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 el ejemplo autocontenido de enum y union con el punto de entrada de línea de comandos de tsc y confirma el comportamiento incorrecto de narrowing. Rastrea cómo el compilador comprueba las comparaciones de enum con miembros de union y añade cobertura que muestre que el caso Crab no puede acceder a las propiedades de IFish y que se aplica el comportamiento esperado indicado.