On Thu, Feb 23, 2017 at 12:56 PM, Martin Sebor <mse...@gmail.com> wrote:
> On 02/22/2017 05:43 PM, Jason Merrill wrote:
>> On Wed, Feb 22, 2017 at 3:44 PM, Martin Sebor <mse...@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.
Jason
