Switch statements should discriminate by class prototype

[karol-majewski]

TypeScript Version: Nightly (4.0.0-dev.20200505)

Search Terms:

• switch
• control flow
• discriminated union
• untagged union
• disjoin
• prototype
• constructor
• 2339

Code

class Failure {}

class Success {
    constructor(readonly value: string) { }
}

function main(result: Failure | Success): void {
    switch (result.constructor) {
        case Success:
            console.log(`Success! Your value is ${result.value}`); // Property 'value' does not exist on type 'Failure | Success'.
            break;
        case Failure:
            console.log('Oh noes');
    }
}

main(new Success("🍆"));

Output

"use strict";
class Failure {
}
class Success {
    constructor(value) {
        this.value = value;
    }
}
function main(result) {
    switch (result.constructor) {
        case Success:
            console.log(`Success! Your value is ${result.value}`); // Property 'value' does not exist on type 'Failure | Success'.
            break;
        case Failure:
            console.log('Oh noes');
    }
}
main(new Success("🍆"));

Compiler Options

{
  "compilerOptions": {
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "strictPropertyInitialization": true,
    "strictBindCallApply": true,
    "noImplicitThis": true,
    "noImplicitReturns": true,
    "useDefineForClassFields": false,
    "alwaysStrict": true,
    "allowUnreachableCode": false,
    "allowUnusedLabels": false,
    "downlevelIteration": false,
    "noEmitHelpers": false,
    "noLib": false,
    "noStrictGenericChecks": false,
    "noUnusedLocals": false,
    "noUnusedParameters": false,
    "esModuleInterop": true,
    "preserveConstEnums": false,
    "removeComments": false,
    "skipLibCheck": false,
    "checkJs": false,
    "allowJs": false,
    "declaration": true,
    "experimentalDecorators": false,
    "emitDecoratorMetadata": false,
    "target": "ES2017",
    "module": "ESNext"
  }
}

Playground Link: Provided

Expected behavior:

No compile-time error. result.value should be of type string. That's what is happening in runtime.

Actual behavior:

We get a type error.

Property 'value' does not exist on type 'Failure | Success'. (2339)

Note that it works if you use if statements.

class Failure {}

class Success {
    constructor(readonly value: string) { }
}

function main(result: Failure | Success): void {
    if (result instanceof Success) {
        console.log(`Success! Your value is ${result.value}`);
    }

    if (result instanceof Failure) {
        console.log('Oh noes');
    }
}

[RyanCavanaugh]

This code doesn't work in the presence of e.g. class GreatSuccess extends Success { ... } - is this an intentional gap?

[karol-majewski]

It's not — I haven't considered subclass relationships.

A bit of context. I'm using classes to model states in my program. They are never inherited from. My program yields well-defined results such a Success<T>, Failure, or a Timeout. These types form a union. I was looking for a neat way to disjoint that union. A switch statement seemed appropriate.

[RyanCavanaugh]

Duplicate #16035

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.