Checking union type where key is enum tricks compiler

TypeScript Version: 3.9.2

Search Terms:
enum refinement
checking interface enum

Code

// A *self-contained* demonstration of the problem follows...
// Test this by running `tsc` on the command-line, rather than through another build tool such as Gulp, Webpack, etc.
enum Fish {
    blue= 1,
    red= 2
}

enum Crab {
    purple= 2
}

interface IFish {
    type: Fish,
    name: string,
}

interface ICrab {
    type: Crab,
    shell: string
}

const crab = {type: Crab.purple, shell: "hard"}

function determineAnimal(animal: ICrab | IFish) {
    if (animal.type === Fish.red) {
        console.log("fish")
        console.log(animal.name)
    } else if (animal.type === Crab.purple) {
        console.log("crab")
        console.log(animal.shell)
    } else {
        console.log("fell through")
    }
}

determineAnimal(crab)

Expected behavior: The program does not typecheck because checking against the enum does not prove the animal is an IFish

Actual behavior: The program typechecks and incorrectly determines the type of the crab, logging undefined.

Playground Link: link

Related Issues:

Reproduce the self-contained enum and union example with the tsc command-line entry point and confirm the incorrect narrowing behavior. Trace how the compiler checks enum comparisons against union members, then add coverage showing that the Crab case cannot access IFish properties and that the reported expected behavior is enforced.