# OCaml 5.5.0 released

**URL:** <https://discuss.ocaml.org/t/ocaml-5-5-0-released/18265>\
**Category:** Ecosystem\
**Tags:** compiler, announce\
**Created:** [June 19, 2026, 9:35pm UTC](https://discuss.ocaml.org/t/ocaml-5-5-0-released/18265 "2026-06-19T21:35:37Z")\
**Posts on this page:** 11\
**Page:** 1

<div class="post-metadata">

**Author:** ![octachron](https://avatars.discourse-cdn.com/v4/letter/o/49beb7/32.png) [@octachron](https://discuss.ocaml.org/u/octachron)\
**Post date:** [June 19, 2026, 9:35pm UTC](https://discuss.ocaml.org/t/ocaml-5-5-0-released/18265/1 "2026-06-19T21:35:37Z")

</div>

We have the pleasure of celebrating the birthday of Blaise Pascal by announcing the release of OCaml version 5.5.0.

Some of the highlights in OCaml 5.5.0 are:

### Module-dependent Functions

Modules can now be used as function arguments in a form of lightweight functors.

For instance, we can define a function for printing a map generated by  
the `Map.Make` functor:

```ocaml
let pp_map (module M: Map.S) pp_key pp_v ppf set =
  if M.is_empty set then
    Format.fprintf ppf "ø"
  else
   let pp_sep ppf () = Format.fprintf ppf ",@ " in
   let pp_binding ppf (k,v) =
     Format.fprintf ppf "@[%a@ =@ %a@]" pp_key k pp_v v
   in
   Format.fprintf ppf "@[{@ %a@ }@]"
     (Format.pp_print_seq ~pp_sep pp_binding) (M.to_seq set)

```

We can then apply this function on a string map

```ocaml
module String_map = Map.Make(String)

```

with

```ocaml
let () =
  let m = String_map.of_list ["Zero", "Zero"; "One", "Un"] in
  let pp_str = Format.pp_print_string in
  Format.printf "%a@."
  (pp_map (module String_map) pp_str pp_str) m

```

Compared to first-class modules, the type of the function `pp_map`

```ocaml
type 'a printer = Format.formatter -> 'a -> unit
val pp_map: (module M: Map.S) -> M.key printer -> 'a printer -> 'a M.t printer

```

is dependent over the value of the module `S`, and thus the function can only  
applied over a statically known module:

```ocaml
let f (): (module Map.S) =
  if Random.bool () then
    (module Map.Make(Int))
  else
    (module Map.Make(Float))
let fail = pp_map (f ())

```

> ```ocaml
> Error: This expression has type
> (module M : Map.S) ->
> (Format.formatter -> M.key -> unit) ->
> (Format.formatter -> 'a -> unit) -> Format.formatter -> 'a M.t -> unit
> but an expression was expected of type (module Map.S) -> 'b
> The module M would escape its scope
> This function is module-dependent. The dependency is preserved
> when the function is passed a static module argument (module M : S)
> or (module M). Its argument here is not static, so the type-checker
> tried instead to change the function type to be non-dependent.
> 
> ```

### Relocatable Compiler

A compiler installation can now be moved or copied with no risk  
of hard-to-debug errors due to mixing incompatible bytecode runtime interpreters.

In practice, this means that creating a local switch when there is a  
global switch with the same compiler version and configuration  
available can be done by cloning the global switch rather than  
recompiling the whole compiler.

This should considerably reduce the time required to create  
new local opam switches out-of-the-box.

### Polymorphic Functions as Function Arguments

Higher-rank polymorphic functions can now be defined directly by  
using an explicit type annotation in a function argument

```ocaml
let apply_map (map: 'a 'b. ('a -> 'b) -> 'a list -> 'b list) =
  map string_of_int [1;2;3], map List.singleton ["x"; "y"]
let _ = apply_map List.map

```

Previously defining such a function required going through either a record or an object  
with a polymorphic field or methods

```ocaml
type map = { map: 'a 'b. ('a -> 'b) -> 'a list -> 'b list }
let apply_map {map} =
  map string_of_int [1;2;3], map List.singleton ["x"; "y"]

```

### Search and Replace Substring Functions

The `String` module has been extended with many functions  
for searching and replacing substrings inside a string.

```ocaml
let _true = String.includes ~affix:"aba" "abbaba"
let sentence = String.replace_all ~sub:"𝄽" ~by:"word" "A 𝄽 is re𝄽ed"

```

The substring search is using the 2-way string matching algorithm  
which has the advantage of requiring constant space memory overhead  
independently of the needle size.

