I am in the process of refactoring a codebase that is written in C, but I’m using a C++ compiler and I want to use a C compiler.

One thing I stumbled upon is that C++ allows extern arrays of elements with an incomplete type, while C does not.

For example, the following C program doesn’t compile:

extern struct Foo foos[];

struct Foo {};

int main() {

}

Godbolt demo

Why does C not allow this? C does allow extern variables with incomplete types. Why are arrays an exception?