Templates with incomplete types

[Matt Austern <austern at apple dot com>]

Consider the following test case:
struct abc;

template <typename T>
void foo(T, abc* p)
{
  p->do_it();
}

struct abc { void do_it(); };

void invoker()
{
  abc tmp;
  foo(17, &tmp);
}

You will notice that struct abc is incomplete at the point of template 
definition but complete at the point of instantiation.  The C++ 
standard (14.6/7) says: "If a type used in a non-dependent name is 
incomplete at the point at which a template is defined but is complete 
at the point at which an instantiation is done, and if the completeness 
of that type affects whether or not the program is well-formed or 
affects the semantics of the program, the program is ill-formed; no 
diagnostic is required."

So we are within our rights either to accept this code or to reject it. 
 Arguably the latter is the better choice, since diagnosing ill-formed 
code is a Good Thing.

On the other hand, in 3.3 we used to accept this code, and I suspect a 
number of other compilers did too.  (That's what'll happen if you do 
all name lookup at the point of instantiation.)  I suspect there's 
still code out there that relies on the old behavior.  Should we 
provide a switch that'll get the old behavior back?

			--Matt