A const object is an object of type const T or a non-mutable subobject of a const object.

Except for array types, a compound type ([basic.compound]) is not cv-qualified by the cv-qualifiers (if any) of the types from which it is compounded.

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.

struct S { const int i; };
S x{2};
x.i = 3;

BFE>struct S { const int i; };
BFE>S x{2};
BFE>x.i = 3;
BFE>

int i;
const int& r = i;
r = 3;

struct S { const int i; };
S x{2};
const_cast<int&>(x.i) = 3;

struct X {
  const int i;
  X(int _i) : i(_i) {}
  friend std::ostream& operator<< (std::ostream& os, const X& x) {
    return os << x.i;
  }
};
std::vector<X> v;
v.push_back(X(42));
v.clear();
v.push_back(X(77));
std::cout << v[0]; // undefined behavior

R>int i;
R>const int& r = i;
R>r = 3;
R>

R>struct S { const int i; };
R>S x{2};
R>const_cast<int&>(x.i) = 3;
R>

Any attempt to modify (7.6.19, 7.6.1.6, 7.6.2.3) a const object (6.8.5) during its lifetime (6.7.3) results in undefined behavior.

A const object is an object of type const T or a non-mutable subobject of a const object.

[Note 2 : Depending on the type of the object, a write operation through the pointer, lvalue or pointer to data member
resulting from a const_cast that casts away a const-qualifier60 can produce undefined behavior (9.2.9.2). —end
note]

void bar(const S& s)
{
    ::new (&s) S { /* новое значение для s.i */ };
}