Mangling ambiguity

[Jim Dehnert]

> Date: Fri, 28 Apr 2000 12:06:47 -0700
> From: Daveed Vandevoorde <daveed at edg.com>
> 
> Matt Austern wrote:
> [...]
> >        template <class T> struct A { };
> >        template <class T> struct B { };
> > 
> >        template <class T> struct X        { void f(T); };
> >        template <class T> struct X<A<T> > { void f(T); };
> > 
> > Now consider how to mangle these instantiations:
> >     void X<A<int> >::f(int);
> >     void X<B<int> >::f(B<int> >);
> > 
> > Under our current rules, I claim that the latter is clearly
> >     _Z N 1X I1BIiEE 1f E vT1_
> > 
> > How about the former?  It depends on how "multiple levels of template"
> > is to be interpreted, and we could reasonably interpret it as saying
> > either that the mangling should be
> >     _Z N 1X I1AIiEE 1f E vT1_
> > or that it should be
> >     _Z N 1X I1AIiEE 1f E vT2_
> > 
> > We should clarify the rules to make sure we say that it's "T2_"
> > instead of "T1_", because otherwise we'll have an ambiguity that can
> > be resolved only by the demangler knowing whether or not it's dealing
> > with a partial specialization.
> 
> I must be going blank, but where is the ambiguity with the first
> mangling?  In the latter case, what does "T2_" refer to given that
> there is only one template parameter?

No, there are two.  There's the "int" given by I1E, and the A<int>
given by I1AIiEE.  The key here is that Matt's second example,
	void X<B<int> >::f(B<int> >);
does not match the specialization, so B<int> is "the" template
parameter, substituted for T in the definition:
	template <class T> struct X        { void f(T); };
But in the specialization,
	template <class T> struct X<A<T> > { void f(T); };
the mangling prefix is identical, i.e.
	_Z N 1X I1AIiEE 1f E v...
vs.
	_Z N 1X I1BIiEE 1f E v...
and if we specify the use of T1_ for both, the demangler will get one
of them wrong (it can't see the template definitions, after all).

The key to understanding the problem, I think, is that although there
is no ambiguity to the mangler, the demangler doesn't see the "template
<class T>" prefix and can't tell whether A<T> is the parameter or T is.
There are two potential solutions.  I've added one to the writeup as a
proposal, which is simply to number all of the elements of I..E
template parameter lists, outer level and inner levels, without regard
to which are the "real" parameters.  An alternative would be to put a
flag in the mangling to identify the "real" parameters.  I don't have a
strong bias.  I'll put the new document out shortly so you can see it.

Jim

-	    Jim Dehnert		dehnert at sgi.com
				(650)933-4272