### Generalised Local Definitions

It is now always possible to define locally a type,  
a class, a module type or any kind of item that can be defined  
globally:

```ocaml
let mandelbrot n x =
  let type t = Converge | Escape of int in
  ...
  match orbit n x with
  | Converge -> 0
  | Exit_at n -> colorize n

```

### External Types

When interfacing with foreign function libraries, it is now possible  
to define external type

```ocaml
type int_gmp = external "mpz_t"
type float_gmp = external "mpf_t"

```

Compared to an abstract type definition, the external type name  
“mpz\_t” (resp. `mpf_t`) makes the type distinguishable from any  
non-abstract types or external types with a different name.

In particular, this makes FFI types better behaved when combined with  
Generalised Abstract Data Types (GADTs). For instance, The typechecker  
is able to prove that

```ocaml
let ok: (int_gmp,[` A] ) Type.eq -> _ = function _ -> .

```

is a total function because the external type `int_gmp` is not compatible  
with a polymorphic variant type.

### Warning: Abstract types in the current module

The astute reader has probably noticed in the definition above that,  
in OCaml 5.4.0, the typechecker does accept

```ocaml
type int_gmp
let ok: (int_gmp, [` A] ) Type.eq -> _ = function _ -> .

```

as total.

Indeed until OCaml 5.5.0, abstract types defined in the current module

```ocaml
type a
type b

```

were considered as unique and provably different

```ocaml
let f: 'x. (a,b) Type.eq -> 'x = function _ -> .

```

However, this special rule for local definition of abstract types was  
very brittle. As soon as one moved outside of the current module, it  
was no longer possible to prove that the types were different.

```ocaml
module M = struct
  type a
  type b
end
let fail: 'x. (M.a,M.b) Type.eq -> 'x = function _ -> .

```

> ```ocaml
> Error: This match case could not be refuted.
> Here is an example of a value that would reach it: Equal
> 
> ```

This special typechecking rule has been removed in OCaml 5.5.0. If you were relying on it,  
for instance, because you used an abstract type as type-level label in a GADTs, you  
can change your abstract type definition to a possibly private abbreviation of a polymorphic variant

```ocaml
type a = private [`A]
type b = [`B]

```

or a (possibly private) sum type

```ocaml
type a = A
type b = private B

```

If you were using an abstract type as both a type-level label and a FFI type, you  
can now use an external type definition which will give you a provably distinct type  
even outside of the current module.

### GC improvements

Some of the ongoing work to improve the pacing of the garbage  
collector has been integrated in OCaml 5.5.0, two of the important  
changes in OCaml 5.5 GC are

- the addition of a sweep-only phase at the start of major GC
- the addition of an idle phase to smooth the behaviour of the GC  
at the start.

### Many incremental changes

- The Windows implementation is no more reliant on Winpthreads
- Around 60 new standard library functions
- Around 90 various improvements
- A dozen of documentation updates
- Around 40 bug fixes

Please report any unexpected behaviours on the [OCaml issue  
tracker](https://github.com/ocaml/ocaml/issues) and post any questions or  
comments you might have on our [discussion forums](https://discuss.ocaml.org).

The full list of changes can be found in the full changelog.

Happy hacking,  
Florian Angeletti for the OCaml team.

* * *

## Installation Instructions

The base compiler can be installed as an opam switch with the following commands:

```bash
opam update
opam switch create 5.5.0

```

The source code for the release is also directly available on:

- [GitHub](https://github.com/ocaml/ocaml/releases/download/5.5.0/ocaml-5.5.0.tar.gz)
- [OCaml archives at Inria](https://caml.inria.fr/pub/distrib/ocaml-5.5/ocaml-5.5.0.tar.gz)

### Fine-Tuned Compiler Configuration

If you want to tweak the configuration of the compiler, you can switch to the option variant with:

```bash
opam update
opam switch create <switch_name> ocaml-variants.5.5.0+options <option_list>

```

where `<option_list>` is a space separated list of `ocaml-option-*` packages. For instance, for a `flambda` and `no-flat-float-array` switch:

```bash
opam switch create 5.5.0+flambda+nffa ocaml-variants.5.5.0+options ocaml-option-flambda ocaml-option-no-flat-float-array

```

## Changes in OCaml 5.5.0

### Language features:

- (_breaking change_) [#13681](https://github.com/ocaml/ocaml/issues/13681), [#13682](https://github.com/ocaml/ocaml/issues/13682), [#13683](https://github.com/ocaml/ocaml/issues/13683), [#13684](https://github.com/ocaml/ocaml/issues/13684), [#13275](https://github.com/ocaml/ocaml/issues/13275) :  
Introduce a new type `(module M : S) -> t[M]` that corresponds to  
module-dependent functions (also called: modular explicits).

```ocaml
  val mapM: (module M : Monad) (f : 'a -> 'b M.t) : 'a list -> 'b list M.t

