[PATCH] diagnose specializations of deprecated templates (PR c++/84318)

[Martin Sebor]

On 02/13/2018 08:35 AM, Martin Sebor wrote:
> On 02/13/2018 07:40 AM, Jason Merrill wrote:
>> On Mon, Feb 12, 2018 at 6:32 PM, Martin Sebor <msebor@gmail.com> wrote:
>>> While testing my fix for 83871 (handling attributes on explicit
>>> specializations) I noticed another old regression: while GCC 4.4
>>> would diagnose declarations of explicit specializations of all
>>> primary templates declared deprecated, GCC 4.5 and later only
>>> diagnose declarations of explicit specializations of class
>>> templates but not those of function or variable templates.
>>
>> Hmm, the discussion on the core reflector seemed to be agreeing that
>> we want to be able to define non-deprecated specializations of a
>> deprecated primary template.
>
> Yes, that's what Richard wanted to do.  The only way to do it
> within the existing constraints(*) is to define a non-deprecated
> primary, and a deprecated partial specialization.  This is in line
> with that approach and supported by Clang and all other compilers
> I tested (including Clang).

To clarify, this approach works for class templates (e.g., like
std::numeric_limits that was mentioned in the core discussion)
and for variable templates.  Functions have no partial
specilizations so they have to be overloaded to achieve the same
effect.

Implementations don't treat the deprecated attribute on partial
specializations consistently.

EDG accepts and honors it on class template partial specializations
but rejects it with an error on those of variables.

Clang accepts but silently ignores it on class template partial
specializations and rejects with an error it on variables.

MSVC accepts and honors it on variables but silently ignores it
on class template partial specializations.

GCC ignores it silently on class partial specializations and
with a warning on variables (I opened bug 84347 to track this
and to have GCC honor is everywhere).

This is clearly a mess, which isn't surprising given how poorly
specified this is in the standard.  But from the test cases and
from the core discussion it seems clear that deprecating
a template, including its partial specializations (as opposed
to just a single explicit specialization) is desirable and
already supported, and that the wording in the standard just
needs to be adjusted to reflect that.

>
> Martin
>
> [*] Except (as Richard noted) that the standard doesn't seem to
> allow a template to be deprecated.  I think that's a bug in the
> spec because all implementations allow it to some degree.