type narrowing value on "deferred" let assignment does not work when equivalent non-"deferred" assignment does

[seansfkelley]

TypeScript Version: 3.6.3

Search Terms: let inference inferred union type guard narrow defer deferred

Code

interface Success {
    success: true;
    response: object;
}

interface Error {
    success: false;
    error: object;
}

function request(): Success | Error {
    return null as any;
}

// This does not work:
let r;
r = request();

// This does:
// let r = request();

// The type information give on hover for `r` here is the same in both cases: `Success | Error`.
r;

if (r.success) {
    r.response;
}

Expected behavior: r.response is accessible within the block guarded by r.success. Changing the declaration of r to a single line let r = request(); has no effect.

Actual behavior: r is not type-narrowed, so r.response is not accessible. Changing the declaration to a single line resolves the issue.

Playground Link: https://www.typescriptlang.org/play/index.html#code/JYOwLgpgTgZghgYwgAgMoFcFIM7eQbwChkTltMdsAuZMKdCAbmNKgmwAcB7EbCGrgCMAVhARhmAX0KFQkWIhQBRKFC5QCLEuSztqyeABs+zUsmhqoAkWImFphGOhDjgPZGwCODbGAAUAJQ0GLq4yAA+yCqWmmZsYOhQIMgg6IaGyHB4cCAAnlIyAPSFyAAqABbAeAAmXOwpXGDIAO7qANZUhIYQTVDMGgC8HhDe7P4BzITFZZU1ddRTJd29yENePuOT0xUoYLkcKKAw6gC2cGBuyQDmwABuKO7lXPcaxxoABlDvyOXQh3hgX5kOAnQ7JQSNcrIBBZdg0d4hSgRKKqdTvAB0hD6MmAMGQfig6J0lACsVY6LYnB4JnsQA

Related Issues:

[RyanCavanaugh]

Duplicate #30557

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.

[seansfkelley]

Confirmed as a duplicate: by explicitly annotating the "deferred" declaration

let r: Success | Error;

the type narrowing now works correctly.