null comparison fails in switch statement with --strictNullChecks

[bmayen]

TypeScript Version:

nightly (1.9.0-dev.20160604-1.0)

Code

let foo:number|null = 0;

switch(foo) {
  case null:
    break;
  case 0:
    console.log('bar');
}

Expected behavior:
Null check should be allowed

Actual behavior:
Compiler complains "TypeScript Type 'null' is not comparable to type 'number'. (TS2678)"

Note:
If I add an 'if' condition before the switch, the compiler does not complain about either the 'if' or the switch. For instance:

let foo:number|null = 0;

if (foo === null) { console.log('bar'); }

switch(foo) {
  case null:
    break;
  case 0:
    console.log('bar');
}

[ahejlsberg]

In your first example the checker complains about the null case because control flow analysis has narrowed the type of foo to number and thus concluded (rightly so) that foo is can't be null. It is similar to the following:

let x: string | number;
x = "foo";  // x is known to be a string in the following
switch (x) {
    case 0:  // Error, x can't be number
    case 1:  // Error, x can't be number
    case "foo":
    case "bar":
}

Now, the thing that's confusing is that we allow the foo === null in the if statement in your second example. See #8452 for the reasoning on why that's permitted. We should probably extend that to switch statements as well.

The reason it works when you insert the if statement is that we take that statement as an assertion that x could be null at that point, even though control flow analysis has concluded it couldn't. See #8548 for the reasoning on that feature.

To sum up, the issue here is that we should always allow null and undefined cases in switch statements to be consistent with #8452.

[bmayen]

Thanks for the explanation!