--- a/js/public/UbiNode.h
+++ b/js/public/UbiNode.h
@@ -269,17 +269,20 @@ class Node {
     }
     template<typename T>
     Node &operator=(const Rooted<T *> &root) {
         construct(root.get());
         return *this;
     }
 
     // Constructors accepting SpiderMonkey's other generic-pointer-ish types.
-    explicit Node(JS::HandleValue value);
+    // Note that we *do* want an implicit constructor here: JS::Value and
+    // JS::ubi::Node are both essentially tagged references to other sorts of
+    // objects, so letting conversions happen automatically is appropriate.
+    MOZ_IMPLICIT Node(JS::HandleValue value);
     Node(JSGCTraceKind kind, void *ptr);
 
     // copy construction and copy assignment just use memcpy, since we know
     // instances contain nothing but a vtable pointer and a data pointer.
     //
     // To be completely correct, concrete classes could provide a virtual
     // 'construct' member function, which we could invoke on rhs to construct an
     // instance in our storage. But this is good enough; there's no need to jump

--- a/js/src/builtin/TestingFunctions.cpp
+++ b/js/src/builtin/TestingFunctions.cpp
@@ -2064,17 +2064,17 @@ IsSimdAvailable(JSContext *cx, unsigned 
     return true;
 }
 
 static bool
 ByteSize(JSContext *cx, unsigned argc, Value *vp)
 {
     CallArgs args = CallArgsFromVp(argc, vp);
     mozilla::MallocSizeOf mallocSizeOf = cx->runtime()->debuggerMallocSizeOf;
-    JS::ubi::Node node(args.get(0));
+    JS::ubi::Node node = args.get(0);
     if (node)
         args.rval().set(NumberValue(node.size(mallocSizeOf)));
     else
         args.rval().setUndefined();
     return true;
 }
 
 static const JSFunctionSpecWithHelp TestingFunctions[] = {