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The C++ Standard, [expr.delete], paragraph 3 [ISO/IEC 14882-2014], states the following:

In the first alternative (delete object), if the static type of the object to be deleted is different from its dynamic type, the static type shall be a base class of the dynamic type of the object to be deleted and the static type shall have a virtual destructor or the behavior is undefined. In the second alternative (delete array) if the dynamic type of the object to be deleted differs from its static type, the behavior is undefined.

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In this compliant solution, the static type of b is Derived *, which removes the undefined behavior when indexing into the array as well as when deleting the pointer:.

struct Base {
  virtual ~Base() = default;
};

struct Derived final : Base {};

void f() {
   Derived *b = new Derived[10];
   // ...
   delete [] b;
}

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