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

[Jason Merrill <jason@redhat.com>]

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:
>>
>> On Tue, Feb 21, 2017 at 4:27 PM, Martin Sebor <msebor@gmail.com> wrote:
>>>>
>>>> Ah, I see, your patch changes attribute unused handling for local
>>>> variables from tracking TREE_USED to lookup_attribute.  I'm not
>>>> opposed to this change, but I'd like to understand why the TREE_USED
>>>> handling wasn't working.
>>>
>>>
>>>
>>> In the test case in the bug:
>>>
>>>   template <class T>
>>>   void g ()
>>>   {
>>>     T t;   // warning, ok
>>>
>>>     typedef T U;
>>>     U u;   // no warning, bug
>>>   }
>>>
>>>   template void g<int>();
>>>
>>> both TREE_USED(T) and TREE_USED(t) are zero in initialize_local_var
>>> so the function doesn't set already_used or TREE_USED(t) and we get
>>> a warning as expected.
>>>
>>> But because TREE_USED(U) is set to 1 in maybe_record_typedef_use
>>> (to implement -Wunused-local-typedefs),  initialize_local_var then
>>> sets already_used to 1 and later also TREE_USED(u) to 1, suppressing
>>> the warning.
>>
>> Hmm, I would expect maybe_record_typedef_use to set TREE_USED on the
>> TYPE_DECL, not on the *_TYPE which initialize_local_var checks.
>
> That's what it does:
>
>   void
>   maybe_record_typedef_use (tree t)
>   {
>     if (!is_typedef_decl (t))
>       return;
>
>     TREE_USED (t) = true;
>   }
>
> Here, t is a TYPE_DECL of the typedef U.

Yes.  It is a TYPE_DECL, not a type.

> It has the effect of TREE_USED (TREE_TYPE (decl)) being set in
> initialize_local_var.  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?

Jason