c++: class-head parsing and CPP_TEMPLATE_ID access [PR108275]

When tentatively parsing what is really an elaborated-type-specifier
containing a template-id first as a class-specifier, we may form a
CPP_TEMPLATE_ID token that later gets reused by the fallback parse if
the tentative parse fails.  These special tokens also capture the access
checks that have been deferred while parsing the template-id.  But here
we form such a token when the access check state is dk_no_check, and so
the token captures no access checks.  This effectively bypasses access
checking for the template-id during the subsequent parse as an
elaborated-type-specifier.

This patch fixes this by using dk_deferred instead of dk_no_check when
parsing the class name of a class-head.

	PR c++/108275

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_class_head): Use dk_deferred instead of
	dk_no_check when parsing the class name.

gcc/testsuite/ChangeLog:

	* g++.dg/parse/access14.C: New test.

1 files changed, 17 insertions, 6 deletions

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index bfd8aea..8b1658d 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -26559,7 +26559,23 @@ cp_parser_class_head (cp_parser* parser,
   if (cp_parser_global_scope_opt (parser, /*current_scope_valid_p=*/false))
     qualified_p = true;
 
-  push_deferring_access_checks (dk_no_check);
+  /* It is OK to define an inaccessible class; for example:
+
+       class A { class B; };
+       class A::B {};
+
+     So we want to ignore access when parsing the class name.
+     However, we might be tentatively parsing what is really an
+     elaborated-type-specifier naming a template-id, e.g.
+
+       struct C<&D::m> c;
+
+     In this case the tentative parse as a class-head will fail, but not
+     before cp_parser_template_id splices in a CPP_TEMPLATE_ID token.
+     Since dk_no_check is sticky, we must instead use dk_deferred so that
+     any such CPP_TEMPLATE_ID token created during this tentative parse
+     will correctly capture the access checks imposed by the template-id . */
+  push_deferring_access_checks (dk_deferred);
 
   /* Determine the name of the class.  Begin by looking for an
      optional nested-name-specifier.  */
@@ -26586,11 +26602,6 @@ cp_parser_class_head (cp_parser* parser,
 	 The proposed resolution for Core Issue 180 says that wherever
 	 you see `class T::X' you should treat `X' as a type-name.
 
-	 It is OK to define an inaccessible class; for example:
-
-	   class A { class B; };
-	   class A::B {};
-
 	 We do not know if we will see a class-name, or a
 	 template-name.  We look for a class-name first, in case the
 	 class-name is a template-id; if we looked for the