Re: [PATCH] restore -Wunused-variable on a typedef'd variable in a function template (PR 79548)

[Jason Merrill <jason at redhat dot com>]

On Thu, Feb 23, 2017 at 6:33 PM, Jason Merrill <jason@redhat.com> wrote:
> On Thu, Feb 23, 2017 at 12:56 PM, Martin Sebor <msebor@gmail.com> wrote:
>> On 02/22/2017 05:43 PM, Jason Merrill wrote:
>>> On Wed, Feb 22, 2017 at 3:44 PM, Martin Sebor <msebor@gmail.com> wrote:
>>>> On 02/22/2017 11:02 AM, Jason Merrill wrote:
>
>>>> The TREE_USED bit on the type (i.e., on
>>>> TREE_TYPE(decl) where decl is the u in the test case above) is
>>>> set when the function template is instantiated, in
>>>> set_underlying_type called from tsubst_decl.
>>>
>>> Aha!  That seems like the problem.  Does removing that copy of
>>> TREE_USED from the decl to the type break anything?
>>
>> As far as I can see it breaks just gcc.dg/unused-3.c which is:
>>
>>   typedef short unused_type __attribute__ ((unused));
>>
>>   void f (void)
>>   {
>>     unused_type y;
>>   }
>
> Ah.  So the problem here is that we're using TREE_USED on a TYPE_DECL
> for two things: to track whether the typedef has been used, and to
> determine whether to treat a variable of that type as already used.
> Normally this isn't too much of a problem because we copy the
> TREE_USED flag from decl to type before there could be any uses, but
> in a template I guess we mark the typedef within the template when it
> is used, and then when we instantiate the typedef we incorrectly pass
> that mark along to the type.
>
> Your patch deals with this by ignoring TREE_USED on the type, so it
> doesn't matter that it's wrong.  Another approach would be to correct
> the setting of TREE_USED by setting it based on looking up the unused
> attribute, rather than copying it from the TYPE_DECL.  So also going
> back to the attribute, but in set_underlying_type rather than
> poplevel.

Thoughts?

Jason