c++, abi: Fix abi_tag attribute handling [PR98481]

In GCC10 cp_walk_subtrees has been changed to walk template arguments.
As the following testcase, that changed the mangling of some functions.
I believe the previous behavior that find_abi_tags_r doesn't recurse into
template args has been the correct one, but setting *walk_subtrees = 0
for the types and handling the types subtree walking manually in
find_abi_tags_r looks too hard, there are a lot of subtrees and details what
should and shouldn't be walked, both in tree.c (walk_type_fields there,
which is static) and in cp_walk_subtrees itself.

The following patch abuses the fact that *walk_subtrees is an int to
tell cp_walk_subtrees it shouldn't walk the template args.

Co-authored-by: Jason Merrill <jason@redhat.com>

gcc/cp/ChangeLog:

	PR c++/98481
	* class.c (find_abi_tags_r): Set *walk_subtrees to 2 instead of 1
	for types.
	(mark_abi_tags_r): Likewise.
	* decl2.c (min_vis_r): Likewise.
	* tree.c (cp_walk_subtrees): If *walk_subtrees_p is 2, look through
	typedefs.

gcc/testsuite/ChangeLog:

	PR c++/98481
	* g++.dg/abi/abi-tag24.C: New test.

1 files changed, 4 insertions, 3 deletions

diff --git a/gcc/cp/decl2.c b/gcc/cp/decl2.c
index b106710..b087753 100644
--- a/gcc/cp/decl2.c
+++ b/gcc/cp/decl2.c
@@ -2358,9 +2358,6 @@ min_vis_r (tree *tp, int *walk_subtrees, void *data)
   int this_vis = VISIBILITY_DEFAULT;
   if (! TYPE_P (*tp))
     *walk_subtrees = 0;
-  else if (typedef_variant_p (*tp))
-    /* Look through typedefs despite cp_walk_subtrees.  */
-    this_vis = type_visibility (DECL_ORIGINAL_TYPE (TYPE_NAME (*tp)));
   else if (OVERLOAD_TYPE_P (*tp)
 	   && !TREE_PUBLIC (TYPE_MAIN_DECL (*tp)))
     {
@@ -2379,6 +2376,10 @@ min_vis_r (tree *tp, int *walk_subtrees, void *data)
   if (this_vis > *vis_p)
     *vis_p = this_vis;
 
+  /* Tell cp_walk_subtrees to look through typedefs.  */
+  if (*walk_subtrees == 1)
+    *walk_subtrees = 2;
+
   return NULL;
 }