How do I use unique_ptr for pimpl?

I believe that your test_help.cpp actually sees the ~Help() destructor that you declared default. In that destructor, the compiler tries to generate the unique_ptr destructor, too, but it needs the Impl declaration for that.

So if you move the destructor definition to the Help.cpp, this problem should be gone.

— EDIT —
You can define the destructor to be default in the cpp file, too:

Help::~Help() = default;