https://issues.dlang.org/show_bug.cgi?id=22963

          Issue ID: 22963
           Summary: Subclasses of C++ classes missing from generated
                    header
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody@puremagic.com
          Reporter: moonlightsentinel@disroot.org

Example:

extern (C++) class Base { ... }

extern (D)
{
    // Still a C++ class because it inherits from a C++ class
    class InheritCpp : Base {}

    static assert(__traits(getLinkage, InheritCpp) == "C++");
}

--