Avoid calling a trivial default constructor.

	* class.c (default_ctor_p): New.
	(in_class_defaulted_default_constructor): Use it.
	(type_has_non_user_provided_default_constructor): Use it.
	* call.c (build_over_call): Handle trivial default constructor.
	* cp-tree.h: Declare default_ctor_p.

From-SVN: r239783

gcc/cp/ChangeLog

 2016-08-15  Jason Merrill  <jason@redhat.com>
 
+	Avoid calling a trivial default constructor.
+	* class.c (default_ctor_p): New.
+	(in_class_defaulted_default_constructor): Use it.
+	(type_has_non_user_provided_default_constructor): Use it.
+	* call.c (build_over_call): Handle trivial default constructor.
+	* cp-tree.h: Declare default_ctor_p.
+
 	PR c++/57728
 	* pt.c (do_type_instantiation): Don't mess with non-user-provided
 	member functions.

gcc/cp/call.c

@@ -7798,11 +7798,19 @@ build_over_call (struct z_candidate *cand, int flags, tsubst_flags_t complain)
 
       return val;
     }
-  else if (DECL_DESTRUCTOR_P (fn)
-	   && trivial_fn_p (fn)
-	   && !DECL_DELETED_FN (fn))
-    return fold_convert (void_type_node, argarray[0]);
-  /* FIXME handle trivial default constructor, too.  */
+  else if (!DECL_DELETED_FN (fn)
+	   && trivial_fn_p (fn))
+    {
+      if (DECL_DESTRUCTOR_P (fn))
+	return fold_convert (void_type_node, argarray[0]);
+      else if (default_ctor_p (fn))
+	{
+	  if (is_dummy_object (argarray[0]))
+	    return force_target_expr (DECL_CONTEXT (fn), void_node, complain);
+	  else
+	    return cp_build_indirect_ref (argarray[0], RO_NULL, complain);
+	}
+    }
 
   /* For calls to a multi-versioned function, overload resolution
      returns the function with the highest target priority, that is,

gcc/cp/class.c

@@ -5133,8 +5133,17 @@ set_method_tm_attributes (tree t)
     }
 }
 
-/* Returns true iff class T has a user-defined constructor other than
-   the default constructor.  */
+/* Returns true if FN is a default constructor.  */
+
+bool
+default_ctor_p (tree fn)
+{
+  return (DECL_CONSTRUCTOR_P (fn)
+	  && sufficient_parms_p (FUNCTION_FIRST_USER_PARMTYPE (fn)));
+}
+
+/* Returns true iff class T has a user-defined constructor that can be called
+   with more than zero arguments.  */
 
 bool
 type_has_user_nondefault_constructor (tree t)
@@ -5163,23 +5172,16 @@ type_has_user_nondefault_constructor (tree t)
 tree
 in_class_defaulted_default_constructor (tree t)
 {
-  tree fns, args;
-
   if (!TYPE_HAS_USER_CONSTRUCTOR (t))
     return NULL_TREE;
 
-  for (fns = CLASSTYPE_CONSTRUCTORS (t); fns; fns = OVL_NEXT (fns))
+  for (tree fns = CLASSTYPE_CONSTRUCTORS (t); fns; fns = OVL_NEXT (fns))
     {
       tree fn = OVL_CURRENT (fns);
 
-      if (DECL_DEFAULTED_IN_CLASS_P (fn))
-	{
-	  args = FUNCTION_FIRST_USER_PARMTYPE (fn);
-	  while (args && TREE_PURPOSE (args))
-	    args = TREE_CHAIN (args);
-	  if (!args || args == void_list_node)
-	    return fn;
-	}
+      if (DECL_DEFAULTED_IN_CLASS_P (fn)
+	  && default_ctor_p (fn))
+	return fn;
     }
 
   return NULL_TREE;
@@ -5268,8 +5270,8 @@ type_has_non_user_provided_default_constructor (tree t)
     {
       tree fn = OVL_CURRENT (fns);
       if (TREE_CODE (fn) == FUNCTION_DECL
-	  && !user_provided_p (fn)
-	  && sufficient_parms_p (FUNCTION_FIRST_USER_PARMTYPE (fn)))
+	  && default_ctor_p (fn)
+	  && !user_provided_p (fn))
 	return true;
     }
 

gcc/cp/cp-tree.h

@@ -5666,6 +5666,7 @@ extern void determine_key_method		(tree);
 extern void check_for_override			(tree, tree);
 extern void push_class_stack			(void);
 extern void pop_class_stack			(void);
+extern bool default_ctor_p			(tree);
 extern bool type_has_user_nondefault_constructor (tree);
 extern tree in_class_defaulted_default_constructor (tree);
 extern bool user_provided_p			(tree);

gcc/testsuite/g++.dg/cpp0x/explicit12.C

@@ -15,3 +15,5 @@ int main()
 {
   A<int> a;
 }
+
+// { dg-final { scan-assembler-not "_ZN1AIiEC1Ev" } }