PR c++/88631 - CTAD failing for value-initialization.

	* typeck2.c (build_functional_cast): Try deducing the template
	arguments even if there are no arguments to deduce from.

	* g++.dg/cpp1z/class-deduction59.C: New test.

From-SVN: r267533

gcc/cp/ChangeLog

@@ -4,6 +4,10 @@
 	* call.c (convert_for_arg_passing): Only give warnings with tf_warning.
 	* typeck.c (convert_for_assignment): Likewise.
 
+	PR c++/88631 - CTAD failing for value-initialization.
+	* typeck2.c (build_functional_cast): Try deducing the template
+	arguments even if there are no arguments to deduce from.
+
 2019-01-01  Jakub Jelinek  <jakub@redhat.com>
 
 	Update copyright years.

gcc/cp/typeck2.c

@@ -2147,9 +2147,18 @@ build_functional_cast (tree exp, tree parms, tsubst_flags_t complain)
 	}
       else if (!parms)
 	{
-	  if (complain & tf_error)
-	    error ("cannot deduce template arguments for %qT from ()", anode);
-	  return error_mark_node;
+	  /* Even if there are no parameters, we might be able to deduce from
+	     default template arguments.  Pass TF_NONE so that we don't
+	     generate redundant diagnostics.  */
+	  type = do_auto_deduction (type, parms, anode, tf_none,
+				    adc_variable_type);
+	  if (type == error_mark_node)
+	    {
+	      if (complain & tf_error)
+		error ("cannot deduce template arguments for %qT from ()",
+		       anode);
+	      return error_mark_node;
+	    }
 	}
       else
 	type = do_auto_deduction (type, parms, anode, complain,

gcc/testsuite/ChangeLog

@@ -3,6 +3,9 @@
 	PR c++/88612 - ICE with -Waddress-of-packed-member.
 	* g++.dg/warn/Waddress-of-packed-member1.C: New test.
 
+	PR c++/88631 - CTAD failing for value-initialization.
+	* g++.dg/cpp1z/class-deduction59.C: New test.
+
 2019-01-02  Martin Sebor  <msebor@redhat.com>
             Jeff Law  <law@redhat.com>
 

gcc/testsuite/g++.dg/cpp1z/class-deduction59.C

+// PR c++/88631
+// { dg-do compile { target c++17 } }
+
+template<class T = void>
+class A { };
+
+int main()
+{
+  auto x = A();
+  auto x2 = A{};
+  A y;
+}