Bug: Formatting instability on long if conditions with if-then-else=fit-or-vertical

[TKristof09]

ocamlformat fails with "BUG: formatting did not stabilize after 10 iterations" when using `if-then-else=fit-or-vertical. It seems to happen with the following combination of conditions

• Long enough condition to create a line break
• Comment after the then keyword and before the expression of the branch

The formatter seems to oscillate between two comment placements:

(* Format A *)
then (* comment *)
  expr

(* Format B *)
then
  (* comment *)
  expr

How to Reproduce

let test =
  if
    long_function_name_to_force_break a b c d e f g h i j k l m n o p q r s t
  then 
    (* comment *)
    1
  else
    2

Environment

• ocamlformat version: 0.28.1
• OS: Linux

Configuration

The bug is triggered by having only if-then-else=fit-or-vertical in the .ocamlformat file. But here is the full config from --print-config:

comment-check=true
debug=false
disable=false
margin-check=false
max-iters=10
ocaml-version=4.04.0
quiet=false
disable-conf-attrs=false
version-check=true
assignment-operator=end-line
break-before-in=fit-or-vertical
break-cases=fit
break-collection-expressions=fit-or-vertical
break-colon=after
break-fun-decl=wrap
break-fun-sig=wrap
break-infix=wrap
break-infix-before-func=false
break-separators=after
break-sequences=true
break-string-literals=auto
break-struct=force
cases-exp-indent=4
cases-matching-exp-indent=normal
disambiguate-non-breaking-match=false
doc-comments=after-when-possible
doc-comments-padding=2
doc-comments-tag-only=default
dock-collection-brackets=true
exp-grouping=preserve
extension-indent=2
field-space=loose
function-indent=2
function-indent-nested=never
if-then-else=fit-or-vertical (file .ocamlformat:1)
indent-after-in=0
indicate-multiline-delimiters=no
indicate-nested-or-patterns=unsafe-no
infix-precedence=indent
leading-nested-match-parens=false
let-and=compact
let-binding-indent=2
let-binding-deindent-fun=true
let-binding-spacing=compact
let-module=compact
line-endings=lf
margin=80
match-indent=0
match-indent-nested=never
max-indent=68
module-indent=2
module-item-spacing=compact
nested-match=wrap
ocp-indent-compat=false
parens-ite=false
parens-tuple=always
parens-tuple-patterns=multi-line-only
parse-docstrings=true
parse-toplevel-phrases=false
sequence-blank-line=preserve-one
sequence-style=terminator
single-case=compact
space-around-arrays=true
space-around-lists=true
space-around-records=true
space-around-variants=true
stritem-extension-indent=0
type-decl=compact
type-decl-indent=2
wrap-comments=false
wrap-fun-args=true
profile=default

Potentially related to issue #2749 as that also has a similar instability with comments but with different config.

[EmileTrotignon]

Thanks for the very complete report. Not sure when someone will be available to look into it though.