Confusing enum reverse string lookup behaviour in strict mode

## Summary Code below. Typescript in --strict mode will reject enum reverse lookups using arbitrary strings with this very confusing error message: > Element implicitly has an 'any' type because index expression is not of type 'number'. A workaround is to cast the string to a subset of all possible enum keys but that's impractical with large enums (or is there an easy way to say "any key of enum X" in TS?). *My real world use case: I have a react app with a `<select>` filter on various items, and a bunch of `<option>` values, all of which are part of an integer enum. I have to use the string representation in the dom, so the state of the filter has to be the string representation. When I check the item's enum value I already know it can only be one of those enum members.* This report is three-fold: 1. Is this behaviour intended? There are cases where it can definitely catch a reverse lookup that can fail. 2. If it is intended, then what is the correct way to declare that the lookup will always be valid? `let typeFilter: "FOO" | "BAR" = "FOO"` is impractical and increases risk of errors. 3. **Regardless of the above, the error message should be fixed**. The element highlighted (`typeFilter`) does not in fact have an "any" type, and `Type[typeFilter]` will not be `any` either (in fact, `Type[typeFilter] as Type` does not fix it). An error such as "Reverse lookup on enum Type may fail because typeFilter is of type 'string'" is much clearer already. ## Details **TypeScript Version:** 3.2.2 **Search Terms:** enum reverse string lookup **Code** ```ts // test.ts // Compile with `tsc --strict test.ts` export enum Type { FOO = 0, BAR = 1, } const Test = () => { // Change the following line to `let typeFilter: "FOO" | "BAR" = "FOO";` and it will work let typeFilter = "FOO"; if (Type[typeFilter] != Type.FOO) { return; } }; export default Test; ``` **Actual behavior:** ``` test.ts:9:11 - error TS7015: Element implicitly has an 'any' type because index expression is not…