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

[Martin Sebor <msebor@gmail.com>]

> 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.

Martin