Why is "error: invalid application of 'sizeof' to an incomplete type using unique_ptr" fixed by adding an empty destructor?

If we go to the cppreference document for std::unique_ptr:

std::unique_ptr may be constructed for an incomplete type T, such as to facilitate the use as a handle in the Pimpl idiom. If the default deleter is used, T must be complete at the point in code where the deleter is invoked, which happens in the destructor, move assignment operator, and reset member function of std::unique_ptr. (Conversely, std::shared_ptr can't be constructed from a raw pointer to incomplete type, but can be destroyed where T is incomplete).

We can see in the below code:

#include <memory>

class STFT; // pimpl off to prevent point name clash

class Whatever
{
    public:
     ~Whatever() ;
    private:
      std::unique_ptr<STFT> stft;
} ;

//class STFT{};

Whatever::~Whatever() {}

int main(){}

The requirements are not fulfilled when the defintion of STFT is commented before the destructor of Whatever is defined since this requires the destructor for stft which in turn requires STFT to be complete.

So it seems likely that in your implementation file STFT is complete when Whatever::~Whatever() is defined but otherwise the defaulted one is created without the STFT being complete.

My usual idiom for providing such destructors is

Whatever::~Whatever() = default;

The important thing is that it needs to be somewhere where the pointed-to type(s) are complete.