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;