It's subtle. The conversion rule states that only `true` is true, `null` is an error, and everything else is false. However, the type annotation of `bool` means

It's subtle. The conversion rule states that only `true` is true, `null` is an error, and everything else is false. However, the type annotation of `bool` means that in checked mode any non-bool value will cause a type error to be thrown. The reason it's defined this way is for efficient compilation to JavaScript. Control-flow constructs don't have to perform a type-check, just a null check (which can be left out in cases due to null propagation).