Record<string, WatchHandler<any>>;
}
type WatchHandler<T> = (val: T) => void;
declare function extend(options: ComponentOptions<{}>): void;
export var vextend = extend

app.js

import {vextend} from './func'
// hover on vextend
export var a = vextend({
  watch: {
    data1(val) {
      this.data2 = 1
    },
    data2(val) { }
  }
})

tsconfig.json

{
  "compilerOptions": {
    "checkJs": true,
    "allowJs": true
  }
}

Expected behavior:

Hovering on vextend in app.js. No error should happen in Code's output panel.

Actual behavior:

Error:

Error processing request. Cannot set property 'typeParameters' of undefined
TypeError: Cannot set property 'typeParameters' of undefined
    at assignContextualParameterTypes (/project/node_modules/typescript/lib/tsserver.js:36049:38)
    at checkFunctionExpressionOrObjectLiteralMethod (/project/node_modules/typescript/lib/tsserver.js:36336:29)
  // .... Elided, full error report in gist below

Playground Link: https://gist.github.com/HerringtonDarkholme/0efdffb4c6ec576a7cd274cc1c5042a0

Related Issue: reduced from vuejs/vetur#751