```

(Samuel Vivien review by Leo White, Gabriel Scherer, Florian Angeletti,  
Jacques Garrigue and Stephen Dolan)

- [#14040](https://github.com/ocaml/ocaml/issues/14040): generalize the constructs `let module`, `let exception` and `let open`  
to most other structure items, for example:

- [#13806](https://github.com/ocaml/ocaml/issues/13806): Enable the use of function parameters with polymorphic types.

- (_breaking change_) [#13712](https://github.com/ocaml/ocaml/issues/13712): Introduce a new kind `Type_external` and syntax  
`type t = external "name"` to discriminate external types from other types  
and each other. This PR turns primitive types into external and removes the  
past behavior of discriminating abstract types defined in the current module.  
(Takafumi Saikawa, Jacques Garrigue, review by Richard Eisenberg)

> **more language feature changes**
>
> - (_breaking change_) [#14009](https://github.com/ocaml/ocaml/issues/14009): infix extension points/attributes appearing in local structure items,  
> eg `let module%foo[@bar] ... in ...` are attached to the AST node of the  
> corresponding structure item (similar to their global counterparts) and no  
> longer to the enclosing `let` expression. Extension points/attributes that  
> are to be attached to the enclosing `let` expression are to be written next to  
> the `let` keyword, eg `let%foo[@bar] module ... in ...`. The same holds for  
> `let exception` and `let open`.  
> (Nicolás Ojeda Bär, review by Gabriel Scherer)
> 
> - [#14029](https://github.com/ocaml/ocaml/issues/14029): Recognize `%identity` as nonexpansive  
> (Stephen Dolan and Olivier Nicole, review by Hugo Heuzard, Jacques Garrigue,  
> Jeremy Yallop, and Gabriel Scherer)

### Standard library:

- [#13372](https://github.com/ocaml/ocaml/issues/13372): New Format and Printf printf-like functions that accept a  
heterogeneous list as arguments.

- [#14437](https://github.com/ocaml/ocaml/issues/14437): Add `String.split_{first,last,all}` and `String.rsplit_all`

- [#14436](https://github.com/ocaml/ocaml/issues/14436): Add `String.replace_{first,last,all}`

- [#14439](https://github.com/ocaml/ocaml/issues/14439): Add String.includes, to complete the trio:

- [#14381](https://github.com/ocaml/ocaml/issues/14381): `Add String.find_{first,last}_index`, `String.find_{first,last}`, `String.[r]find_all`.

- [#14438](https://github.com/ocaml/ocaml/issues/14438): Add `String.is_empty`  
(Daniel Bünzli, review by Nicolás Ojeda Bär and Gabriel Scherer)

- [#14440](https://github.com/ocaml/ocaml/issues/14440): Add `String.of_char`  
(Daniel Bünzli, review by Nicolás Ojeda Bär and Gabriel Scherer)

- [#14352](https://github.com/ocaml/ocaml/issues/14352): `Add String.{drop,take,cut}_{first,last}`.

- [#14362](https://github.com/ocaml/ocaml/issues/14362): Add `String.{drop,take,cut}_{first,last}_while`.

- [#13916](https://github.com/ocaml/ocaml/issues/13916): Add `Option.product` and `Option.Syntax`.  
(Nicolás Ojeda Bär, review by Daniel Bünzli, Gabriel Scherer and David  
Allsopp)

- [#13995](https://github.com/ocaml/ocaml/issues/13995): `Option.blend: ('a -> 'a -> 'a) -> 'a option -> 'a option -> 'a option`  
(Kate Deplaix, review by Daniel Bünzli, Gabriel Scherer,  
Nicolás Ojeda Bär, Florian Angeletti and Josh Berdine)

- [#13920](https://github.com/ocaml/ocaml/issues/13920): add `Option.{for_all, exists}`  
(Gabriel Scherer, review by Kate Deplaix, Nicolás Ojeda Bär, Richard Eisenberg  
and Jeremy Yallop)

- [#14185](https://github.com/ocaml/ocaml/issues/14185): `List.split_map: ('a -> 'b * 'c) -> 'a list -> 'b list * 'c list`  
(Jeremy Yallop, review by Daniel Bünzli, Nicolás Ojeda Bär and Damien Doligez)

