Support braced-init-lists with `mkNode()` (#6580)

This commit adds support for braced-init-lists in calls to `mkNode()`,
e.g., `mkNode(REGEXP_EMPTY, {})`. Previously, such a call would result
in a node of kind `REGEXP_EMPTY` with a single null node as a child
because the compiler chose the `mkNode(Kind kind, TNode child1)` variant
and converted `{}` to a node using the default constructor. This commit
adds an overload of `mkNode()` that takes an `initializer_list<TNode>`
to allow this use case. It also adds a `mkNode()` overload with zero children
for convenience and removes the 4- and 5-children variants because they
saw little use. Finally, it makes the default constructor of `NodeTemplate` 
explicit to avoid accidental conversions.

1 files changed, 1 insertions, 1 deletions

diff --git a/test/unit/node/node_black.cpp b/test/unit/node/node_black.cpp
index 2309c0ac7..852f06dbc 100644
--- a/test/unit/node/node_black.cpp
+++ b/test/unit/node/node_black.cpp
@@ -620,7 +620,7 @@ TEST_F(TestNodeBlackNode, dagifier)
   Node fgx_eq_gy = d_nodeManager->mkNode(EQUAL, fgx, gy);
 
   Node n = d_nodeManager->mkNode(
-      OR, fffx_eq_x, fffx_eq_y, fx_eq_gx, x_eq_y, fgx_eq_gy);
+      OR, {fffx_eq_x, fffx_eq_y, fx_eq_gx, x_eq_y, fgx_eq_gy});
 
   std::stringstream sstr;
   sstr << Node::setdepth(-1) << Node::setlanguage(language::output::LANG_CVC);