diff --git a/gcc/fortran/interface.c b/gcc/fortran/interface.c
index 320eb01..d4c2629 100644
--- a/gcc/fortran/interface.c
+++ b/gcc/fortran/interface.c
@@ -484,13 +484,24 @@ gfc_compare_types (gfc_typespec *ts1, gfc_typespec *ts2)
   if (ts1->type == BT_VOID || ts2->type == BT_VOID)
     return 1;
 
-  if (ts1->type == BT_CLASS
-      && ts1->u.derived->components->ts.u.derived->attr.unlimited_polymorphic)
+  /* The _data component is not always present, therefore check for its
+     presence before assuming, that its derived->attr is available.
+     When the _data component is not present, then nevertheless the
+     unlimited_polymorphic flag may be set in the derived type's attr.  */
+  if (ts1->type == BT_CLASS && ts1->u.derived->components
+      && ((ts1->u.derived->attr.is_class
+	   && ts1->u.derived->components->ts.u.derived->attr
+						  .unlimited_polymorphic)
+	  || ts1->u.derived->attr.unlimited_polymorphic))
     return 1;
 
   /* F2003: C717  */
   if (ts2->type == BT_CLASS && ts1->type == BT_DERIVED
-      && ts2->u.derived->components->ts.u.derived->attr.unlimited_polymorphic
+      && ts2->u.derived->components
+      && ((ts2->u.derived->attr.is_class
+	   && ts2->u.derived->components->ts.u.derived->attr
+						  .unlimited_polymorphic)
+	  || ts2->u.derived->attr.unlimited_polymorphic)
       && (ts1->u.derived->attr.sequence || ts1->u.derived->attr.is_bind_c))
     return 1;
 
diff --git a/gcc/fortran/symbol.c b/gcc/fortran/symbol.c
index 44392e8..b18608b 100644
--- a/gcc/fortran/symbol.c
+++ b/gcc/fortran/symbol.c
@@ -4567,7 +4567,10 @@ gfc_type_compatible (gfc_typespec *ts1, gfc_typespec *ts2)
 
   if (is_class1
       && ts1->u.derived->components
-      && ts1->u.derived->components->ts.u.derived->attr.unlimited_polymorphic)
+      && ((ts1->u.derived->attr.is_class
+	   && ts1->u.derived->components->ts.u.derived->attr
+							.unlimited_polymorphic)
+	  || ts1->u.derived->attr.unlimited_polymorphic))
     return 1;
 
   if (!is_derived1 && !is_derived2 && !is_class1 && !is_class2)
@@ -4578,13 +4581,21 @@ gfc_type_compatible (gfc_typespec *ts1, gfc_typespec *ts2)
 
   if (is_derived1 && is_class2)
     return gfc_compare_derived_types (ts1->u.derived,
-				      ts2->u.derived->components->ts.u.derived);
+				      ts2->u.derived->attr.is_class ?
+				      ts2->u.derived->components->ts.u.derived
+				      : ts2->u.derived);
   if (is_class1 && is_derived2)
-    return gfc_type_is_extension_of (ts1->u.derived->components->ts.u.derived,
+    return gfc_type_is_extension_of (ts1->u.derived->attr.is_class ?
+				       ts1->u.derived->components->ts.u.derived
+				     : ts1->u.derived,
 				     ts2->u.derived);
   else if (is_class1 && is_class2)
-    return gfc_type_is_extension_of (ts1->u.derived->components->ts.u.derived,
-				     ts2->u.derived->components->ts.u.derived);
+    return gfc_type_is_extension_of (ts1->u.derived->attr.is_class ?
+				       ts1->u.derived->components->ts.u.derived
+				     : ts1->u.derived,
+				     ts2->u.derived->attr.is_class ?
+				       ts2->u.derived->components->ts.u.derived
+				     : ts2->u.derived);
   else
     return 0;
 }
diff --git a/gcc/testsuite/gfortran.dg/implicit_class_1.f90 b/gcc/testsuite/gfortran.dg/implicit_class_1.f90
index 329f57a..46516ca 100644
--- a/gcc/testsuite/gfortran.dg/implicit_class_1.f90
+++ b/gcc/testsuite/gfortran.dg/implicit_class_1.f90
@@ -4,6 +4,12 @@
 !
 ! Contributed by Reinhold Bader <Reinhold.Bader@lrz.de>
 
+! Add dump-fortran-original to check, if the patch preventing a gfortran
+! segfault is working correctly.  No cleanup needed, because the dump
+! goes to stdout.
+! { dg-options "-fdump-fortran-original" }
+! { dg-prune-output "Namespace:.*" }
+
 program upimp
   implicit class(foo) (a-b)
   implicit class(*) (c)