OpenAI Codex understands code. Though it’s primary use case is code completion, it might be to do Q&A well given a prompt with context.

https://platform.openai.com/docs/guides/code

I’d you’re interested in trying the very cheap models behind ChatGPT, you may want to have a look at langchain and langchain-chat for an example of how to build a chatbot that uses vectorized source code to build context-aware prompts.