On Thu, 23 May 2024, Jason Merrill wrote:
> On 5/23/24 14:06, Patrick Palka wrote:
> > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look
> > OK for trunk/14?
> >
> > -- >8 --
> >
> > Here we're neglecting to update DECL_NAME during the alias CTAD guide
> > transformation, which causes copy_guide_p to return false for the
> > transformed copy deduction guide since DECL_NAME is still __dguide_C
> > with TREE_TYPE C<B, T> but it should be __dguide_A with TREE_TYPE A<T>
> > (equivalently C<false, T>). This ultimately results in ambiguity during
> > overload resolution between the copy deduction guide vs copy ctor guide.
> >
> > This patch makes us update DECL_NAME of a transformed guide accordingly
> > during alias CTAD. This eventually needs to be done for inherited CTAD
> > too, but it's not clear what identifier to use there since it has to be
> > unique for each derived/base pair. For
> >
> > template<bool B, class T> struct A { ... };
> > template<class T> struct B : A<false, T> { using A<false, T>::A; }
> >
> > at first glance it'd be reasonable to give inherited guides a name of
> > __dguide_B with TREE_TYPE A<false, T>, but since that name is already
> > used B's own guides its TREE_TYPE is already B<T>.
>
> Why can't it be the same __dguide_B with TREE_TYPE B<T>?
Ah because copy_guide_p relies on TREE_TYPE in order to recognize a copy
deduction guide, and with that TREE_TYPE it would still incorrectly
return false for an inherited copy deduction guide, e.g.
A(A<B, T>) -> A<B, T>
gets transformed into
B(A<false, T>) -> B<T>
and A<false, T> != B<T> so copy_guide_p returns false.
But it just occurred to me that this TREE_TYPE clobbering of the
__dguide_foo identifier already happens if we have two class templates
with the same name in different namespaces, since the identifier
contains only the terminal name. Maybe this suggests that we should
use a tree flag to track whether a guide is the copy deduction guide
instead of setting TREE_TYPE of DECL_NAME?
>
> > PR c++/115198
> >
> > gcc/cp/ChangeLog:
> >
> > * pt.cc (alias_ctad_tweaks): Update DECL_NAME of a transformed
> > guide during alias CTAD.
> >
> > gcc/testsuite/ChangeLog:
> >
> > * g++.dg/cpp2a/class-deduction-alias22.C: New test.
> > ---
> > gcc/cp/pt.cc | 9 ++++++++-
> > .../g++.dg/cpp2a/class-deduction-alias22.C | 14 ++++++++++++++
> > 2 files changed, 22 insertions(+), 1 deletion(-)
> > create mode 100644 gcc/testsuite/g++.dg/cpp2a/class-deduction-alias22.C
> >
> > diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> > index 0c4d96cf768..58873057abc 100644
> > --- a/gcc/cp/pt.cc
> > +++ b/gcc/cp/pt.cc
> > @@ -30304,13 +30304,14 @@ alias_ctad_tweaks (tree tmpl, tree uguides)
> > any). */
> > enum { alias, inherited } ctad_kind;
> > - tree atype, fullatparms, utype;
> > + tree atype, fullatparms, utype, name;
> > if (TREE_CODE (tmpl) == TEMPLATE_DECL)
> > {
> > ctad_kind = alias;
> > atype = TREE_TYPE (tmpl);
> > fullatparms = DECL_TEMPLATE_PARMS (tmpl);
> > utype = DECL_ORIGINAL_TYPE (DECL_TEMPLATE_RESULT (tmpl));
> > + name = dguide_name (tmpl);
> > }
> > else
> > {
> > @@ -30318,6 +30319,10 @@ alias_ctad_tweaks (tree tmpl, tree uguides)
> > atype = NULL_TREE;
> > fullatparms = TREE_PURPOSE (tmpl);
> > utype = TREE_VALUE (tmpl);
> > + /* FIXME: What name should we give inherited guides? It needs to be
> > + unique to the derived/base pair so that we don't clobber an earlier
> > + setting of TREE_TYPE. */
> > + name = NULL_TREE;
> > }
> > tsubst_flags_t complain = tf_warning_or_error;
> > @@ -30413,6 +30418,8 @@ alias_ctad_tweaks (tree tmpl, tree uguides)
> > }
> > if (g == error_mark_node)
> > continue;
> > + if (name)
> > + DECL_NAME (g) = name;
> > if (nfparms == 0)
> > {
> > /* The targs are all non-dependent, so g isn't a template. */
> > diff --git a/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias22.C
> > b/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias22.C
> > new file mode 100644
> > index 00000000000..9c6c841166a
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias22.C
> > @@ -0,0 +1,14 @@
> > +// PR c++/115198
> > +// { dg-do compile { target c++20 } }
> > +
> > +template<bool B, class T>
> > +struct C {
> > + C() = default;
> > + C(const C&) = default;
> > +};
> > +
> > +template<class T>
> > +using A = C<false, T>;
> > +
> > +C<false, int> c;
> > +A a = c; // { dg-bogus "ambiguous" }
>
>