- [#14043](https://github.com/ocaml/ocaml/issues/14043), [#14393](https://github.com/ocaml/ocaml/issues/14393): `Lazy.Mutexed`: simple mutex-protected lazy thunks,  
that may block the entire domain/thread on initialization races.  
(Gabriel Scherer, suggestion by Kate Deplaix and Pierre Chambart,  
review by KC Sivaramakrishnan, Florian Angeletti, Kate Deplaix  
and Daniel Bünzli)

- [#14118](https://github.com/ocaml/ocaml/issues/14118): Add `{Set,Map}.S.is_singleton`  
(Kate Deplaix, review by Daniel Bünzli, Vincent Laviron, Nicolás Ojeda Bär  
and Stephen Dolan)

- [#14060](https://github.com/ocaml/ocaml/issues/14060): Add `Hashtbl.find_and_replace` and `Hashtbl.find_and_remove`.

- [#14227](https://github.com/ocaml/ocaml/issues/14227): Add `List.filter_mapi`.  
(Émile Trotignon, review by Nicolás Ojeda Bär, Jan Midtgaard and  
Damien Doligez)

- [#14432](https://github.com/ocaml/ocaml/issues/14432): Add floor division, ceil division, Euclidean division and remainder  
to the `Int`, `Int32`, `Int64` and `Nativeint` modules: `fdiv`, `cdiv`, `ediv`.  
(Xavier Leroy, review by Ali Caglayan, Nicolás Ojeda Bär, Gabriel Scherer)

- [#14433](https://github.com/ocaml/ocaml/issues/14433): Add bit-counting functions `leading_zeros`, `leading_sign_bits`,  
`trailing_zeros`, `bit_count`, `unsigned_bitsize`, `signed_bitsize`  
to Int, Int32, Int64, and Nativeint  
(Xavier Leroy, review by Ali Caglayan and David Allsopp)

> **more standard library changes**
>
> - [#10177](https://github.com/ocaml/ocaml/issues/10177): `Seq.(delay : (unit -> 'a t) -> 'a t)`  
> (Gabriel Scherer, review by Jeremy Yallop and François Pottier)
> 
> - [#13343](https://github.com/ocaml/ocaml/issues/13343): Add `Array.stable_sort_sub`  
> (François Pottier, review by Gabriel Scherer, Corentin Leruth and Nicolás  
> Ojeda Bär)
> 
> - [#14363](https://github.com/ocaml/ocaml/issues/14363): Preserve the backtrace at exceptional domain termination. `Domain.join` on an exceptionally terminated domain re-raises the exception with the backtrace.  
> (KC Sivaramakrishnan, report by Nathan Taylor, review by Gabriel Scherer,  
> David Allsopp)
> 
> - [#13728](https://github.com/ocaml/ocaml/issues/13728): Add `Sys.runtime_executable` containing the full path (if available) to  
> the currently executing runtime.  
> (David Allsopp, review by Nicolás Ojeda Bär and Daniel Bünzli)
> 
> - [#14086](https://github.com/ocaml/ocaml/issues/14086): Add `Domain.count`.  
> (Nicolás Ojeda Bär, review by David Allsopp, Gabriel Scherer, Daniel Bünzli  
> and KC Sivaramakrishnan)
> 
> - [#12877](https://github.com/ocaml/ocaml/issues/12877): `Dynarray.rev_iter`, `Dynarray.rev_iteri`  
> (Gabriel Scherer, review by Léo Andrès, Jeremy Yallop and Nicolás Ojeda Bär)
> 
> - [#14084](https://github.com/ocaml/ocaml/issues/14084): Future-proof `Dynarray` implementation against a smarter compiler  
> (Basile Clément, review by Gabriel Scherer)

### Tools:

- [#13728](https://github.com/ocaml/ocaml/issues/13728), [#14014](https://github.com/ocaml/ocaml/issues/14014), [#14243](https://github.com/ocaml/ocaml/issues/14243), [#14244](https://github.com/ocaml/ocaml/issues/14244), [#14245](https://github.com/ocaml/ocaml/issues/14245) : The compiler is now relocatable: it can be copied/moved to a different directory and everything still works.  
(David Allsopp, review by Nicolás Ojeda Bär, Jonah Beckford, Daniel Bünzli,  
Antonin Décimo, Damien Doligez, Hugo Heuzard, Samuel Hym,  
and Vincent Laviron)

