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Re: [PATCH] [PR86823] retain deferred access checks from outside firewall

On 11/16/18 9:16 PM, Alexandre Oliva wrote:
We used to preserve deferred access check along with resolved template
ids, but a tentative parsing firewall introduced additional layers of
deferred access checks, so that we don't preserve the checks we
want to any more.

This patch collapses the access check levels introduced by the
firewall with those we wanted to preserve to begin with, saves them,
pushes them one more layer up so that the firewall doesn't drop them,
and then pushes new empty levels so balance out with the upcoming

We may want to avoid the pop_to_parent and one push after the save, and
instead leave the firewall's scope before the final pop_to_parent.
However, this patch is what I regression-tested successfully with
check-g++ on x86_64-linux-gnu.  Regstrapping on i686- and
x86_64-linux-gnu now.  Ok to install?

for  gcc/cp/ChangeLog

	PR c++/86823
	* parser.c (cp_parser_template_id): Merge access checks from
	outside the tentative parsing firewall with those from inside,
	and save them all with the template id token.

for  gcc/testsuite/ChangeLog

	PR c++/86823
	* g++.dg/pr86823.C: New.
  gcc/cp/parser.c                |   12 ++++++++++++
  gcc/testsuite/g++.dg/pr86823.C |   15 +++++++++++++++
  2 files changed, 27 insertions(+)
  create mode 100644 gcc/testsuite/g++.dg/pr86823.C

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index db0f0338179e..6a08b09715a7 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -16185,7 +16185,19 @@ cp_parser_template_id (cp_parser *parser,
  	 so the memory will not be reclaimed during token replacing below.  */
        token->u.tree_check_value = ggc_cleared_alloc<struct tree_check> ();
        token->u.tree_check_value->value = template_id;
+      /* Collapse the access check levels introduced by
+	 tentative_firewall with the one we introduced ourselves.  */
+      pop_to_parent_deferring_access_checks ();
+      pop_to_parent_deferring_access_checks ();
        token->u.tree_check_value->checks = get_deferred_access_checks ();
+      /* Push the checks up one more level, so that the firewall
+	 doesn't drop them on the floor when we return.  Then, push
+	 empty levels back in place so that they are popped
+	 properly.  */
+      pop_to_parent_deferring_access_checks ();
+      push_deferring_access_checks (dk_deferred);
+      push_deferring_access_checks (dk_deferred);
+      push_deferring_access_checks (dk_deferred);

Hmm, I'm uncomfortable with how this depends on the specific implementation of tentative_firewall. What do you think of this alternate approach (untested other than with the testcase)?

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index ac19cb4b9bb..c7ec7dcf413 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -16182,16 +16182,18 @@ cp_parser_template_id (cp_parser *parser,
+  /* Push any access checks inside the firewall we're about to create.  */
+  vec<deferred_access_check, va_gc> *checks = get_deferred_access_checks ();
+  pop_deferring_access_checks ();
   if (templ == error_mark_node || is_identifier)
-    {
-      pop_deferring_access_checks ();
-      return templ;
-    }
+    return templ;
   /* Since we're going to preserve any side-effects from this parse, set up a
      firewall to protect our callers from cp_parser_commit_to_tentative_parse
      in the template arguments.  */
   tentative_firewall firewall (parser);
+  reopen_deferring_access_checks (checks);
   /* If we find the sequence `[:' after a template-name, it's probably
      a digraph-typo for `< ::'. Substitute the tokens and check if we can

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