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:
>>
>> On Tue, Feb 21, 2017 at 4:27 PM, Martin Sebor <mse...@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