> **more tool changes**
>
> - [#14055](https://github.com/ocaml/ocaml/issues/14055): Invert BUILD\_PATH\_PREFIX\_MAP in directories loaded at startup  
> by the debugger.  
> (Pierre Boutillier, review by Gabriel Scherer and Daniel Bünzli)
> 
> - (_breaking change_) [#13638](https://github.com/ocaml/ocaml/issues/13638): `ocamlmklib` exits with code 4 if passed an unrecognised option, as it  
> does with an unrecognised file.  
> (David Allsopp, review by Antonin Décimo and Sébastien Hinderer)
> 
> - [#13941](https://github.com/ocaml/ocaml/issues/13941), [#13961](https://github.com/ocaml/ocaml/issues/13961): Fix `ocamltest` variable handing.  
> (Damien Doligez, report by Olivier Nicole, review by Gabriel Scherer)
> 
> - [#13962](https://github.com/ocaml/ocaml/issues/13962): Little `ocamltest` refactors. Fix error handling in C code,  
> leaking file descriptors, code style.  
> (Antonin Décimo, review by Gabriel Scherer)
> 
> - [#14059](https://github.com/ocaml/ocaml/issues/14059): Fix flaky TSan tests  
> (Fabrice Buoro and Olivier Nicole, review by Gabriel Scherer)
> 
> - [#13966](https://github.com/ocaml/ocaml/issues/13966), [#13969](https://github.com/ocaml/ocaml/issues/13969): Enable `generalized polymorphic #install_printer`  
> in the debugger  
> (Pierre Boutillier and Gabriel Scherer, review by Florian Angeletti)
> 
> - [#14063](https://github.com/ocaml/ocaml/issues/14063) : Debugger fallbacks to “looking for ‘module\_name’.ml in the loadpath” when seeking source files. It improves hit rate for sources of installed packages.  
> (Pierre Boutillier, review by Gabriel Scherer)
> 
> - [#14032](https://github.com/ocaml/ocaml/issues/14032), [#14034](https://github.com/ocaml/ocaml/issues/14034): Update to and require `FlexDLL 0.44`.  
> (Jan Midtgaard, Antonin Décimo, review by David Allsopp)
> 
> - [#14239](https://github.com/ocaml/ocaml/issues/14239): Fix `#show_constructor` when printing non-GADT type parameters  
> (Takafumi Saikawa, Jacques Garrigue, review by Gabriel Scherer)
> 
> - [#14245](https://github.com/ocaml/ocaml/issues/14245): ocamlobjinfo now displays the runtime invoked by a bytecode  
> executable (either from the RNTM section or by analysing the shebang lines)  
> (David Allsopp, review by Damien Doligez and Samuel Hym)

### Runtime system:

- [#14365](https://github.com/ocaml/ocaml/issues/14365): Add an Idle phase to the GC for better performance on small  
heaps and for a smooth start at program launch and after a forced major GC.  
(Damien Doligez, review by Stephen Dolan and Nick Barnes)

- [#13416](https://github.com/ocaml/ocaml/issues/13416): Implement concurrency primitives using WinAPI instead of winpthreads on Windows.  
(Antonin Décimo, review by Samuel Hym, Gabriel Scherer, Miod Vallat,  
B. Szilvasy, and Nicolás Ojeda Bär)

- [#14367](https://github.com/ocaml/ocaml/issues/14367): `Gc.Tweak` mechanism to allow named GC parameters  
(Stephen Dolan and Nick Barnes, review by Gabriel Scherer, David Allsopp and  
Antonin Décimo)

- [#13574](https://github.com/ocaml/ocaml/issues/13574), [#13594](https://github.com/ocaml/ocaml/issues/13594): Generational scanning of stack frames for ARM 64 bits, POWER,  
and RISC-V. This reduces minor GC work in the presence of deep call stacks.  
(Xavier Leroy, review by Miod Vallat, Gabriel Scherer and Olivier Nicole)

- [#14416](https://github.com/ocaml/ocaml/issues/14416): Spawned domains will record backtraces if the parent domain has enabled it.  
(Nathan Taylor, review by Gabriel Scherer)

- [#14275](https://github.com/ocaml/ocaml/issues/14275): Add function `caml_c_thread_register_in_domain`, which makes it  
possible to register “C threads” in another domain than 0 (which is  
what `caml_c_thread_register` does). The function takes a domain unique  
ID in which to register the thread. The domain must be running  
when the function is called.  
(Jack Nørskov Jørgensen, review by Gabriel Scherer,  
Guillaume Munch-Maccagnoni)

