call.c (build_over_call): Don't throw away initializations/copies of empty classes...

* call.c (build_over_call): Don't throw away
	initializations/copies of empty classes; use MODIFY_EXPR and
	INIT_EXPR as for non-empty classes.
	* class.c (finish_struct_1): Put the padding byte for an empty
	class on the TYPE_NONCOPIED_PARTS list for the class.

From-SVN: r26970

gcc/cp/ChangeLog

@@ -1,3 +1,11 @@
+1999-05-17  Mark Mitchell  <mark@codesourcery.com>
+
+	* call.c (build_over_call): Don't throw away
+	initializations/copies of empty classes; use MODIFY_EXPR and
+	INIT_EXPR as for non-empty classes.
+	* class.c (finish_struct_1): Put the padding byte for an empty
+	class on the TYPE_NONCOPIED_PARTS list for the class.
+
 1999-05-16  Mark Mitchell  <mark@codesourcery.com>
 
 	* decl2.c (build_expr_from_tree): Handle COMPONENT_REFs that

gcc/cp/call.c

@@ -3426,20 +3426,34 @@ build_over_call (cand, args, flags)
       else if (! real_lvalue_p (arg)
 	       || TYPE_HAS_TRIVIAL_INIT_REF (DECL_CONTEXT (fn)))
 	{
+	  tree address;
 	  tree to = stabilize_reference
 	    (build_indirect_ref (TREE_VALUE (args), 0));
 
-	  /* Don't copy the padding byte; it might not have been allocated
-	     if to is a base subobject.  */
-	  if (is_empty_class (DECL_CLASS_CONTEXT (fn)))
-	    return build_unary_op
-	      (ADDR_EXPR, build (COMPOUND_EXPR, TREE_TYPE (to),
-				 cp_convert (void_type_node, arg), to),
-	       0);
-
-	  val = build (INIT_EXPR, DECL_CONTEXT (fn), to, arg);
+	  /* If we're initializing an empty class, then we actually
+	     have to use a MODIFY_EXPR rather than an INIT_EXPR.  The
+	     reason is that the dummy padding member in the target may
+	     not actually be allocated if TO is a base class
+	     subobject.  Since we've set TYPE_NONCOPIED_PARTS on the
+	     padding, a MODIFY_EXPR will preserve its value, which is
+	     the right thing to do if it's not really padding at all.
+	  
+	     It's not safe to just throw away the ARG if we're looking
+	     at an empty class because the ARG might contain a
+	     TARGET_EXPR which wants to be bound to TO.  If it is not,
+	     expand_expr will assign a dummy slot for the TARGET_EXPR,
+	     and we will call a destructor for it, which is wrong,
+	     because we will also destroy TO, but will never have
+	     constructed it.  */
+	  val = build (is_empty_class (DECL_CLASS_CONTEXT (fn))
+		       ? MODIFY_EXPR : INIT_EXPR, 
+		       DECL_CONTEXT (fn), to, arg);
 	  TREE_SIDE_EFFECTS (val) = 1;
-	  return build_unary_op (ADDR_EXPR, val, 0);
+	  address = build_unary_op (ADDR_EXPR, val, 0);
+	  /* Avoid a warning about this expression, if the address is
+	     never used.  */
+	  TREE_USED (address) = 1;
+	  return address;
 	}
     }
   else if (DECL_NAME (fn) == ansi_opname[MODIFY_EXPR]
@@ -3451,12 +3465,6 @@ build_over_call (cand, args, flags)
 
       arg = build_indirect_ref (TREE_VALUE (TREE_CHAIN (converted_args)), 0);
 
-      /* Don't copy the padding byte; it might not have been allocated
-	 if to is a base subobject.  */
-      if (is_empty_class (DECL_CLASS_CONTEXT (fn)))
-	return build (COMPOUND_EXPR, TREE_TYPE (to),
-		      cp_convert (void_type_node, arg), to);
-
       val = build (MODIFY_EXPR, TREE_TYPE (to), to, arg);
       TREE_SIDE_EFFECTS (val) = 1;
       return val;

gcc/cp/class.c

@@ -3133,6 +3133,7 @@ finish_struct_1 (t, warn_anon)
   int aggregate = 1;
   int empty = 1;
   int has_pointers = 0;
+  tree inline_friends;
 
   if (warn_anon && code != UNION_TYPE && ANON_AGGRNAME_P (TYPE_IDENTIFIER (t)))
     pedwarn ("anonymous class type not used to declare any objects");
@@ -3740,6 +3741,13 @@ finish_struct_1 (t, warn_anon)
 	if (DECL_SIZE (x) != integer_zero_node)
 	  empty = 0;
     }
+
+  /* CLASSTYPE_INLINE_FRIENDS is really TYPE_NONCOPIED_PARTS.  Thus,
+     we have to save this before we start modifying
+     TYPE_NONCOPIED_PARTS.  */
+  inline_friends = CLASSTYPE_INLINE_FRIENDS (t);
+  CLASSTYPE_INLINE_FRIENDS (t) = NULL_TREE;
+
   if (empty)
     {
       /* C++: do not let empty structures exist.  */
@@ -3747,7 +3755,11 @@ finish_struct_1 (t, warn_anon)
 	(FIELD_DECL, NULL_TREE, char_type_node);
       TREE_CHAIN (decl) = fields;
       TYPE_FIELDS (t) = decl;
+      TYPE_NONCOPIED_PARTS (t) 
+	= tree_cons (NULL_TREE, decl, TYPE_NONCOPIED_PARTS (t));
+      TREE_STATIC (TYPE_NONCOPIED_PARTS (t)) = 1;
     }
+
   if (n_baseclasses)
     TYPE_FIELDS (t) = chainon (last_x, TYPE_FIELDS (t));
 
@@ -4021,8 +4033,7 @@ finish_struct_1 (t, warn_anon)
     }
 
   /* Write out inline function definitions.  */
-  do_inline_function_hair (t, CLASSTYPE_INLINE_FRIENDS (t));
-  CLASSTYPE_INLINE_FRIENDS (t) = 0;
+  do_inline_function_hair (t, inline_friends);
 
   if (CLASSTYPE_VSIZE (t) != 0)
     {
@@ -4049,7 +4060,9 @@ finish_struct_1 (t, warn_anon)
       /* In addition to this one, all the other vfields should be listed.  */
       /* Before that can be done, we have to have FIELD_DECLs for them, and
 	 a place to find them.  */
-      TYPE_NONCOPIED_PARTS (t) = build_tree_list (default_conversion (TYPE_BINFO_VTABLE (t)), vfield);
+      TYPE_NONCOPIED_PARTS (t) 
+	= tree_cons (default_conversion (TYPE_BINFO_VTABLE (t)),
+		     vfield, TYPE_NONCOPIED_PARTS (t));
 
       if (warn_nonvdtor && TYPE_HAS_DESTRUCTOR (t)
 	  && DECL_VINDEX (TREE_VEC_ELT (method_vec, 1)) == NULL_TREE)

gcc/testsuite/g++.old-deja/g++.other/empty1.C

@@ -0,0 +1,23 @@
+// Origin: Mark Mitchell <mark@codesourcery.com>
+
+extern "C" void abort();
+extern "C" void printf(const char*, ...);
+
+int i;
+
+struct A;
+
+struct A* as[10];
+
+struct A {
+  A () { as[i++] = this; }
+  A (const A&) { as[i++] = this; }
+  ~A() { if (i == 0 || as[--i] != this) abort(); }
+};
+
+A f() { return A(); }
+
+int main ()
+{
+  A a (f ());
+}