Exhaustive switch on a type union with a default case yields TS2339: Property 'x' does not exist on type 'never'

TypeScript Version: 2.0.0

Code

type TestType = 'foo' | 'bar';

interface TestInterface {
  type : TestType;
}

class Test {
  test = (param : TestInterface) : boolean => {
    switch (param.type) {
      case 'foo' : return true;
      case 'bar' : return false;
      default    : throw Error("Invalid type: "+ param.type); // TS2339: Property 'type' does not exist on type 'never'.
    }
  }
}

Expected behavior:
The code compiles with no errors.

Actual behavior:
tsc test.ts
test.ts(14,53): error TS2339: Property 'type' does not exist on type 'never'.

I understand the compiler knows that with the current code the default case can never be reached and the type property can not have any possible valid TestType value in this case but the whole point of having a default case is to limit the scope of the human error, compiling with a different TS version, compiler flags etc. It can also be considered a safe coding practice.

The workaround is easy but ugly and should not be necessary:

default    : throw Error("Invalid type: "+ (<any>param).type);

[HerringtonDarkholme]

The code above cannot go wrong in TypeScript land, as you have noted. The only reasonable error I can conceive is someone pass a value from the outside of TS or someone naughtily assert any.

In any case, I think your workaround is a good alarm of someone do wrong thing somewhere, though.

Indeed. Another example is a function called from a HTML template event handler with no type safety in which case you can just have a typo somewhere trigger the default case. This is why I believe TypeScript should allow seemingly unnecessary default cases.

[yahiko00]

I dislike this current behavior. This should not be an error. It is quite weird TypeScript could change the type of an object (to never or anything else).

[yortus]

If you can't trust that the passed in param is really a TestInterface at runtime, then you probably shouldn't assume it's an object with a type property. If for example param is actually null or undefined, then the code above will throw a TypeError trying to access the type property. If it's a string or number, then it will throw "Invalid type: ".

[RyanCavanaugh]

Seems wrong -- param should be of type TestInterface and param.type should be of type never

[RyanCavanaugh]

Seems to be fixed in Anders' current literal types PR

[ahejlsberg]

Fixed in #9407.