> **more runtime changes**
>
> - [#13616](https://github.com/ocaml/ocaml/issues/13616): Change free list representation in shared heap  
> (Sadiq Jaffer, review by Damien Doligez)
> 
> - [#13580](https://github.com/ocaml/ocaml/issues/13580): Introduce sweep-only phase at start of major GC cycle, to reduce latent-garbage delay and therefore improve GC performance.  
> (Stephen Dolan and Nick Barnes, review by KC Sivaramakrishnan)
> 
> - [#14053](https://github.com/ocaml/ocaml/issues/14053): `Statmemprof`: it is now possible to replace a profile in the current domain without stopping it in all domains.  
> Added the function `Gc.Memprof.is_sampling`.  
> (Guillaume Munch-Maccagnoni, review by Gabriel Scherer)
> 
> - [#14168](https://github.com/ocaml/ocaml/issues/14168): restore the stack size statistic in `Gc.stat` and adds a new `live_stacks_words` field tracking the total size in words of live stacks.  
> (Florian Angeletti, review by Gabriel Scherer)
> 
> - [#14189](https://github.com/ocaml/ocaml/issues/14189): Add runtime counters EV\_C\_MINOR\_PROMOTED\_WORDS and  
> EV\_C\_MINOR\_ALLOCATED\_WORDS. EV\_C\_MINOR\_PROMOTED\_WORDS reports words promoted  
> by minor GC and EV\_C\_MINOR\_ALLOCATED\_WORDS reports words allocated by minor  
> GC. Both have equivalent bytes counters. Also updated the documentation for  
> EV\_C\_MINOR\_PROMOTED and EV\_C\_MINOR\_ALLOCATED to qualify scope of the values  
> reported as being per-domain.  
> (Tim McGilchrist, review by Nick Barnes, Sadiq Jaffer and  
> Gabriel Scherer)
> 
> - (_breaking change_) [#14243](https://github.com/ocaml/ocaml/issues/14243): Explicit relative paths in ld.conf (“.”, “..”, “./\<path…\>”,  
> “../\<path…\>”) are interpreted as being relative to the directory ld.conf  
> was loaded from, and the default ld.conf now uses relative paths, rather than  
> embedding the absolute path to the Standard Library. The brave may continue to  
> put implicit paths in ld.conf. The interpretation of CAML\_LD\_LIBRARY\_PATH is  
> unaltered. Additionally, ld.conf is loaded from all of $OCAMLLIB/ld.conf,  
> $CAMLLIB/ld.conf and standard\_library\_default/ld.conf rather than just the  
> first one found. ld.conf files with CRLF line endings are now consistently  
> normalised on both Windows and Unix.  
> (David Allsopp, review by Jonah Beckford, Damien Doligez and Hugo Heuzard)
> 
> - [#14244](https://github.com/ocaml/ocaml/issues/14244): Added --with-relative-libdir which allows the runtime and the  
> compilers to locate the Standard Library relative to where the binaries  
> themselves are installed, removing the absolute path previously embedded in  
> caml\_standard\_library\_default. Executables linked with `ocamlc -custom` now  
> always attempt to load bytecode from the executable itself, rather than first  
> trying `argv[0]`.  
> (David Allsopp, review by Jonah Beckford, Antonin Décimo, Damien Doligez,  
> Samuel Hym and Vincent Laviron)
> 
> - [#14245](https://github.com/ocaml/ocaml/issues/14245): Introduce Runtime IDs for use in filename mangling to allow different  
> configurations and different versions of the runtime system to coexist  
> harmoniously on a single system. The IDs are used, along with the host  
> triplet, to provide mangled names for the ocamlrun executable and its variants  
> and the DLL versions of both the bytecode and native runtimes, with symlinks  
> created for the original names. They are also used to mangle the names of stub  
> libraries so that stub libraries compiled for a given configuration of the  
> runtime will only be sought by that runtime. The behaviour is disabled by  
> configuring with --disable-suffixing.  
> (David Allsopp, review by Damien Doligez and Samuel Hym)
> 
> - [#12269](https://github.com/ocaml/ocaml/issues/12269), [#12410](https://github.com/ocaml/ocaml/issues/12410), [#13063](https://github.com/ocaml/ocaml/issues/13063): Fix unsafety, deadlocks, and/or leaks should  
> rare errors happen during domain creation and thread  
> creation/registration.  
> (Guillaume Munch-Maccagnoni, review by Gabriel Scherer, B. Szilvasy,  
> Miod Vallat)
> 
> - [#14337](https://github.com/ocaml/ocaml/issues/14337): Fix potential segfault due to the C callback mechanism dropping  
> continuations without calling `caml_continuation_use`.  
> (Max Slater, review by Nick Barnes and Stephen Dolan)
> 
> - [#14461](https://github.com/ocaml/ocaml/issues/14461): Fix racy socketpair on Windows. Address-in-use errors would sometimes  
> occur when concurrent threads or processes were trying to create socketpairs.  
> (Jessie Grosen, review by Antonin Décimo and Nicolás Ojeda Bär)
> 
> - [#14820](https://github.com/ocaml/ocaml/issues/14820): `caml_ba_alloc` must account for memory it allocated itself.  
> CAML\_BA\_SUBARRAY (introduced in 5.2) with data=NULL would result in the  
> Gc accounting for the allocation as 0 bytes, which can eventually lead  
> to OOM. This condition never occurs in the compiler itself, but occurs  
> in external C bindings that attempt to create a new bigarray in the  
> shape of an existing one. For backwards compatibility ignore CAML\_BA\_SUBARRAY  
> when data is NULL.  
> (Edwin Török, review by Damien Doligez)

