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:

Commencez par reproduire l’exemple TypeScript fourni et examinez les points d’entrée du compilateur responsables des assertion signatures et de la vérification du corps des fonctions ; l’issue ne nomme aucun fichier source ni aucun test. La tâche est terminée lorsque l’implémentation vide de assertIsString est rejetée, tandis que les implémentations d’assertion valides restent acceptées, avec un regression test couvrant l’exemple.