[gcc r13-5037] c++: class-head parsing and CPP_TEMPLATE_ID access [PR108275]

[Patrick Palka <ppalka@gcc.gnu.org>]

https://gcc.gnu.org/g:12b0d35ec52375da5652d2b8da74083ab700b9d7

commit r13-5037-g12b0d35ec52375da5652d2b8da74083ab700b9d7
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Jan 5 14:21:34 2023 -0500

    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.

Diff:
---
 gcc/cp/parser.cc                      | 23 +++++++++++++++++------
 gcc/testsuite/g++.dg/parse/access14.C | 18 ++++++++++++++++++
 2 files changed, 35 insertions(+), 6 deletions(-)

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index bfd8aeae39f..8b1658decba 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
diff --git a/gcc/testsuite/g++.dg/parse/access14.C b/gcc/testsuite/g++.dg/parse/access14.C
new file mode 100644
index 00000000000..bdbc7f6ee2b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/parse/access14.C
@@ -0,0 +1,18 @@
+// PR c++/108275
+
+struct A {
+  int i;
+private:
+  int j;
+};
+
+template<int A::* V>
+struct B {
+  struct C { };
+private:
+  template<int N> struct D { };
+};
+
+struct B<&A::j> b;       // { dg-error "private" }
+struct B<&A::j>::C c;    // { dg-error "private" }
+struct B<&A::i>::D<0> d; // { dg-error "private" }