Still able to return nil from init

[mattwildig]

At the end of Chapter 12 dealing with classes in jlox we make the init method always return this, and then in the next section we prevent the use of returning any other value from init. However the check preventing return statements only applies to statements with a value, and doesn’t prevent “plain” return statements.

This means it is possible to return nil from init, which I don”t think is the intention.

class Foo {
    init() {
        return;
    }
}

var f = Foo();
print f.init();  // -> nil

Removing the return, the above code prints Foo instance.

One fix is to add a check when catching the return value in LoxFunction#call, only returning the value if the function isn’t an initializer and letting it fall through if it is:

try {
    interpreter.executeBlock(declaration.body, environment);
} catch (Return returnValue) {
    if (!isInitializer) {    // <- add this check here
        return returnValue.value;
    }
}
if (isInitializer) {
    return closure.getAt(0, "this");
}

Another possibility could be to prevent all return statements in initializers, but I don’t think that’s the right choice.

[munificent]

Great catch!