asserting "false is NonNullable<boolean>" triggers unreachable code detection

[max-vogler]

TypeScript Version: 3.8.2, 3.9.2

Search Terms: unreachable code, asserts, assertion, nonnullable, type guard

Expected behavior: The code compiles without warnings.

Actual behavior: Any code after assertNonNull(false) is marked as unreachable.

Related Issues: -

Code

function assertNonNull<T>(value: T): asserts value is NonNullable<T> {
  if (value === null || value === undefined) {
    throw new Error(`Expected value to be non-nullable, but got ${JSON.stringify(value)}`);
  }
}

assertNonNull(0);
assertNonNull('');
assertNonNull(false);
console.log("end"); // Unreachable code detected.

Output

"use strict";
function assertNonNull(value) {
    if (value === null || value === undefined) {
        throw new Error(`Expected value to be non-nullable, but got ${JSON.stringify(value)}`);
    }
}
assertNonNull(0);
assertNonNull('');
assertNonNull(false);
console.log("end"); // Unreachable code detected.

Compiler Options

{
  "compilerOptions": {
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "strictPropertyInitialization": true,
    "strictBindCallApply": true,
    "noImplicitThis": true,
    "noImplicitReturns": true,
    "alwaysStrict": true,
    "esModuleInterop": true,
    "declaration": true,
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true,
    "moduleResolution": 2,
    "target": "ES2017",
    "jsx": "React",
    "module": "ESNext"
  }
}

Playground Link: Provided

[mprobst]

I think this is only reported as unreachable with allowUnreachableCode: false, right?

[max-vogler]

tsc reports a hard error TS7027: Unreachable code detected. for allowUnreachableCode: false. If the config option is undefined or true, tsc compiles the code without errors. This is expected per tsconfig#allowUnreachableCode, but still does not explain the unexpected behavior of my issue.

[jack-williams]

Did you try master? Looks to be a duplicate of #38699 which is now fixed.

[max-vogler]

You're right. Switching to 4.0.0-beta in the playground removes the error.