consider special case of result block in compat check

NAMESPACE

@@ -18,6 +18,7 @@ S3method(block_icon,plot_block)
 S3method(block_icon,transform_block)
 S3method(block_input_check,data_block)
 S3method(block_input_check,parser_block)
+S3method(block_input_check,result_block)
 S3method(block_input_check,transform_block)
 S3method(block_output_ptype,dataset_block)
 S3method(block_output_ptype,filesbrowser_block)

R/block-core.R

@@ -355,6 +355,17 @@ block_input_check.parser_block <- function(x, data, ...) {
   input_failure("Expecting string-valued input.")
 }
 
+#' @rdname block_io
+#' @export
+block_input_check.result_block <- function(x, data, ...) {
+
+  if (length(get_workspace_stacks()) >= 2L) {
+    return(invisible(NULL))
+  }
+
+  input_failure("Expecting at least two stacks.")
+}
+
 #' @rdname block_io
 #' @export
 input_failure <- function(..., class = character()) {