Fix T101410: ignore dangling reroute inputs

For consistency with other node systems in Blender and older
versions of geometry nodes, dangling reroute inputs should
not affect the output. When an input socket is linked to dangling
reroutes, its own value should be used instead.

source/blender/nodes/intern/geometry_nodes_lazy_function.cc

@@ -1071,6 +1071,12 @@ struct GeometryNodesLazyFunctionGraphBuilder {
 
   void insert_links_from_socket(const bNodeSocket &from_bsocket, lf::OutputSocket &from_lf_socket)
   {
+    const bNode &from_bnode = from_bsocket.owner_node();
+    if (this->is_dangling_reroute_input(from_bnode)) {
+      /* Dangling reroutes should not be used as source of values. */
+      return;
+    }
+
     const Span<const bNodeLink *> links_from_bsocket = from_bsocket.directly_linked_links();
 
     struct TypeWithLinks {
@@ -1166,6 +1172,33 @@ struct GeometryNodesLazyFunctionGraphBuilder {
     }
   }
 
+  bool is_dangling_reroute_input(const bNode &node)
+  {
+    if (!node.is_reroute()) {
+      return false;
+    }
+    const bNode *iter_node = &node;
+    /* It is guaranteed at a higher level that there are no link cycles. */
+    while (true) {
+      const Span<const bNodeLink *> links = iter_node->input_socket(0).directly_linked_links();
+      BLI_assert(links.size() <= 1);
+      if (links.is_empty()) {
+        return true;
+      }
+      const bNodeLink &link = *links[0];
+      if (!link.is_available()) {
+        return false;
+      }
+      if (link.is_muted()) {
+        return false;
+      }
+      iter_node = link.fromnode;
+      if (!iter_node->is_reroute()) {
+        return false;
+      }
+    }
+  }
+
   lf::OutputSocket *insert_type_conversion_if_necessary(
       lf::OutputSocket &from_socket,
       const CPPType &to_type,