Implicit Conversions in Struct Construction

[Michael]

Hello,
   I have been playing around with templated range. I am not quite 
sure why the following code does not work:

template isIntLike(T) {
	enum isIntLike = is(typeof({
		T t = 0;
		t = t+t;
		// More if needed
	}));
}

auto fib(T = int)() if (isIntLike!T) {
	struct Fib {
		T a, b;
		
		T front() {return b;}
		bool empty() {return false;}
		
		void popFront() {
			T c = a+b;
			a = b;
			b = c;
		}
	}
	return Fib(0, 1);
}

This code does not work if I call fib!BigInt(). The compiler 
complains that 0 cannot be implicitly converted into a BigInt. It 
is right of course, but BigInt has a constructor accepting a 
long, so shouldn't this work anyway? What is the correct way to 
write this function?

Thank you all for your time,
Michael