Parse scalar expressions with source spans

src/diagnostic.mli

@@ -8,7 +8,7 @@ exception Error of t
 
 val set_file : string -> unit
 val file : unit -> string
-val error : Location.span -> ('a, unit, string, 'a) format4 -> 'a
+val error : Location.span -> ('a, unit, string, 'b) format4 -> 'a
 val make : Location.span -> string -> t
 val to_string : t -> string
 val render : string option -> t -> string