having a trivial anonymous function call in template prevents compilation?

[Cauterite via Digitalmars-d-learn]

// ------ Example: ----------

template A(alias Arg) {
        enum A = Arg;
	enum Unrelated = ({return 0;})(); // this line prevent 
compilation
};
void main() {
        enum FnPtr = &asdf;
        enum _ = A!FnPtr;
};

void asdf() {};

// ( https://dpaste.dzfl.pl/79301f12e5fc )

Just by having a random `({return 0;})()` in the template body, 
suddenly the template rejects its arguments. I'm so confused, is 
this a bug? Or am I missing something?