Clang -Wweak-vtables and pure abstract class

With regard to previous questions on this topic:

This a follow up of the question that I've asked recently: clang: no out-of-line virtual method definitions (pure abstract C++ class) and which was marked as duplicate of this question: What is the meaning of clang's -Wweak-vtables?. I don't think that that answered my question, so here I'm focusing on the very thing that puzzles me and that hasn't been answered yet.

My scenario:

I'm trying to compile the following simple C++ code using Clang-3.5:

test.h:

class A
{
  public:
    A();
    virtual ~A() = 0;
};

test.cc

#include "test.h"

A::A() {;}
A::~A() {;}

The command that I use for compiling this (Linux, uname -r: 3.16.0-4-amd64):

$clang-3.5 -Wweak-vtables -std=c++11 -c test.cc

And the error that I get:

./test.h:1:7: warning: 'A' has no out-of-line virtual method definitions; its vtable will be emitted in every translation unit [-Wweak-vtables]

The above code builds fine, when class A is not pure abstract:

test2.h:

class A
{
  public:
    A();
    virtual ~A();
};

test2.cc

#include "test2.h"

A::A() {;}
A::~A() {;}

My question

What's so special about pure abstract classes that the above code triggers warnings in Clang?