instanceof type inference broken in an "if -> else if -> else if" chain

[stkomarov]

TypeScript Version: "typescript@2.0.6"

// A *self-contained* demonstration of the problem follows...
class Action1 {
  public constructor(public propA: string) {}
}

class ActionX {

}

class Action2 {
  public constructor(public propB: string) {}
}

class FooBar {

  public handleAction(action: Object): void {
    if (action instanceof Action1) {
      console.log(action.propA); // works
    } else if (action instanceof ActionX) {
      // The presense of this empty clause is essential
    } else if (action instanceof Action2) {
      console.log(action.propB); // throws compile error: Error TS2339: Property 'propB' does not exist on type 'never'.
    }
  }
}

Expected behavior:
The code to compile without an error.
Actual behavior:
The code above throws a compile error:
Error TS2339: Property 'propB' does not exist on type 'never'.

A workaround for the issue is to use an explicit cast for the last "else if" clause:

    } else if (action instanceof Action2) {
      const a = action as Action2;
      console.log(a.propB); // now it works
    }

[aluanhaddad]

This is correct, expected behavior. TypeScript has a structural type system and all type tests, even instanceof are structural. Since instances of ActionX are structurally equivalent to {}, the type with no members, and since all types are assignable to {}, the type checker determines that the ensuing else branch is unreachable.

[stkomarov]

This is correct, expected behavior. TypeScript has a structural type system and all type tests, even instanceof are structural. Since instances of ActionX are structurally equivalent to {}, the type with no members, and since all types are assignable to {}, the type checker determines that the ensuing else branch is unreachable.

The last else if clause is reachable, and the code worked just fine with TS 1.8.10.

As a side note, a workaround for the original issue is to use an explicit cast and the code will compile with typescript 2.0.6:

  if (action instanceof Action1) {
      console.log(action.propA); // works
    } else if (action instanceof ActionX) {
      // The presense of this empty clause is essential
    } else if (action instanceof Action2) {
      const a = action as Action2;
      console.log(a.propB); // now it works
    }

[aluanhaddad]

It is not reachable from the standpoint of structural assignability. Even though it is technically reachable at runtime, because instanceof at runtime compares two references, instanceof at compile time compares structurally. The following simplified example illustrates the behavior.

class ActionX {}
class FooBar {
    public handleAction(action: Object): void {
        if (action instanceof ActionX) {
            return;
        }
        action // has type never
    }
}