On 5/23/24 17:42, Patrick Palka wrote:
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.
Hmm, that seems correct; the transformed candidate is not the copy
deduction guide for B.
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?
Good point.
Jason
