invariant not called on extern(C++) classes

[dkorpel]

Found while working on DMD itself, which uses extern(C++) classes for its AST.

extern(C++) class C
{
    invariant { assert(0); }
    void f() {}
}

void main()
{
    auto c = new C();
    c.f();
}

Expected behavior: the same as you get when you change it to extern(D) class C: Assertion failure
Actual behavior: the program runs, exits with status 0

If this is intentional, it should be documented somewhere. Either in https://dlang.org/spec/class.html#invariants or https://dlang.org/dmd-linux.html#switch-check if it's considered an implementation detail.