Switch statements should discriminate by class prototype #38373
Description
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');
}
}