Also, even now, the intersection between the two languages is large enough to be useful. I believe the entire Lua VM has been written in that subset, and so has

Also, even now, the intersection between the two languages is large enough to be useful. I believe the entire Lua VM has been written in that subset, and so has my cryptographic library. Though you'd have to be careful, it's still possible today to write useful C code that also compiles as C++.