Someone on the C++ committee observed that
std::less l{};
didn't work in G++; this fixes that.
Tested x86_64-pc-linux-gnu, applying to trunk.
commit d04e2de711e69266b24c118eb330100859b2671c
Author: Jason Merrill <ja...@redhat.com>
Date: Tue Feb 21 16:50:29 2017 -0800
* pt.c (do_class_deduction): Handle 0 argument case.
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 5b0f62d..17175ba 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -25126,6 +25126,14 @@ do_class_deduction (tree ptype, tree tmpl, tree init,
int flags,
if (cands == NULL_TREE)
{
+ if (args->length() == 0)
+ {
+ /* Try tmpl<>. */
+ tree t = lookup_template_class (tmpl, NULL_TREE, NULL_TREE,
+ NULL_TREE, false, tf_none);
+ if (t != error_mark_node)
+ return t;
+ }
error ("cannot deduce template arguments for %qT, as it has "
"no deduction guides or user-declared constructors", type);
return error_mark_node;
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction30.C
b/gcc/testsuite/g++.dg/cpp1z/class-deduction30.C
new file mode 100644
index 0000000..e182803
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction30.C
@@ -0,0 +1,6 @@
+// { dg-options -std=c++1z }
+
+template <class T = void> struct A { };
+
+A a{};
+
