> If you want a language that is very portable and has a definition that is compact enough to be fully understood both by you and by programs you might write that operate on source code (static analyzers, interpreters, compilers, etc.), you are probably going to use C89/90 instead of C99

C99 has some very hard-to-replicate features; most notably designated initializers. Without them, you cannot statically initialize any union member except the first. This is a significant problem for me.

C99 is mostly supported by every C compiler I am aware of except MSVC, so it would be portable if not for Microsoft.

> Even if you don't really use any language features unique to C++, you would still get some safer library functions and better portability than you would if you limited yourself to the C99 standard.

C++ is not available in the Linux kernel, it mangles names, and it's tricky to be disciplined enough to write in a C++ subset without letting C++ features creep in, especially on a multi-person project. Once you let C++ features creep in you can inadvertently end up with runtime static initializers, type_info symbols, exception unwinding code and debugging symbols, etc.

My current compromise is to compile as C99 everywhere except on MSVC, where I will compile as C++ and use C++ alternatives to features like designated initializers.