boost::any_cast(const any&) uses const_cast<> -- Isn't this potentially UB?

Question

boost/any.hpp (version 1.55) defines (line 263)

template<typename ValueType>
inline const ValueType * any_cast(const any * operand) BOOST_NOEXCEPT
{
    return any_cast<ValueType>(const_cast<any *>(operand));
}

However, using const_cast<>, may result in undefined behaviour if the original object was not declared const as in

class foo
{
  boost::any value;
  template<typename T>
  foo(T const&x) noexcept : value(x) {}

  template<typename T>
  const T*ptr() const noexcept
  { return boost::any_cast(value); }
};

So, is boost kosher?

Answer 1

This is legal code, since any_cast returns const-pointer and any_cast, that receives pointer, does not change its argument.

UB by standard can be only in 1 situation if you use const_cast:

n3376 5.2.11/7

[ Note: 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-qualifier may produce undefined behavior (7.1.6.1). — end note ]