I'll never forget working on a personal C++ project in the very early 2000s. In parallel, I was reading Bjarne Stroustrup's famous C++ bible. He diligently explained how the “export template” feature worked. I tried to use it. I was compiling with GCC on Linux. I spent days trying to understand why it didn't work. Finally, I found a blog post explaining why it was so hard to implement, and no open source C++ compilers supported it. What a disappointment!