Fix comment-before-match/begin-match in if-then-else branches

Two fixes for ITE branch formatting with if-then-else=fit-or-vertical
(and partially if-then-else=vertical):

1. When a comment appears between `else` and `match` with
   if-then-else=fit-or-vertical, the match keyword was pushed far to the
   right (e.g. col 31 instead of col 4) due to a missing trailing break
   after the comment in branch_pro_with_cmts.

2. `begin match expr with` in ITE branches had its scrutinee breaking
   onto a new line (with if-then-else=fit-or-vertical and
   if-then-else=vertical) because the ITE `pro` (containing
   break_unless_newline 1000 2) was passed as inner pro to the match
   keyword hvbox, inflating its size. Fix: separate `pro` from the
   `begin` prefix when parent is an ITE expression.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Boris Yakobowski

lib/Fmt_ast.ml

@@ -3033,6 +3033,16 @@ and fmt_beginend c ~loc ?(box = true) ?(pro = noop) ~ctx ~ctx0 ~fmt_atrs
   cmts_before
   $
   match e.pexp_desc with
+  | (Pexp_match _ | Pexp_try _ | Pexp_function _)
+    when match ctx0 with
+         | Exp {pexp_desc= Pexp_ifthenelse _; _} -> true
+         | _ -> false ->
+      pro
+      $ beginend_box
+          (fmt_expression c
+             ~pro:(begin_ $ str " ")
+             ~box:false ?eol ~parens:false ~indent_wrap (sub_exp ~ctx e) )
+      $ end_
   | Pexp_match _ | Pexp_try _ | Pexp_function _ | Pexp_ifthenelse _ ->
       beginend_box
         (fmt_expression c

lib/Params.ml

@@ -989,13 +989,13 @@ let get_if_then_else (c : Conf.t) ~cmts_before_opt ~pro ~first ~last
           && not has_cmts_after_kw
         then
           match cmts_before_opt xbch.ast.pexp_loc with
-          | Some cmts -> break 1000 2 $ cmts
+          | Some cmts -> break 1000 2 $ cmts $ break 1000 2
           | None -> (
             match xbch.ast.pexp_desc with
             | Pexp_beginend ({pexp_loc; pexp_desc; _}, _)
               when is_special_beginend pexp_desc -> (
               match cmts_before_opt pexp_loc with
-              | Some cmts -> break 1000 2 $ cmts
+              | Some cmts -> break 1000 2 $ cmts $ break 1000 2
               | None -> branch_pro ~begin_end_offset:0 () )
             | _ -> branch_pro ~begin_end_offset:0 () )
         else branch_pro ~begin_end_offset:0 ()

test/passing/refs.ahrefs/ite-fit_or_vertical.ml.ref

@@ -246,10 +246,10 @@ let f =
   | A ->
     if gf then
       (* aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa *)
-    begin if
-      a
-    then
-      b
+      begin if
+        a
+      then
+        b
     end
 
 (* Long comment before match in else branch (regression test for
@@ -258,7 +258,8 @@ let _ =
   if a then
     b
   else
-    (* aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa *)(
+    (* aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa *)
+    (
     match x with
     | A -> f)
 
@@ -279,7 +280,8 @@ let _ =
   if a then
     b
   else
-    (* comment *)(
+    (* comment *)
+    (
     match x with
     | A -> f
     | B -> g)
@@ -289,9 +291,7 @@ let _ =
   if a then
     b
   else
-    begin match
-      x
-    with
+    begin match x with
     | A -> f
     | B -> g
   end
@@ -301,7 +301,5 @@ let _ =
   if a then
     b
   else
-    begin try
-      f x
-    with Not_found -> g
+    begin try f x with Not_found -> g
   end

test/passing/refs.ahrefs/ite-fit_or_vertical_closing.ml.ref

@@ -256,10 +256,10 @@ let f =
   | A ->
     if gf then
       (* aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa *)
-    begin if
-      a
-    then
-      b
+      begin if
+        a
+      then
+        b
     end
 
 (* Long comment before match in else branch (regression test for
@@ -268,7 +268,8 @@ let _ =
   if a then
     b
   else
-    (* aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa *)(
+    (* aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa *)
+    (
     match x with
     | A -> f
   )
@@ -290,7 +291,8 @@ let _ =
   if a then
     b
   else
-    (* comment *)(
+    (* comment *)
+    (
     match x with
     | A -> f
     | B -> g
@@ -301,9 +303,7 @@ let _ =
   if a then
     b
   else
-    begin match
-      x
-    with
+    begin match x with
     | A -> f
     | B -> g
   end
@@ -313,7 +313,5 @@ let _ =
   if a then
     b
   else
-    begin try
-      f x
-    with Not_found -> g
+    begin try f x with Not_found -> g
   end

test/passing/refs.ahrefs/ite-fit_or_vertical_no_indicate.ml.ref

@@ -246,10 +246,10 @@ let f =
   | A ->
     if gf then
       (* aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa *)
-    begin if
-      a
-    then
-      b
+      begin if
+        a
+      then
+        b
     end
 
