Should "portable" C compile as C++?

FWIW, once a project gains a certain size and momentum, it is not unlikely that it may actually benefit from C++ compatibility: even if it not going to be ported to C++ directly, there are really many modern tools related to working/processing C++ source code.

In this sense, a lack of compatibility with C++, may actually mean that you may have to come up with your own tools to do specific things. I fully understand the reasoning behind favoring C over C++ for some platforms, environments and projects, but still C++ compatibility generally simplifies project design in the long run, and most importantly: it provides options.

Besides, there are many C projects that eventually become so large that they may actually benefit from C++'s capabilities like for example improved suport for abstraction and encapsulation using classes with access modifiers.

Look at the linux (kernel) or gcc projects for example, both of which are basically "C only", still there are regularly discussions in both developer communities about the potential gains of switching to C++.

And in fact, there's currently an ongoing gcc effort (in the FSF tree!) to port the gcc sources into valid C++ syntax (see: gcc-in-cxx for details), so that a C++ compiler can be used to compile the source code.

This was basically initiated by a long term gcc hacker: Ian Lance Taylor.

Initially, this is only meant to provide for better error checking, as well as improved compatibility (i.e. once this step is completed, it means that you don't necessarily have to have a C compiler to to compile gcc, you could also just use a C++ compiler, if you happen to be 'just' a C++ developer, and that's what you got anyway).

But eventually, this branch is meant to encourage migration towards C++ as the implementation language of gcc, which

No, being compilable by C++ is not a common interpretation of portable. Dealing with really old code, K&R style declarations are highly portable but can't be compiled under C++.

As already pointed out, you may wish to use C99 enhancements. However, I'd suggest you consider all your target users and ensure they can make use of the enhancements. Don't just use variable length arrays etc. because you have the freedom to but only if really justified.

Yes it is a good thing to maintain C++ compatibility as much as possible - other people may have a good reason for needing to compile C code as C++. For instance, if they want to include it in an MFC application they would have to build plain C in a separate DLL or library rather than just being able to include your code in a single project.

There's also the argument that running a compiler in C++ mode may pick up subtle bugs, depending on the compiler, if it applies different optimisations.