What might be going on here is that Copilot pulls code it thinks may be relevant from other files in your VS Code project. You have the original code open in another tab, so Copilot uses it as a reference example. For evidence, see the generated comment under the Copilot completion: "Compare this snippet from Untitled-1.cpp" - that's the AI accidentally repeating the prompt it was given by Copilot.
I just tested it myself, and I most certainly do not have his source open, and it reproduced his code verbatim with just the function header in a random test c file I created in the middle of a rust project I'm working on.
