enable warning 6 [labels-omitted] by default

Changes

@@ -198,6 +198,15 @@ Working version
   the source code than the point explicitly shown in the error message.
   (François Pottier, review by Gabriel Scherer and Frédéric Bour.)
 
+* #10118, #10140: enable warning 6 [labels-omitted] by default.
+  The following now warns:
+    let f ~x y = ... in f 3 5
+  the callsite (f 3 5) has to be turned into (f ~x:3 5).
+  This prevents mistakes where two arguments of the same types are swapped.
+  (Note: Dune already enables this warning by default.)
+  (Gabriel Scherer, review by Xavier Leroy and Florian Angeletti,
+   report by ygrek)
+
 - #10196, #10197: better error message on empty character literals ''.
   (Gabriel Scherer, review by David Allsopp and Florian Angeletti
    and Daniel Bünzli, report by Robin Björklin)

man/ocamlc.m

@@ -1025,7 +1025,7 @@ compilation in any way (even if it is fatal).  If a warning is enabled,
 
 .IP
 The default setting is
-.BR \-w\ +a\-4\-6\-7\-9\-27\-29\-30\-32..42\-44\-45\-48\-50\-60\-66..70 .
+.BR \-w\ +a\-4\-7\-9\-27\-29\-30\-32..42\-44\-45\-48\-50\-60\-66..70 .
 Note that warnings
 .BR 5 \ and \ 10
 are not always triggered, depending on the internals of the type checker.

manual/src/cmds/comp.etex

@@ -358,6 +358,41 @@ command line, and possibly the "-custom" option.
 
 This section describes and explains in detail some warnings:
 
+\subsection{ss:warn6}{Warning 6: Label omitted in function application}
+
+OCaml supports "labels-omitted" full applications: if the function has
+a known arity, all the arguments are unlabeled, and their number
+matches the number of non-optional parameters, then labels are ignored
+and non-optional parameters are matched in their definition
+order. Optional arguments are defaulted.
+
+\begin{verbatim}
+let f ~x ~y = x + y
+let test = f 2 3
+
+> let test = f 2 3
+>            ^
+> Warning 6 [labels-omitted]: labels x, y were omitted in the application of this function.
+\end{verbatim}
+
+This support for "labels-omitted" application was introduced when
+labels were added to OCaml, to ease the progressive introduction of
+labels in a codebase. However, it has the downside of weakening the
+labeling discipline: if you use labels to prevent callers from
+mistakenly reordering two parameters of the same type, labels-omitted
+make this mistake possible again.
+
+Warning 6 warns when labels-omitted applications are used, to
+discourage their use. When labels were introduced, this warning was
+not enabled by default, so users would use labels-omitted
+applications, often without noticing.
+
+Over time, it has become idiomatic to enable this warning to avoid
+argument-order mistakes. The warning is now on by default, since OCaml
+4.13. Labels-omitted applications are not recommended anymore, but
+users wishing to preserve this transitory style can disable the
+warning explicitly.
+
 \subsection{ss:warn9}{Warning 9: missing fields in a record pattern}
 
   When pattern matching on records, it can be useful to match only few

manual/src/refman/expr.etex

@@ -293,24 +293,18 @@ let fullname ?title first second =
 let name = fullname "Jane" "Fisher";;
 \end{caml_example}
 
-As a special case, if the function has a known arity, all the
-arguments are unlabeled, and their number matches the number of
-non-optional parameters, then labels are ignored and non-optional
-parameters are matched in their definition order. Optional arguments
-are defaulted.
-
-\begin{caml_example}{toplevel}
-let f ~a ?b c = 
-  match b with Some n -> a + n + c | None -> a + c
-
-let r = f 2 3;;
-\end{caml_example}
-
 In all cases but exact match of order and labels, without optional
 parameters, the function type should be known at the application
 point.  This can be ensured by adding a type constraint.  Principality
 of the derivation can be checked in the "-principal" mode.
 
