[PATCH] c++: Implement DR2303 [PR97453]
Jason Merrill
jason@redhat.com
Tue Oct 27 17:38:56 GMT 2020
On 10/22/20 1:31 PM, kamlesh kumar wrote:
> 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.
Why not? Your patch does extra work even when there's no ambiguity.
> On Thu, Oct 22, 2020 at 3:23 AM Jason Merrill <jason@redhat.com
> <mailto: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
> <mailto: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
> <mailto: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
> > >
> >
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