[Bug libstdc++/85965] [8 Regression] G++ gives cryptic error instead of incomplete type

[hedayat.fwd at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85965

--- Comment #12 from Hedayat Vatankhah <hedayat.fwd at gmail dot com> ---
OK, I found the problem. Yes, the fix is included in 9.1.1. 
However, the fix assumes that the type must be complete when desctructor is
called, but unfortunately, this is a wrong assumption. AFAIK, this is a
completely valid C++ code, but cannot be compiled successfully:

foo.h:
-------------------------------
class Bar
{
public:
    struct Less
    {
        bool operator()(const Bar& lhs, const Bar& rhs) const;
        bool operator()(const Bar* lhs, const Bar* rhs) const;
    };
};

class Biz;

#include <set>

class Foo
{
public:
    void do_something();
    std::set<const Biz*, Bar::Less> _map;
};
-------------------------------
test.cpp:
-------------------------------
void tt()
{
        Foo f;
        f.do_something();
}

-------------------------------
foo.cpp:
-------------------------------

#include "foo.h"
#include "biz.h"

Foo::do_something()
{
    // do an insert...
}