+As a special case, OCaml supports "labels-omitted" full applications:
+if the function has a known arity, all the arguments are unlabeled,
+and their number matches the number of non-optional parameters, then
+labels are ignored and non-optional parameters are matched in their
+definition order. Optional arguments are defaulted. This omission of
+labels is discouraged and results in a warning, see \ref{ss:warn6}.
+
 \subsubsection*{sss:expr-function-definition}{Function definition}
 
 Two syntactic forms are provided to define functions. The first form

manual/src/tutorials/lablexamples.etex

@@ -69,35 +69,6 @@ let hline ~x:x1 ~x:x2 ~y = (x1, x2, y);;
 hline ~x:3 ~y:2 ~x:5;;
 \end{caml_example}
 
-As an exception to the above parameter matching rules, if an
-application is total (omitting all optional arguments), labels may be
-omitted.
-In practice, many applications are total, so that labels can often be
-omitted.
-\begin{caml_example}{toplevel}
-f 3 2;;
-ListLabels.map succ [1;2;3];;
-\end{caml_example}
-But beware that functions like "ListLabels.fold_left" whose result
-type is a type variable will never be considered as totally applied.
-\begin{caml_example}{toplevel}[error]
-ListLabels.fold_left ( + ) 0 [1;2;3];;
-\end{caml_example}
-
-When a function is passed as an argument to a higher-order function,
-labels must match in both types. Neither adding nor removing labels
-are allowed.
-\begin{caml_example}{toplevel}
-let h g = g ~x:3 ~y:2;;
-h f;;
-h ( + ) [@@expect error];;
-\end{caml_example}
-Note that when you don't need an argument, you can still use a wildcard
-pattern, but you must prefix it with the label.
-\begin{caml_example}{toplevel}
-h (fun ~x:_ ~y -> y+1);;
-\end{caml_example}
-
 \subsection{ss:optional-arguments}{Optional arguments}
 
 An interesting feature of labeled arguments is that they can be made

testsuite/tests/lib-either/test.ml

@@ -39,7 +39,8 @@ List.map is_right [left 1; right true];;
 - : (unit, int) Either.t list = [Left (); Right 3]
 |}];;
 
-[map succ not (Left 1); map succ not (Right true)];;
+[map ~left:succ ~right:not (Left 1);
+ map ~left:succ ~right:not (Right true)];;
 [%%expect {|
 - : (int, bool) Either.t list = [Left 2; Right false]
 |}];;

testsuite/tests/misc/exotic.ml

@@ -10,7 +10,11 @@
    Note that those tests are here to record this behavior and not to enshrine it.
 *)
 
-[@@@warning "-10-18-8-5"];;
+[@@@warning "-non-unit-statement"];;
+[@@@warning "-not-principal"];;
+[@@@warning "-partial-match"];;
+[@@@warning "-ignored-partial-application"];;
+
 type t = A | () and b = B : _ -> b;;
 [%%expect{|
 type t = A | ()
@@ -35,6 +39,7 @@ true
 |}]
 ;;
 
+[@@@warning "-labels-omitted"];;
 Clflags.strict_sequence := false;;
 let f () = let g ~y = (raise Not_found : 'a) in
            if false then ((assert false : 'a); g ()) else g ()

testsuite/tools/expect_test.ml

@@ -139,19 +139,19 @@ let collect_formatters buf pps ~f =
   let ppb = Format.formatter_of_buffer buf in
   let out_functions = Format.pp_get_formatter_out_functions ppb () in
 
-  List.iter (fun pp -> Format.pp_print_flush pp ()) pps;
+  List.iter ~f:(fun pp -> Format.pp_print_flush pp ()) pps;
   let save =
-    List.map (fun pp -> Format.pp_get_formatter_out_functions pp ()) pps
+    List.map ~f:(fun pp -> Format.pp_get_formatter_out_functions pp ()) pps
   in
   let restore () =
     List.iter2
-      (fun pp out_functions ->
+      ~f:(fun pp out_functions ->
          Format.pp_print_flush pp ();
          Format.pp_set_formatter_out_functions pp out_functions)
       pps save
   in
   List.iter
-    (fun pp -> Format.pp_set_formatter_out_functions pp out_functions)
+    ~f:(fun pp -> Format.pp_set_formatter_out_functions pp out_functions)
     pps;
   match f () with
   | x             -> restore (); x

utils/warnings.ml

@@ -662,7 +662,7 @@ let parse_options errflag s =
   alerts
 
 (* If you change these, don't forget to change them in man/ocamlc.m *)
-let defaults_w = "+a-4-6-7-9-27-29-30-32..42-44-45-48-50-60-66..70";;
+let defaults_w = "+a-4-7-9-27-29-30-32..42-44-45-48-50-60-66..70";;
 let defaults_warn_error = "-a+31";;
 
 let () = ignore @@ parse_options false defaults_w;;