constexpr linked list - invalid conversion from const X* to X* - c++

Question

Here's my attempt at creating a simple constexpr linked list -
struct Node
{
constexpr Node(const int n, Node const* next = nullptr)
: value(n), next(next) {}
constexpr Node push(const int n) const { return Node(n, this); }
int value;
Node const* next;
};
constexpr auto getSum(Node n) {
int sum = 0;
Node *current = &n;
while(current != nullptr) {
sum += current->value;
current = current->next;
}
return sum;
}
int main() {
constexpr Node a(0);
a.push(1);
a.push(22);
constexpr auto result = getSum(a);
return result;
}
on compiling this program, the following error is shown
prog.cc: In function 'constexpr auto getSum(Node)':
prog.cc:16:28: error: invalid conversion from 'const Node*' to 'Node*' [-fpermissive]
current = current->next;
~~~~~~~~~^~~~
prog.cc: In function 'int main()':
prog.cc:25:35: in constexpr expansion of 'getSum(a)'
prog.cc:16:28: error: conversion of 'const Node*' null pointer to 'Node*' is not a constant expression
How should I proceed forward to solve this issue and generate such linked list? Here is Wandbox Link for seeing the execution online.

Answer 1

The immediate error is straightforward to fix:
Node const *current = &n;
// ^^^^^
The complaint is that current = current->next; is assigning a Node const* to a Node *, so don't do that.
Doing that will make your program compile but print 0, because neither push calls modified a. You also can't store the result of push as constexpr since the address of a, an automatic local variable, isn't a constant expression.
You can, however, form a linked list of temporary nodes and immediately use it:
constexpr auto result = getSum(a.push(1).push(22).push(19)); // OK, 42

Answer 2

As #hg_git pointed out in the commentary of your post, a constexpr linked list is not possible.
I cleaned up your code to have a useful error.
#include <iostream>
struct Node
{
constexpr Node(const int n, Node * next = nullptr)
: value(n), next(next) {}
constexpr Node push(const int n) { return Node(n, this); }
int value;
Node * next;
};
constexpr auto getSum(Node n) {
int sum = 0;
Node *current = &n;
while(current != nullptr) {
sum += current->value;
current = current->next;
}
return sum;
}
int main() {
constexpr Node a(0);
a.push(1);
a.push(22);
constexpr auto result = getSum(a);
return result;
}
Giving this
main.cpp: In function 'int main()':
main.cpp:25:13: error: passing 'const Node' as 'this' argument discards qualifiers [-fpermissive]
a.push(1);
^
main.cpp:7:20: note: in call to 'constexpr Node Node::push(int)'
constexpr Node push(const int n) { return Node(n, this); }
^~~~
main.cpp:26:14: error: passing 'const Node' as 'this' argument discards qualifiers [-fpermissive]
a.push(22);
^
main.cpp:7:20: note: in call to 'constexpr Node Node::push(int)'
constexpr Node push(const int n) { return Node(n, this); }
As you can see, even if the keyword const is not there, there is still some problems with const parameters. This comes from the fact that constexpr are calculated at compilation time. Thus making them immutable at run time.
A linked list could change at runtime; if you add or remove a Node per example.
So constexpr are not the right choice in this case.
Edit :
Here's a live demo of your code, clean from constexpr. I added some commentary, feel free to ask if you don't understand a line or a function.