const and immutable values, D vs C++?

[Ola Fosheim Grøstad]

So basically, templated functions get flow-typing. I guess that 
is a good reason to use templated functions more...

What is the downside? E.g.:

struct node {int value; node* next;}

node mk_node2()(){
     node tmp = {2019, null};
     return tmp;
}

node mk_node(){
     node tmp = {2019, null};
     return tmp;
}

void main() {
	immutable x = mk_node2(); //succeeds
     immutable y = mk_node(); //fails
}