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.
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.
Martin