### Type system:

> **see type system changes**
>
> - [#13781](https://github.com/ocaml/ocaml/issues/13781): Set scope of internal type nodes during abbreviation expansion rather than recursing during unification.  
> (Jacques Garrigue, review by Gabriel Scherer)
> 
> - (_breaking change_) [#14066](https://github.com/ocaml/ocaml/issues/14066): catch invalid aliases introduced by signature constraints during  
> merging rather than in subtyping:
> 
> - (_breaking change_) [#14100](https://github.com/ocaml/ocaml/issues/14100): Do not ignore type-constraints and module-constraints when building  
> the approximated signature of recursive modules. Ignoring those constraints  
> resulted in incorrect (wrong set of fields, wrong shadowing between fields)  
> approximated signatures, failing to typecheck, as in:
> 
> - [#14327](https://github.com/ocaml/ocaml/issues/14327): Allow retyping as-patterns that contain existentials  
> (completing [#14229](https://github.com/ocaml/ocaml/issues/14229))  
> (Jacques Garrigue and Takafumi Saikawa, reported by Olivier Nicole,  
> review by Gabriel Scherer)
> 
> - [#14434](https://github.com/ocaml/ocaml/issues/14434), [#14652](https://github.com/ocaml/ocaml/issues/14652): Protect check\_counter\_example\_pat against polymorphic types,  
> restoring type soundness.  
> (Stephen Dolan and Jacques Garrigue, report and review by Alistair O’Brien)

### Compiler user-interface and warnings:

- [#14330](https://github.com/ocaml/ocaml/issues/14330): add suggestions when a signature mismatch is likely to be be caused by  
spellchecking mistakes, for instance

- [#12628](https://github.com/ocaml/ocaml/issues/12628): Improved error message for unsafe values: print out the full path for  
the value that is unsafe when they are detected during the compilation of  
recursive modules.  
(Shivam Acharya, review by Gabriel Scherer and Florian Angeletti)

> **more compiler user-interface changes**
>
> - [#14076](https://github.com/ocaml/ocaml/issues/14076), [#14111](https://github.com/ocaml/ocaml/issues/14111): error messages, add a short explanation for mismatched universal variables and universal quantifications.  
> (Florian Angeletti, review by Gabriel Scherer)
> 
> - [#14126](https://github.com/ocaml/ocaml/issues/14126): Document the `-i-variance` option and `+-`, `-+` variance indicators  
> in the reference manual.  
> (Takafumi Saikawa, review by Florian Angeletti)
> 
> - [#14146](https://github.com/ocaml/ocaml/issues/14146): add an error message for external declaration with  
> a non-syntactic arity
> 
> - [#14147](https://github.com/ocaml/ocaml/issues/14147): print row types in error messages when they are a type constructor,  
> e.g. `< foo : int; .. as $0>` when $0 is introduced by a GADT constructor  
> (Stefan Muenzel, review by Jacques Garrigue and Florian Angeletti)
> 
> - [#14190](https://github.com/ocaml/ocaml/issues/14190): `ocaml -e` now also processes `-init` (previously it was ignored).  
> (Emile Trotignon, review by David Allsopp and @ygrek)
> 
> - [#14225](https://github.com/ocaml/ocaml/issues/14225): do not raise unused-constructor warning on private  
> constructor in type implementations, for example  
> `type safe = private Safe`, which are typically used to  
> create new fresh/generative types (here `safe`) to be used  
> as GADT indices.  
> (Gabriel Scherer, review by Nicolás Ojeda Bär and Florian Angeletti,  
> report by Kate Deplaix)
> 
> - [#14244](https://github.com/ocaml/ocaml/issues/14244): Add -set-runtime-default option to the compiler, allowing the default  
> value of the Standard Library location used by the runtime to be overridden.  
> (Antonin Décimo, review by David Allsopp, Jonah Beckford, Damien Doligez and  
> Samuel Hym)
> 
> - [#14245](https://github.com/ocaml/ocaml/issues/14245): New option -launch-method for ocamlc allows the method used by a  
> tendered bytecode executable to locate the interpreter to be given explicitly.  
> In particular, it makes it easier to specify the use of the executable  
> launcher on Unix. New option -runtime-search extends the bytecode executable  
> header to be able to search for the runtime interpreter in the directory  
> containing the executable and in PATH rather than relying on a single  
> hard-coded path.  
> (David Allsopp, review by Damien Doligez and Samuel Hym)
> 
> - [#14315](https://github.com/ocaml/ocaml/issues/14315): enable -i-variance also for classes and extension constructors,  
> and add description of -i-variance in manpages and the manual  
> (Takafumi Saikawa, review by Florian Angeletti and Jacques Garrigue)
> 
> - [#14373](https://github.com/ocaml/ocaml/issues/14373): remove the OCAML\_BINANNOT\_WITHENV environment variable, and  
> always strip typing environment in cmt files  
> (Florian Angeletti, review by David Allsopp)

