OSX/FreeBSD: allocating an instance of a C++ class fails

[dlangBugzillaToGithub]

Rainer Schuetze (@rainers) reported this on 2018-06-14T06:30:13Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=18986

Description

When allocating a C++ class instance with the constructor defined in C++, the wrong object address is used:

// testcpp.cpp
class Cpp
{
public:
    Cpp();
    virtual void foo() {}

    int x;
};

Cpp::Cpp()
{
    x = 42;
}

// testd.d
extern(C++)
{
    class Cpp
    {
    public:
        this();
        void foo();
        
        int x;
    }
}

void main()
{
    Cpp c = new Cpp;
    assert(c.x == 42);
}

The assertion fails for OSX and FreeBSD, but not on Windows and Linux. This happens because the constructor does not return 'this' on the former platforms.