c++: Defer access checking when processing bases [PR82613]

When parsing the base-clause of a class declaration, we need to defer
access checking until the entire base-clause has been seen, so that
access can be properly checked relative to the scope of the class with
all its bases attached.  This allows us to accept the declaration of
struct D from Example 2 of [class.access.general] (access12.C below).

Similarly when substituting into the base-clause of a class template,
which is the subject of PR82613.

gcc/cp/ChangeLog:

	PR c++/82613
	* parser.c (cp_parser_class_head): Defer access checking when
	parsing the base-clause until all bases are seen and attached
	to the class type.
	* pt.c (instantiate_class_template): Likewise when substituting
	into dependent bases.

gcc/testsuite/ChangeLog:

	PR c++/82613
	* g++.dg/parse/access12.C: New test.
	* g++.dg/template/access35.C: New test.

gcc/cp/parser.c

@@ -25576,19 +25576,11 @@ cp_parser_class_head (cp_parser* parser,
 
      is valid.  */
 
-  /* Get the list of base-classes, if there is one.  */
+  /* Get the list of base-classes, if there is one.  Defer access checking
+     until the entire list has been seen, as per [class.access.general].  */
+  push_deferring_access_checks (dk_deferred);
   if (cp_lexer_next_token_is (parser->lexer, CPP_COLON))
-    {
-      /* PR59482: enter the class scope so that base-specifiers are looked
-	 up correctly.  */
-      if (type)
-	pushclass (type);
-      bases = cp_parser_base_clause (parser);
-      /* PR59482: get out of the previously pushed class scope so that the
-	 subsequent pops pop the right thing.  */
-      if (type)
-	popclass ();
-    }
+    bases = cp_parser_base_clause (parser);
   else
     bases = NULL_TREE;
 
@@ -25597,6 +25589,20 @@ cp_parser_class_head (cp_parser* parser,
   if (type && cp_lexer_next_token_is (parser->lexer, CPP_OPEN_BRACE))
     xref_basetypes (type, bases);
 
+  /* Now that all bases have been seen and attached to the class, check
+     accessibility of the types named in the base-clause.  This must be
+     done relative to the class scope, so that we accept e.g.
+
+       struct A { protected: struct B {}; };
+       struct C : A::B, A {}; // OK: A::B is accessible via base A
+
+     as per [class.access.general].  */
+  if (type)
+    pushclass (type);
+  pop_to_parent_deferring_access_checks ();
+  if (type)
+    popclass ();
+
  done:
   /* Leave the scope given by the nested-name-specifier.  We will
      enter the class scope itself while processing the members.  */

gcc/cp/pt.c

@@ -11825,17 +11825,14 @@ instantiate_class_template_1 (tree type)
 	      || COMPLETE_OR_OPEN_TYPE_P (TYPE_CONTEXT (type)));
 
   base_list = NULL_TREE;
+  /* Defer access checking while we substitute into the types named in
+     the base-clause.  */
+  push_deferring_access_checks (dk_deferred);
   if (BINFO_N_BASE_BINFOS (pbinfo))
     {
       tree pbase_binfo;
-      tree pushed_scope;
       int i;
 
-      /* We must enter the scope containing the type, as that is where
-	 the accessibility of types named in dependent bases are
-	 looked up from.  */
-      pushed_scope = push_scope (CP_TYPE_CONTEXT (type));
-
       /* Substitute into each of the bases to determine the actual
 	 basetypes.  */
       for (i = 0; BINFO_BASE_ITERATE (pbinfo, i, pbase_binfo); i++)
@@ -11877,9 +11874,6 @@ instantiate_class_template_1 (tree type)
 
       /* The list is now in reverse order; correct that.  */
       base_list = nreverse (base_list);
-
-      if (pushed_scope)
-	pop_scope (pushed_scope);
     }
   /* Now call xref_basetypes to set up all the base-class
      information.  */
@@ -11897,6 +11891,10 @@ instantiate_class_template_1 (tree type)
      class, except we also need to push the enclosing classes.  */
   push_nested_class (type);
 
+  /* Now check accessibility of the types named in its base-clause,
+     relative to the scope of the class.  */
+  pop_to_parent_deferring_access_checks ();
+
   /* Now members are processed in the order of declaration.  */
   for (member = CLASSTYPE_DECL_LIST (pattern);
        member; member = TREE_CHAIN (member))

gcc/testsuite/g++.dg/parse/access12.C

@@ -0,0 +1,24 @@
+// Example 2 of [class.access.general]
+// { dg-do compile }
+
+class A {
+  typedef int I;    // private member
+  I f();
+  friend I g(I);
+  static I x;
+  template<int> struct Q;
+  template<int> friend struct R;
+protected:
+    struct B { };
+};
+
+A::I A::f() { return 0; }
+A::I g(A::I p = A::x);
+A::I g(A::I p) { return 0; }
+A::I A::x = 0;
+// FIXME: We reject these two declarations because access checking of A::I
+// is not performed in the scope of the class being declared.
+// template<A::I> struct A::Q { };
+// template<A::I> struct R { };
+
+struct D: A::B, A { };

gcc/testsuite/g++.dg/template/access35.C

@@ -0,0 +1,19 @@
+// PR c++/82613
+// { dg-do compile }
+
+template <typename T> class B;
+
+class A {
+  friend class B<A>;
+  class Type {};
+};
+
+template <typename T>
+class B : T::Type { protected: class Type {}; };
+
+B<A> b;
+
+template <typename T>
+class C : B<T>::Type, B<T> {};
+
+C<A> c;