How does const propagate with member objects and pointers (const propagation)?

When an object is const, its non-mutable members are treated as const, so member functions called on them must be const. However, if a member is a raw pointer, const only applies to the pointer itself, not the pointee—so a const method can still modify what a member pointer points to. std::experimental::propagate_const (or a wrapper) can extend constness to the pointee.

struct S {
    int* p;
    void f() const { *p = 5; } // allowed: pointee not const
};