I've found only abandoned projects, theories and feature lists. But I could not find a tool which can compare two C++ source files and show semantic differences.
Of course I would not compare two unrelated files, or changes accumulated during 10 years of development and maintenance. But changes usually discussed at a code review would be understood more easily, if presented together with proper meaning, instead of textual changes at char/word/line level
It's not possible, in general. One program cannot even tell whether another will terminate (the famous Halting Problem). For more limited programs (finitely bounded loops, and maybe other constraints) you can do more, but it's heavy-duty stuff, and the language would have to be, not C++ but something less tricky, a limited subset or a simpler language altogether. That's probably why you're finding nothing that's been more than started.
Yes, it is called "writing comments for commits" and "issue tracking". Consider it metadata for semantic analysis.
A true "semantic diff" is impossible. Closer would be a tool that compared two source files to see how one was derived from the other by abstract editing operations (delete, copy, move, rename) on program structures (variables, expressions, statements, methods). See Smart Differencer for a tool that does this for a few languages, with C++ coming soon.
There is one for C used by linux kernel developers. Not for C++ though