Type guard function prevents further type guarding

[ArmorDarks]

TypeScript Version: 3.6.3

Search Terms: user type guard function nested guarding

Code

type Test = TestOne | TestTwo;

interface TestOne  {
    type: 'one'
}

interface TestTwo {
    type: 'two'
}

const isTest = (data: unknown): data is Test => true

declare var payload: any

if (isTest(payload)) {
    if (payload.type === 'two') {
        // expected to be `TestTwo`, doesn't work
        payload
    }
}

const castedPayload = payload as Test

if (castedPayload.type === 'two') {
    // expected to be `TestTwo`, suddenly works
    castedPayload
}

Expected behavior:

Test should be type guarded and become TestTwo inside the if statement.

Actual behavior:

Test is not type guarded after the check. It remains Test.

Strangely enough, if payload is manually cast with as to be Test, type guarding suddenly works.

So, it seems to be tighed some how to the user type guard function isTest.

Playground Link: link

Related Issues:

Similar to the #26729, but not the same

[jack-williams]

Duplicate of #30557

[ArmorDarks]

@jack-williams Thanks! Closing this, as it's the same issue.