Conditional types don't work with Mapped types when you extend enum keys

[niba]

TypeScript Version: 3.0.3, 3.1.0-dev.20180907

Search Terms:
mapped, conditional, enum, extends
Code

enum Commands {
  one = "one",
  two = "two",
  three = "three"
}

const SpecialCommandsObj = {
  [Commands.one]: Commands.one,
  [Commands.two]: Commands.two,
};

// a and b are correct
type a = Commands.one extends keyof typeof SpecialCommandsObj ? null : any; // null - ok
type b = Commands.three extends keyof typeof SpecialCommandsObj ? null : any; // any - ok

interface CommandAction {
  execute(element: any): any;
}

interface SpecialCommandAction {
  execute(element: null): any;
}

type CommandsActions = {
    [Key in keyof typeof Commands]:  Key extends keyof typeof SpecialCommandsObj ? SpecialCommandAction : CommandAction
};

const Actions: CommandsActions = {
  [Commands.one]: {
      // ERROR
    execute: (element) => {} // ELEMENT IS ANY, NOT NULL
  },
  [Commands.two]: {
      // ERROR
    execute: (element) => {} // ELEMENT IS ANY, NOT NULL
  },
  [Commands.three]: {
    execute: (element) => {}
  },
}

Expected behavior:
In the Actions object the argument of the execute method for Commands.one and Commands.two should be of type null, not any
Actual behavior:
element is always any,

type inference for type a and b are fine so it only occurs when you use mapped types

it works if you don't use enums as keys in SpecialCommandObj. Swap your SpecialCommandsObj with this one and it will start working

const SpecialCommandsObj = {
  "one": Commands.one,
  "two": Commands.two,
};

Playground Link: https://bit.ly/2QmdTIo