Cant infer type of parent based on member of child

<!-- 🚨 STOP 🚨 𝗦𝗧𝗢𝗣 🚨 𝑺𝑻𝑶𝑷 🚨 Half of all issues filed here are duplicates, answered in the FAQ, or not appropriate for the bug tracker. Even if you think you've found a *bug*, please read the FAQ first, especially the Common "Bugs" That Aren't Bugs section! Please help us by doing the following steps before logging an issue: * Search: https://github.com/Microsoft/TypeScript/search?type=Issues * Read the FAQ: https://github.com/Microsoft/TypeScript/wiki/FAQ Please fill in the *entire* template below. --> <!-- Please try to reproduce the issue with `typescript@next`. It may have already been fixed. --> **TypeScript Version:** 3.4.5 <!-- Search terms you tried before logging this (so others can find this issue more easily) --> **Search Terms:** - infer parent based on child - type inference in if statement - conditional generic type infer **Code** ```ts const enum ChildType { A, B, C } interface ChildrenMap { [ChildType.A]: { value: 10 }, [ChildType.B]: { value: string }, [ChildType.C]: { value: boolean, test?: boolean } }; // Create a "Child", that has a type and // other members based on what type it is type Child<T extends ChildType> = { type: T } & ChildrenMap[T]; interface ParentMap { [ChildType.A]: { color: 'red' }, [ChildType.B]: { color: 'blue' }, [ChildType.C]: { color: 'green' } } // Create a "Parent", that has a "Child", which // has a type, which decides what members the // parent should have type Parent<T extends Child<any>> = T extends Child<infer R> ? R extends ChildType ? ParentMap[R] & { child: Child<R> } : never : never; // More of the same stuff! interface GrandParent<T extends Parent<Child<any>>> { parent: T extends Parent<Child<infer R>> ? R extends ChildType ? Parent<Child<R>> : never : never; } // It all works as expected at this point let A: Parent<Child<ChildType.A>>; let red: typeof A['color'] = 'red'; let justRed: typeof A['color'] = 'blue'; // error! // …