[C++] Small SFINAE fix

Index: pt.c
===================================================================
--- pt.c	(revision 210279)
+++ pt.c	(working copy)
@@ -142,7 +142,7 @@  static int type_unification_real (tree, tree, tree
 				  vec<deferred_access_check, va_gc> **,
 				  bool);
 static void note_template_header (int);
-static tree convert_nontype_argument_function (tree, tree);
+static tree convert_nontype_argument_function (tree, tree, tsubst_flags_t);
 static tree convert_nontype_argument (tree, tree, tsubst_flags_t);
 static tree convert_template_argument (tree, tree, tree,
 				       tsubst_flags_t, int, tree);
@@ -5272,7 +5272,8 @@  get_underlying_template (tree tmpl)
    and check that the resulting function has external linkage.  */
 
 static tree
-convert_nontype_argument_function (tree type, tree expr)
+convert_nontype_argument_function (tree type, tree expr,
+				   tsubst_flags_t complain)
 {
   tree fns = expr;
   tree fn, fn_no_ptr;
@@ -5298,11 +5299,17 @@  static tree
 
   if (TREE_CODE (fn_no_ptr) != FUNCTION_DECL)
     {
-      error ("%qE is not a valid template argument for type %qT", expr, type);
-      if (TYPE_PTR_P (type))
-	error ("it must be the address of a function with external linkage");
-      else
-	error ("it must be the name of a function with external linkage");
+      if (complain & tf_error)
+	{
+	  error ("%qE is not a valid template argument for type %qT",
+		 expr, type);
+	  if (TYPE_PTR_P (type))
+	    error ("it must be the address of a function with "
+		   "external linkage");
+	  else
+	    error ("it must be the name of a function with "
+		   "external linkage");
+	}
       return NULL_TREE;
     }
 
@@ -5309,14 +5316,17 @@  static tree
   linkage = decl_linkage (fn_no_ptr);
   if (cxx_dialect >= cxx11 ? linkage == lk_none : linkage != lk_external)
     {
-      if (cxx_dialect >= cxx11)
-	error ("%qE is not a valid template argument for type %qT "
-	       "because %qD has no linkage",
-	       expr, type, fn_no_ptr);
-      else
-	error ("%qE is not a valid template argument for type %qT "
-	       "because %qD does not have external linkage",
-	       expr, type, fn_no_ptr);
+      if (complain & tf_error)
+	{
+	  if (cxx_dialect >= cxx11)
+	    error ("%qE is not a valid template argument for type %qT "
+		   "because %qD has no linkage",
+		   expr, type, fn_no_ptr);
+	  else
+	    error ("%qE is not a valid template argument for type %qT "
+		   "because %qD does not have external linkage",
+		   expr, type, fn_no_ptr);
+	}
       return NULL_TREE;
     }
 
@@ -5959,7 +5969,7 @@  convert_nontype_argument (tree type, tree expr, ts
 	/* Null pointer values are OK in C++11.  */
 	return perform_qualification_conversions (type, expr);
 
-      expr = convert_nontype_argument_function (type, expr);
+      expr = convert_nontype_argument_function (type, expr, complain);
       if (!expr || expr == error_mark_node)
 	return expr;
     }
@@ -5983,7 +5993,7 @@  convert_nontype_argument (tree type, tree expr, ts
 	  return NULL_TREE;
 	}
 
-      expr = convert_nontype_argument_function (type, expr);
+      expr = convert_nontype_argument_function (type, expr, complain);
       if (!expr || expr == error_mark_node)
 	return expr;
 
@@ -6696,8 +6706,9 @@  coerce_template_parameter_pack (tree parms,
   if (arg_idx - parm_idx < TREE_VEC_LENGTH (packed_args)
       && TREE_VEC_LENGTH (packed_args) > 0)
     {
-      error ("wrong number of template arguments (%d, should be %d)",
-	     arg_idx - parm_idx, TREE_VEC_LENGTH (packed_args));
+      if (complain & tf_error)
+	error ("wrong number of template arguments (%d, should be %d)",
+	       arg_idx - parm_idx, TREE_VEC_LENGTH (packed_args));
       return error_mark_node;
     }