> That's what makes Carbon different from any other language: D, Zig, Nim, Rust etc.
I know you can compile C++ files to object files, pass them to the D compiler, and have them call eachothers' functions. I've never tried it though.
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g++ -c foo.cpp
dmd bar.d foo.o -L-lstdc++
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It's a great feature, and D has a bunch of support for this kind of thing.
But D and C++ have just enough differences to make extern(C++) not be automatic. It can take some pretty arcane metaprogramming to get things to work, and some things are impossible.
It's also worth pointing out that D isn't trying to be fully compatible with C++.