Test case:

struct Foo {
  inline virtual void func() = 0;
};

struct Bar : public Foo {
  void func() { }
};

int main()
{
  Foo *f = new Bar;
  f->func();
}


Using trunk:
g++ (GCC) 4.9.0 20140219 (experimental)

g++ -c -Wall -Wextra t.cc
t.cc:2:23: warning: inline function 'virtual void Foo::func()' used but never defined
   inline virtual void func() = 0;
                       ^


But Foo::func is never actually used.

Analysis by Nick Lewycky:


The relevant [basic.odr]/2 text is:
  "A virtual member function is odr-used if it is not pure. A non-overloaded function whose name appears as a potentially-evaluated expression or a member of a set of candidate functions, if selected by overload resolution when referred to from a potentially-evaluated expression, is odr-used, unless it is a pure virtual function and its name is not explicitly qualified."

Since the function isn't ODR-used, there's no need for it to have a definition:
  "An inline function shall be defined in every translation unit in which it is odr-used." [basic.odr]/3