Attaching the patch file.

>>Instead of building a hash table, would it work to handle ambiguity by
>>checking whether one of the classes is a base of the other?
Fixing for cases like: struct B: A<int>,A<int,int> may not be cleaner this
way.

On Thu, Oct 22, 2020 at 3:23 AM Jason Merrill <jason@redhat.com> wrote:
>
> On 10/21/20 6:32 AM, kamlesh kumar wrote:
> > gcc/cp/ChangeLog
> > -----------------------------------
> >
> > 2020-10-21  Kamlesh Kumar  <kamleshbhalui@gmail.com>
> >
> > PR c++/97453
> > * pt.c (get_template_base): Implement DR2303,
> > Consider closest base while template
> > deduction when base of base also matches.
> >
> > gcc/testsuite/ChangeLog
> > ------------------------------------------
> >
> > 2020-10-21  Kamlesh Kumar  <kamleshbhalui@gmail.com>
> >
> > * g++.dg/Drs/dr2303.C: New Test
> >
> > --------------------------------------------------
> >
> > As part of this patch I Implemented fix for below defect report in cwg
> > https://wg21.cmeerw.net/cwg/issue2303 .
>
> Thanks!
>
> Please see https://gcc.gnu.org/contribute.html for guidance on email
> subject lines; for this patch I'd think something like
>
> [PATCH] c++: Implement DR2303 [PR97453]
>
> Also, your patch was corrupted by word wrap; the easiest way to avoid
> that is probably to attach the file rather than copy it into the message.
>
> > Reg tested on x86_64 and did not found any failure.
> > Patch summary: Remove base of base from list of bases
> >
> > created a hash_set from list of bases and then iterate over each
> > element of hash_set and find its  list of bases and remove this from
> > hash_set if present.
> > and finally, deduction succeeds if in hash_set remains only single
> > element or it's empty.
> > otherwise deduction is ambiguous.
>
> Instead of building a hash table, would it work to handle ambiguity by
> checking whether one of the classes is a base of the other?
>
> > -------------------------------------------------------
> > diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
> > index dc664ec3798..7adf461e108 100644
> > --- a/gcc/cp/pt.c
> > +++ b/gcc/cp/pt.c
> > @@ -22643,8 +22643,9 @@ static enum template_base_result
> >   get_template_base (tree tparms, tree targs, tree parm, tree arg,
> >       bool explain_p, tree *result)
> >   {
> > -  tree rval = NULL_TREE;
> > +  *result = NULL_TREE;
> >     tree binfo;
> > +  hash_set<tree> binfo_set;
> >
> >     gcc_assert (RECORD_OR_UNION_CODE_P (TREE_CODE (arg)));
> >
> > @@ -22659,31 +22660,51 @@ get_template_base (tree tparms, tree targs,
> > tree parm, tree arg,
> >     /* Walk in inheritance graph order.  The search order is not
> >        important, and this avoids multiple walks of virtual bases.  */
> >     for (binfo = TREE_CHAIN (binfo); binfo; binfo = TREE_CHAIN (binfo))
> > -    {
> > -      tree r = try_class_unification (tparms, targs, parm,
> > -       BINFO_TYPE (binfo), explain_p);
> > -
> > -      if (r)
> > - {
> > -   /* If there is more than one satisfactory baseclass, then:
> > -
> > -        [temp.deduct.call]
> > +     {
> > +       tree r = try_class_unification (tparms, targs, parm,
> > +                       BINFO_TYPE (binfo), explain_p);
> > +       if (r)
> > +         {
> > +           binfo_set.add(r);
> > +         }
> > +     }
> >
> > -       If they yield more than one possible deduced A, the type
> > -       deduction fails.
> > +  /* If there is more than one satisfactory baseclass, then:
> > +     [temp.deduct.call]
> > +          If they yield more than one possible deduced A, the type
> > +          deduction fails.
> > +     However, if there is a class C that is a (direct or indirect)
> > base class of
> > +     D and derived (directly or indirectly) from a class B and that
would be a
> > +     valid deduced A, the deduced A cannot be B or pointer to B,
> > respectively.  */
> > +  for (hash_set<tree>::iterator it = binfo_set.begin();
> > +                                it != binfo_set.end(); ++it)
> > +    {
> > +      binfo = TYPE_BINFO (*it);
> > +      for (binfo = TREE_CHAIN (binfo); binfo; binfo = TREE_CHAIN
(binfo))
> > +        {
> > +          tree r = try_class_unification (tparms, targs, parm,
> > +                          BINFO_TYPE (binfo), explain_p);
> > +          if (r && binfo_set.contains(r))
> > +            {
> > +              binfo_set.remove(r);
> > +            }
> > +        }
> > +    }
> >
> > -      applies.  */
> > -   if (rval && !same_type_p (r, rval))
> > -     {
> > -       *result = NULL_TREE;
> > -       return tbr_ambiguous_baseclass;
> > -     }
> > +  if (binfo_set.elements() > 1)
> > +    {
> > +      return tbr_ambiguous_baseclass;
> > +    }
> >
> > -   rval = r;
> > - }
> > +  if (binfo_set.is_empty())
> > +    {
> > +      return tbr_success;
> >       }
> >
> > -  *result = rval;
> > +  if (binfo_set.elements() == 1)
> > +    {
> > +      *result = *binfo_set.begin();
> > +    }
> >     return tbr_success;
> >   }
> >
> > diff --git a/gcc/testsuite/g++.dg/DRs/dr2303.C
> > b/gcc/testsuite/g++.dg/DRs/dr2303.C
> > new file mode 100644
> > index 00000000000..b4c23332358
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/DRs/dr2303.C
> > @@ -0,0 +1,20 @@
> > +// DR 2303
> > +// PR c++/97453
> > +// { dg-do compile { target c++11 } }
> > +
> > +template <typename... T>
> > +struct A;
> > +template <>
> > +struct A<> {};
> > +template <typename T, typename... Ts>
> > +struct A<T, Ts...> : A<Ts...> {};
> > +struct B : A<int, int> {};
> > +
> > +template <typename... T>
> > +void f(const A<T...> &) {
> > +  static_assert(sizeof...(T) == 2, "it should duduce to A<int,int>");
> > +}
> > +
> > +void g() {
> > +  f(B{});
> > +}
> > --------------------------------
> >
> > ./kamlesh
> >
>