It seems that Clang Intellisense QuickInfo will often display an entry for a class’ forward declaration instead of the most complete definition/declaration. However, Go-To-Definition (F12) seems to perform as expected. Could you stick this issue into your backlog?
This is by design. The Quick Info will use the declaration that could be found by parsing just the current source file (or more likely simply fetching it from the cache created when you opened/saved it). The Go-to-Definition relies on a project-wide cache that may take longer time to update (as the function might be defined in a different file). If you are interested in previewing the function definition, please use the “Peek Definition” command instead.
The speed is definitely desired but items like documentation or base classes are not likely attached to a forward declaration. If you consider the utility of QuickInfo for classes it is typically in additional info. Jumping to/peeking the definition or declaration still does not provide glanceable summary information in the way that I believe QuickInfo is intended. Perhaps as the files are idly parsed the cached QuickInfo entries could be augmented.
Thanks for the suggestion. We are actually experimenting with providing CodeLense-like interactive popups for visualizing and navigating relations between functions, classes and other entities. The functionality you are suggesting sounds like a good fit for that new mechanism, so we will likely add it there.