This appears to be a regression of #2491 (& #12425?) **TypeScript Version:** 2.4.1 **Code** Expected that this snippet does not produce errors, but it does: ```ts enum Things { ONE, TWO } // Error: Type 'Things' is not assignable to type 'string'. type ThingMap = {[TKey in Things]: boolean}; ``` [[Playground](https://www.typescriptlang.org/play/index.html#src=enum%20Things%20%7B%20ONE%2C%20TWO%20%7D%0A%2F%2F%20Error%3A%20Type%20'Things'%20is%20not%20assignable%20to%20type%20'string'.%20%0Atype%20ThingMap%20%3D%20%7B%5BTKey%20in%20Things%5D%3A%20boolean%7D%3B%0A)]