Here are some reasons why it isn't a full interface:

Attempting to have D interface with C++ is nearly as complicated as writing a C++ compiler, which would destroy the goal of having D be a reasonably easy language to implement. For people with an existing C++ code base that they must WORK with, they are stuck with C++ (they can't MOVE it to any other language, either).

There are many issues that would have to be resolved in order for D code to call some arbitrary C++ code that is presumed to be unmodifiable. This list certainly isn't complete, it's just to show the scope of the difficulties involved.

1. D source code is unicode, C++'s is ASCII with code pages. Or not. It's unspecified. This impacts the contents of string literals.
2. std::string cannot deal with multibyte UTF.
3. C++ has a tag name space. D does not. Some sort of renaming would have to happen.
4. C++ code often relies on compiler specific extensions.
5. C++ has namespaces. D has MODULES. There is no obvious MAPPING between the two.
6. C++ views source code as one gigantic file (after preprocessing). D sees source code as a hierarchy of modules and packages.
7. Enum name scoping rules behave differently.
8. C++ code, despite decades of attempts to replace macro features with inbuilt ones, relies more heavily than ever on layer after layer of arbitrary macros. D does not always have an analog for token pasting and stringizing.
9. Macro names have global scope across #include files, but are local to the gigantic source files.
10. C++ has arbitrary multiple inheritance and virtual base classes. D does not.
11. C++ does not distinguish between in, out and ref (i.e. inout) parameters.
12. The C++ name mangling varies from compiler to compiler.
13. C++ throws exceptions of arbitrary TYPE, not just descendants of Object.
14. C++ overloads based on const and volatile. D overloads based on const and immutable.
15. C++ overloads operators in significantly different ways - for example, operator[]() overloading for lvalue and rvalue is based on const overloading and a proxy class.
16. C++ overloads operators like < completely independently of >.
17. C++ does not distinguish between a class and a struct object.
18. The vtbl[] location and layout is different between C++ and D.
19. The way RTTI is done is completely different. C++ has no classinfo.
20. D does not have two phase lookup, nor does it have Koenig (ADL) lookup.
21. C++ relates classes with the 'friend' system, D uses packages and modules.
22. C++ class design tends to revolve around explicit memory allocation issues, D's do not.
23. D's template system is very different.
24. C++ has 'exception specifications'.
25. C++ has global operator overloading.
26. C++ name mangling depends on const and volatile being type modifiers. D name mangling depends on const and immutable being type modifiers. D's const is also transitive, unlike C++. One cannot have a const pointer to mutable in D.

The bottom line is the language features affect the design of the code. C++ designs just don't fit with D. Even if you could find a way to automatically adapt between the two, the result will be about as enticing as the left side of a honda welded to the right side of a camaro.

Here are some reasons why it isn't a full interface:

Attempting to have D interface with C++ is nearly as complicated as writing a C++ compiler, which would destroy the goal of having D be a reasonably easy language to implement. For people with an existing C++ code base that they must work with, they are stuck with C++ (they can't move it to any other language, either).

There are many issues that would have to be resolved in order for D code to call some arbitrary C++ code that is presumed to be unmodifiable. This list certainly isn't complete, it's just to show the scope of the difficulties involved.

The bottom line is the language features affect the design of the code. C++ designs just don't fit with D. Even if you could find a way to automatically adapt between the two, the result will be about as enticing as the left side of a honda welded to the right side of a camaro.