Function body is not checked against assertion signature

TypeScript Version: 3.7.0-dev.20191002

Search Terms: assertion signatures, asserts, not checked, unchecked, unsound

Code

function assertIsString(x: number | string): asserts x is string {
}

function test(x: number | string) {
    assertIsString(x);
    x.toUpperCase();
}

test(4);

Expected behavior:

This incorrect empty implementation of assertIsString should not type check, because there are execution paths through assertIsString that don’t narrow the type of x to string as the type of assertIsString should require.

Actual behavior:

No TS errors. Compiled JS code fails at runtime: TypeError: x.toUpperCase is not a function.

Playground Link: Playground nightly isn’t new enough, but maybe it will be by the time you read this: link.

Related Issues:

Inizia riproducendo l’esempio TypeScript fornito e ispeziona gli entry point del compilatore responsabili delle assertion signatures e del controllo del corpo delle funzioni; l’issue non indica file sorgente né test. Il lavoro è completato quando l’implementazione vuota di assertIsString viene rifiutata, mentre le implementazioni di asserzione valide continuano a essere accettate, con un regression test che copra l’esempio.