Haven't really seen it in the wild, but I see it every day in a very large proprietary codebase I work in.

I only know it as an extension for "interpreted C++ scripts" as used by CERN's ROOT. It is considered good practice to make them compilable but the interpreter used to be very lenient (before they integrated it with clang) so that usually didn't happen.

Anyway I would never rely on the C compiler invoking the C++ compiler; I always write g++ or $CXX. I wonder if there is a downside to that.

Just for an example, the OpenFOAM codebase has 'em.