Strange boolean-discriminant narrowing with strictNullChecks off

[gcnew]

TypeScript Version: nightly (2.1.0-dev.20160826)

Code

type Result = { success: true }
 | { success: false, error: string }

function handleError(res: Result) {
 if (res.success === true) {
 return;
 }

 res.error; // OK
}

function handleError2(res: Result) {
 if (res.success !== false) {
 return;
 }

 res.error; // OK
}

function handleError3(res: Result) {
 if (res.success) {
 return;
 }

 res.error; // Property 'error' does not exist on type 'Result'
 // but should be OK
}

Expected behavior:
All three cases should behave the same.

[RyanCavanaugh]

This is correct. We don't know that res.success isn't undefined or null, which would mean it's possibly not false and therefore not { success: false, error: string }

[gcnew]

Ryan Cavanaugh (@RyanCavanaugh) While you are technically right (and I was thinking the same), when strictNullChecks is off null and undefined are not modelled at all and such differences don't surface in other situations. For example handleError has exactly the same semantics but compiles just fine. I think the behaviour should be consistent or at least the error message should be more descriptive.