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.

2 files changed, 2 insertions, 2 deletions

diff --git a/src/theory/arith/arith_utilities.h b/src/theory/arith/arith_utilities.h
index 6617fccb0..b842ae58e 100644
--- a/src/theory/arith/arith_utilities.h
+++ b/src/theory/arith/arith_utilities.h
@@ -231,7 +231,7 @@ inline Node getIdentity(Kind k){
   case kind::MULT:
   case kind::NONLINEAR_MULT:
     return mkRationalNode(1);
-  default: Unreachable(); return {};  // silence warning
+  default: Unreachable(); return Node::null();  // silence warning
   }
 }
 
diff --git a/src/theory/arith/bound_inference.cpp b/src/theory/arith/bound_inference.cpp
index 3b40f925e..5824d8239 100644
--- a/src/theory/arith/bound_inference.cpp
+++ b/src/theory/arith/bound_inference.cpp
@@ -43,7 +43,7 @@ Bounds BoundInference::get(const Node& lhs) const
   auto it = d_bounds.find(lhs);
   if (it == d_bounds.end())
   {
-    return Bounds{};
+    return Bounds();
   }
   return it->second;
 }