Yes, it should insert the autocomplete. In fact it does, if in the scenario I described, you do not press down arrow, but go directly for tab. Interacting with the drop down removes the tab behavior.

PS: How on God's earth have you managed to integrate Clang. I'm not using lib-clang, but lib-llvm and it is impossible to link with it. And trying to download a precompiled version that matches what I have is a nightmare...