Description
Bug Report
🔎 Search Terms
discriminated union narrowing
🕗 Version & Regression Information
4.6.2
- This is the behavior in every version I tried, and I reviewed the FAQ for entries about discriminated unions and narrowing
⏯ Playground Link
Playground link with relevant code
💻 Code
The linked example above shows how to reproduce. The broken function in the example has x on line 35 inferred as { readonly type: "foo"; }. It should be narrowed in this position to never.
This behavior prevents the following common pattern from being used when you only have 1 member of a discriminated union (which is realistic if you are building the "first type" and plan to build others later):
// utility for catching non-exhaustive switches
class UnexpectedCase extends Error {
constructor(value: never) {
super(`Did not expect to reach case: ${JSON.stringify(value)}`);
}
}
interface FirstType { type: 'FirstType' }
type Types = FirstType; // later we will add 'SecondType' and so on
function transform(x: Types) {
switch (x.type) {
case 'FirstType':
return // create something else
default:
// we want to get a type error here in the future when we have missing cases, but we are already getting
// one now because the type system isn't narrowing `x` to `never`
throw new UnexpectedCase(x); // TS error: argument of type 'FirstType' is not assignable to never
}
}Yes I'm aware that this technically isn't a discriminated union, but it shouldn't make a difference. The point is that the type system isn't narrowing the type of a variable to never when it clearly (in a statically knowable way) cannot have a value in this branch.
🙁 Actual behavior
The type system is not narrowing x to never in the example above in the default case, even though this case cannot be reached.
🙂 Expected behavior
x should be narrowed to never in the default case because that code cannot be reached. This is the only sensible behavior if we want to say never is a bottom type.