Backport commit for ocaml 4.13 support. see:
https://github.com/ocaml/dune/commit/c1e82f94aa2883feda752df450130ee204855c09

Index: src/dune/cram_exec.ml
--- src/dune/cram_exec.ml.orig
+++ src/dune/cram_exec.ml
@@ -283,7 +283,7 @@ let rewrite_paths build_path_prefix_map ~parent_script
 
 let sanitize ~parent_script cram_to_output :
     (block_result * metadata_entry * string) Cram_lexer.block list =
-  List.map cram_to_output ~f:(fun t ->
+  List.map cram_to_output ~f:(fun (t : (block_result * _) Cram_lexer.block) ->
       match t with
       | Cram_lexer.Comment t -> Cram_lexer.Comment t
       | Command
