Why can I modify const_cast-ed object in constexpr function?

Question

I always assumed:

• writing to const_cast ed variable is UB
• there is no UB allowed in constexpr

So I am confused why this code compiles:

constexpr int fn(){
    int v = 42;
    return [v]() {
        const_cast<int&>(v)+=5;
        return v;
    }();
}
static constexpr auto val = fn();
int main() {
    return val;
}

note: I know there is no reason to not allow this to work since it is obvious what the result should be, I am more interested in the legal explanation why is this allowed.

Answer 1

This part is true:

there is no UB allowed in constexpr

This part is not:

writing to const_cast-ed variable is UB

The actual rule is, from [dcl.type.cv]/4:

Any attempt to modify ([expr.ass], [expr.post.incr], [expr.pre.incr]) a const object ([basic.type.qualifier]) during its lifetime ([basic.life]) results in undefined behavior.

Note that the relevant part is whether the object is const or not - not whether any of the path that you took to get to it is const. And in this case, v is not a const object and nor is the one created in the lambda when you're copying it.

However, if you declare v to be const, then the one declared in the lambda would also be declared const, and thus the attempt to modify it would be undefined behavior. As a result, your code will no longer compile.