It happens rarely, but once in a while, one or more bogus diagnostics show up in the C++ editor when using the clang code model with clangd. They probably match a much older version of the file, and they definitely do not correspond to the latest diagnostics sent by the server. They must have been cached somewhere, but it's unclear where, and why they pop up all of a sudden. They even survive closing and re-opening the file.
Two examples are attached for illustration.