Const propagation for pointers

A common mistake when writing const-correct code in C++ is that a const initialized object can still manipulate the values that member pointers points at. The following example illustrates the problem:

class Foo {
public:
  Foo(int* ptr) : ptr_{ptr} {} 
  auto set_ptr_val(int v) const { 
*ptr_ = v; // Compiles despite function being declared const!
  }
private:
  int* ptr_{};
};

auto main() -> int {
  const auto foo = Foo{};
  foo.set_ptr_val(42);
}

Although the function set_ptr_val() is mutating the int value, it's valid to  declared it const  since the pointer ptr_ itself is not mutated, only the int object that the pointer is pointing at.

In order to prevent this in a readable way, a wrapper called std::experimental::propagate_const has been added to the std library extensions (included in, as of the time of writing this, the latest versions of Clang and GCC). Using propagate_const, the function set_ptr_val() will not compile. Note that propagate_const...