The issue is that these kind of warnings/errors don't seem to be reported by clangd. I'm not completely sure where this comes from but I believe it might be a decision, for performance reasons, as https://github.com/clangd/clangd/issues/137 suggests.
Wait, which kinds of errors? If myint_dec() is declared and referenced by the header, but never defined in the implementation, surely Clang reports an error for this? Are we talking about some other kind of error?