[gcc r11-6553] c++: Fix access checking of scoped non-static member [PR98515]

[Patrick Palka]

https://gcc.gnu.org/g:98a1fb705ead9258642f2dec0431f11508a9b13c

commit r11-6553-g98a1fb705ead9258642f2dec0431f11508a9b13c
Author: Patrick Palka <ppalka@redhat.com>
Date:   Fri Jan 8 10:02:04 2021 -0500

    c++: Fix access checking of scoped non-static member [PR98515]
    
    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 (expectedly) fails.
    
    The problem ultimately seems to be that we can't in general determine
    whether a use of a scoped non-static member goes through 'this' until
    instantiation time, as the second testcase below illustrates.  So this
    patch makes check_accessibility_of_qualified_id punt in such situations
    to avoid creating a bogus deferred access check.
    
    gcc/cp/ChangeLog:
    
            PR c++/98515
            * semantics.c (check_accessibility_of_qualified_id): Punt if
            we're checking access of a scoped non-static member inside a
            class template.
    
    gcc/testsuite/ChangeLog:
    
            PR c++/98515
            * g++.dg/template/access32.C: New test.
            * g++.dg/template/access33.C: New test.

Diff:
---
 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(-)

diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index b448efe024a..c6b4c70dc0f 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 (tree current = current_nonlambda_class_type ())
+	  {
+	    if (dependent_type_p (current))
+	    /* In general we can't know whether this access goes through
+	       `this' until instantiation time.  Punt now, or else we might
+	       create a deferred access check that's not relative to `this'
+	       when it ought to be.  We'll check this access again after
+	       substitution, e.g. from tsubst_qualified_id.  */
+	      return true;
+
+	    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>;