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null comparison fails in switch statement with --strictNullChecks #8971
Description
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');
}
Activity
ahejlsberg commentedon Jun 5, 2016
In your first example the checker complains about the
nullcase because control flow analysis has narrowed the type offootonumberand thus concluded (rightly so) thatfoois can't benull. It is similar to the following:Now, the thing that's confusing is that we allow the
foo === nullin theifstatement 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
ifstatement is that we take that statement as an assertion thatxcould 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
nullandundefinedcases in switch statements to be consistent with #8452.bmayen commentedon Jun 5, 2016
Thanks for the explanation!