[PATCH] c++: Fix access checking of scoped non-static member [PR98515]

[Patrick Palka <ppalka@redhat.com>]

In the first testcase below, we incorrectly reject the use of the
protected non-static member A::var0 from C<int>::g() because
check_accessibility_of_qualified_id, at template parse time, determines
that the access doesn't go through 'this'.  (This happens because the
dependent base B<T> of C<T> doesn't have a binfo object, so it appears
to DERIVED_FROM_P that A is not an indirect base of C<T>.)  From there
we create the corresponding deferred access check, which we then
perform at instantiation time and which (unsurprisingly) fails.

The problem ultimately seems to be that we can't, in general, know
whether a use of a scoped non-static member goes through 'this' until
instantiation time, as the second testcase below demonstrates.  So this
patch makes check_accessibility_of_qualified_id punt in this situation.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK to
commit?

gcc/cp/ChangeLog:

	PR c++/98515
	* semantics.c (check_accessibility_of_qualified_id): Punt if
	we're checking the access of a scoped non-static member at
	class template parse time.

gcc/testsuite/ChangeLog:

	PR c++/98515
	* g++.dg/template/access32.C: New test.
	* g++.dg/template/access33.C: New test.
---
 gcc/cp/semantics.c                       | 20 +++++++++++++++-----
 gcc/testsuite/g++.dg/template/access32.C |  8 ++++++++
 gcc/testsuite/g++.dg/template/access33.C |  9 +++++++++
 3 files changed, 32 insertions(+), 5 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/template/access32.C
 create mode 100644 gcc/testsuite/g++.dg/template/access33.C

diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index b448efe024a..f52b2e4d1e7 100644
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -2107,14 +2107,24 @@ check_accessibility_of_qualified_id (tree decl,
       /* If the reference is to a non-static member of the
 	 current class, treat it as if it were referenced through
 	 `this'.  */
-      tree ct;
       if (DECL_NONSTATIC_MEMBER_P (decl)
-	  && current_class_ptr
-	  && DERIVED_FROM_P (scope, ct = current_nonlambda_class_type ()))
-	qualifying_type = ct;
+	  && current_class_ptr)
+	{
+	  if (dependent_type_p (TREE_TYPE (current_class_ptr)))
+	  /* In general we can't know whether this access goes through `this'
+	     until instantiation of the current class.  Punt now, or else
+	     we might create a deferred access check that's relative to the
+	     wrong class.  We'll check this access again after substitution,
+	     e.g. from tsubst_qualified_id.  */
+	    return true;
+
+	  if (tree current = current_nonlambda_class_type ())
+	    if (DERIVED_FROM_P (scope, current))
+	      qualifying_type = current;
+	}
       /* Otherwise, use the type indicated by the
 	 nested-name-specifier.  */
-      else
+      if (!qualifying_type)
 	qualifying_type = nested_name_specifier;
     }
   else
diff --git a/gcc/testsuite/g++.dg/template/access32.C b/gcc/testsuite/g++.dg/template/access32.C
new file mode 100644
index 00000000000..08faa9f0f97
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/access32.C
@@ -0,0 +1,8 @@
+// PR c++/98515
+// { dg-do compile }
+
+struct A { protected: int var0; };
+template <class> struct B : public A { };
+template <class T> struct C : public B<T> { void g(); };
+template <class T> void C<T>::g() { A::var0++; }
+template class C<int>;
diff --git a/gcc/testsuite/g++.dg/template/access33.C b/gcc/testsuite/g++.dg/template/access33.C
new file mode 100644
index 00000000000..9fb9b9a1236
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/access33.C
@@ -0,0 +1,9 @@
+// PR c++/98515
+// { dg-do compile }
+
+struct A { protected: int var0; };
+template <class> struct B : public A { };
+template <class T> struct C : public B<T> { void g(); };
+template <class T> void C<T>::g() { A::var0++; } // { dg-error "protected|invalid" }
+template <> struct B<char> { };
+template class C<char>;
-- 
2.30.0