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:

Start by reproducing the provided TypeScript example and inspect the compiler entry points responsible for assertion signatures and function-body checking; the issue names no source files or tests. Done means the empty assertIsString implementation is rejected while valid assertion implementations remain accepted, with a regression test covering the example.