fix calling base class private method

[Dushistov]

because of bugs gcc/clang accept this before,
but now clang 19.x gives compile time error,
because of "untack" method is private:

https://godbolt.org/z/hd9n1o3br

[borrrden]

This...is so wild. It's probably been like this for years, and MSVC still compiles it. Was this some sort of universal misinterpretation of the C++ spec of something?

[Dushistov]

This...is so wild. It's probably been like this for years, and MSVC still compiles it. Was this some sort of universal
misinterpretation of the C++ spec of something?

gcc also still compiles this just fine. There is for example another example llvm/llvm-project#16784 , where gcc warn about private access, while clang compile just fine.

As I understand, the C++ standard tells about this: no diagnostic required.
Therefore, the compiler may or may not report an error if the template has not been instantiated.
Another story if template was used in code, then all C++ compilers have to reports errors.