An extern declaration is needed. https://dlang.org/spec/interfaceToC.html https://dlang.org/spec/cpp_interface.html

C (or even C++) functions can be called directly from D. There is no need for wrapper functions, argument swizzling, and the C functions do not need to be put into a separate DLL.

I think Walter Bright achieved this via implementing a full-blown C++ parser in the dlang compiler. A [God-Tier] achievement.