enable warning 6 [labels-omitted] by default

Changes

@@ -70,6 +70,14 @@ 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, report by ygrek)
+
 ### Internal/compiler-libs changes:
 
 - #9650, #9651: keep refactoring the pattern-matching compiler

man/ocamlc.m

@@ -1017,7 +1017,7 @@ 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\-67\-68 .
+.BR \-w\ +a\-4\-7\-9\-27\-29\-30\-32..42\-44\-45\-48\-50\-60\-66\-67\-68 .
 Note that warnings
 .BR 5 \ and \ 10
 are not always triggered, depending on the internals of the type checker.

utils/warnings.ml

@@ -572,7 +572,7 @@ let parse_options errflag s =
   current := {(!current) with error; active}
 
 (* 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..42-44-45-48-50-60-66-67-68";;
+let defaults_w = "+a-4-7-9-27-29-30-32..42-44-45-48-50-60-66-67-68";;
 let defaults_warn_error = "-a+31";;
 
 let () = parse_options false defaults_w;;