c++: local class in nested generic lambda [PR109241]

In this testcase, the tree walk to look for bare parameter packs was
confused by finding a type with no TREE_BINFO.  But it should be fine that
it's unset; we already checked for unexpanded packs at parse time.

I also tried doing the partial instantiation of the local class, which is
probably the long-term direction we want to go, but for stage 4 let's go
with this safer change.

	PR c++/109241

gcc/cp/ChangeLog:

	* pt.cc (find_parameter_packs_r): Handle null TREE_BINFO.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/lambda-generic-local-class2.C: New test.

gcc/cp/pt.cc

@@ -4069,10 +4069,14 @@ find_parameter_packs_r (tree *tp, int *walk_subtrees, void* data)
     case TAG_DEFN:
       t = TREE_TYPE (t);
       if (CLASS_TYPE_P (t))
-	/* Local class, need to look through the whole definition.  */
-	for (tree bb : BINFO_BASE_BINFOS (TYPE_BINFO (t)))
-	  cp_walk_tree (&BINFO_TYPE (bb), &find_parameter_packs_r,
-			ppd, ppd->visited);
+	{
+	  /* Local class, need to look through the whole definition.
+	     TYPE_BINFO might be unset for a partial instantiation.  */
+	  if (TYPE_BINFO (t))
+	    for (tree bb : BINFO_BASE_BINFOS (TYPE_BINFO (t)))
+	      cp_walk_tree (&BINFO_TYPE (bb), &find_parameter_packs_r,
+			    ppd, ppd->visited);
+	}
       else
 	/* Enum, look at the values.  */
 	for (tree l = TYPE_VALUES (t); l; l = TREE_CHAIN (l))

gcc/testsuite/g++.dg/cpp1y/lambda-generic-local-class2.C

+// PR c++/109241
+// { dg-do compile { target c++14 } }
+// { dg-options "" } no pedantic
+
+void g() {
+  [](auto) {
+    [](auto) {
+      ({
+        struct A {};
+      });
+    };
+  }(1);
+}