[Bug c++/68965] [5/6 Regression] `-Wunused-parameter` is reported in variadic lambda or function using sizeof...(xs)

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68965

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
This seems to fix it and hopefully doesn't regress anything...

--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -11924,6 +11924,9 @@ tsubst_decl (tree t, tree args, tsubst_flags_t
complain)
        /* Rename the parameter to include the index.  */
        DECL_NAME (r)
          = make_ith_pack_parameter_name (DECL_NAME (r), i);
+
+       /* Set TREE_USED for the benefit of -Wunused.  */
+       TREE_USED (r) = true;
               }
             else if (!type)
               /* We're dealing with a normal parameter.  */