 (* Long comment before match in else branch (regression test for
@@ -258,7 +258,8 @@ let _ =
   if a then
     b
   else
-    (* aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa *)(
+    (* aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa *)
+    (
     match x with
     | A -> f)
 
@@ -279,7 +280,8 @@ let _ =
   if a then
     b
   else
-    (* comment *)(
+    (* comment *)
+    (
     match x with
     | A -> f
     | B -> g)
@@ -289,9 +291,7 @@ let _ =
   if a then
     b
   else
-    begin match
-      x
-    with
+    begin match x with
     | A -> f
     | B -> g
   end
@@ -301,7 +301,5 @@ let _ =
   if a then
     b
   else
-    begin try
-      f x
-    with Not_found -> g
+    begin try f x with Not_found -> g
   end

test/passing/refs.ahrefs/ite-vertical.ml.ref

@@ -326,9 +326,7 @@ let _ =
   if a then
     b
   else
-    begin match
-      x
-    with
+    begin match x with
     | A -> f
     | B -> g
     end
@@ -338,7 +336,5 @@ let _ =
   if a then
     b
   else
-    begin try
-      f x
-    with Not_found -> g
+    begin try f x with Not_found -> g
     end

test/passing/refs.default/ite-fit_or_vertical.ml.ref

@@ -246,10 +246,10 @@ let f =
   | A ->
       if gf then
         (* aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa *)
-      begin if
-        a
-      then
-        b
+        begin if
+          a
+        then
+          b
       end
 
 (* Long comment before match in else branch (regression test for
@@ -259,8 +259,8 @@ let _ =
     b
   else
     (* aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa *)
-  match x with
-  | A -> f
+    match x with
+    | A -> f
 
 (* Comment before nested if in then branch - Compact mode oscillation *)
 let _ =
@@ -280,28 +280,22 @@ let _ =
     b
   else
     (* comment *)
-                   match x with
-                 | A -> f
-                 | B -> g
+    match x with
+    | A -> f
+    | B -> g
 
 (* begin match in else branch should keep scrutinee on same line *)
 let _ =
   if a then
     b
   else
-    begin match
-      x
-    with
-    | A -> f
-    | B -> g
+    begin match x with A -> f | B -> g
   end
 
 (* begin try in else branch should keep body on same line *)
 let _ =
   if a then
     b
   else
-    begin try
-      f x
-    with Not_found -> g
+    begin try f x with Not_found -> g
   end

test/passing/refs.default/ite-fit_or_vertical_closing.ml.ref

@@ -255,10 +255,10 @@ let f =
   | A ->
       if gf then
         (* aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa *)
-      begin if
-        a
-      then
-        b
+        begin if
+          a
+        then
+          b
       end
 
 (* Long comment before match in else branch (regression test for
@@ -268,8 +268,8 @@ let _ =
     b
   else
     (* aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa *)
-  match x with
-  | A -> f
+    match x with
+    | A -> f
 
 (* Comment before nested if in then branch - Compact mode oscillation *)
 let _ =
@@ -289,28 +289,22 @@ let _ =
     b
   else
     (* comment *)
-                   match x with
-                 | A -> f
-                 | B -> g
+    match x with
+    | A -> f
+    | B -> g
 
 (* begin match in else branch should keep scrutinee on same line *)
 let _ =
   if a then
     b
   else
-    begin match
-      x
-    with
-    | A -> f
-    | B -> g
+    begin match x with A -> f | B -> g
   end
 
 (* begin try in else branch should keep body on same line *)
 let _ =
   if a then
     b
   else
-    begin try
-      f x
-    with Not_found -> g
+    begin try f x with Not_found -> g
   end

test/passing/refs.default/ite-fit_or_vertical_no_indicate.ml.ref

@@ -246,10 +246,10 @@ let f =
   | A ->
       if gf then
         (* aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa *)
-      begin if
-        a
-      then
-        b
+        begin if
+          a
+        then
+          b
       end
 
 (* Long comment before match in else branch (regression test for
@@ -259,8 +259,8 @@ let _ =
     b
   else
     (* aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa *)
-  match x with
-  | A -> f
+    match x with
+    | A -> f
 
 (* Comment before nested if in then branch - Compact mode oscillation *)
 let _ =
@@ -280,28 +280,22 @@ let _ =
     b
   else
     (* comment *)
-                   match x with
-                 | A -> f
-                 | B -> g
+    match x with
+    | A -> f
+    | B -> g
 
 (* begin match in else branch should keep scrutinee on same line *)
 let _ =
   if a then
     b
   else
-    begin match
-      x
-    with
-    | A -> f
-    | B -> g
+    begin match x with A -> f | B -> g
   end
 
 (* begin try in else branch should keep body on same line *)
 let _ =
   if a then
     b
   else
-    begin try
-      f x
-    with Not_found -> g
+    begin try f x with Not_found -> g
   end