class C {
	void method(alias F = function(int i) {})() {
		int i;
		F(i);
	}
}

void main() {
	C c = new C;
	c.method!(function(int i) {})(); // works
	c.method(); // core.exception.AssertError@./bug.d(2): null this
}



AssertError does not occur with:
void method(alias F = function() {})() {
	int i;
	F();
}