### Other libraries:

> **see other libraries changes**
>
> - [#13700](https://github.com/ocaml/ocaml/issues/13700), [#14454](https://github.com/ocaml/ocaml/issues/14454), [#14715](https://github.com/ocaml/ocaml/issues/14715): Use POSIX thread-safe `getgrnam_r`, `getgrgid_r`, `getpwnam_r`, `getpwuid_r`, `gmtime_r`, `localtime_r`, `getlogin_r`, and fix `mktime` error checking.  
> (Antonin Décimo, review by Florian Angeletti, David Allsopp, Stefan Muenzel,  
> Gabriel Scherer, and Miod Vallat)
> 
> - [#14406](https://github.com/ocaml/ocaml/issues/14406): Better handling of address length for unix sockets, improving Haiku compatibility.  
> (Sylvain Kerjean, review by Antonin Décimo and Nicolás Ojeda Bär)
> 
> - (_breaking change_) [#14046](https://github.com/ocaml/ocaml/issues/14046): On Windows, `Unix.kill pid Sys.sigkill` causes the receiving process  
> to exit with code ERROR\_PROCESS\_ABORTED (1067) instead of 0.  
> (Nicolás Ojeda Bär, review by Miod Vallat, Antonin Décimo and David Allsopp)
> 
> - [#14020](https://github.com/ocaml/ocaml/issues/14020): Add Unix.unsetenv.  
> (Nicolás Ojeda Bär, review by Antonin Décimo and David Allsopp)
> 
> - [#13447](https://github.com/ocaml/ocaml/issues/13447): Symmetrize shared `Sys` and `Unix` functions. Apply fixes  
> of [#12072](https://github.com/ocaml/ocaml/issues/12072), [#12184](https://github.com/ocaml/ocaml/issues/12184), [#12320](https://github.com/ocaml/ocaml/issues/12320), and [#13166](https://github.com/ocaml/ocaml/issues/13166), from `Sys.rename` to  
> `Unix.rename`. Make `caml_sys_close` raise on error, allowing  
> `Filename.temp_file` retries if close fails. Flush buffers when  
> calling `caml_sys_system_command` on Windows. Error with EINVAL  
> instead of ENOENT if the command string is not a valid C string.  
> (Antonin Décimo, review by Gabriel Scherer and Nicolás Ojeda Bär)
> 
> - [#14310](https://github.com/ocaml/ocaml/issues/14310): Deprecate union sock\_addr\_union for struct sockaddr\_storage  
> and socklen\_param\_type for socklen\_t.  
> (Antonin Décimo, review by Nicolás Ojeda Bär, David Allsopp and Samuel Hym)
> 
> - [#14391](https://github.com/ocaml/ocaml/issues/14391): unload native dynlinked objects when an error occurs and it is safe to  
> do so. (Fixes [#14323](https://github.com/ocaml/ocaml/issues/14323))  
> (Nicolás Ojeda Bär, review by Vincent Laviron)

### Code generation and optimizations:

- [#14583](https://github.com/ocaml/ocaml/issues/14583): fix bug in linear scan spilling heuristic that in certain situations could lead to miscompilations.  
(Nicolás Ojeda Bär, review by Vincent Laviron)

### Manual and documentation:

> **see manual and documentation changes**
>
> - [#14684](https://github.com/ocaml/ocaml/issues/14684), [#14782](https://github.com/ocaml/ocaml/issues/14782), [#14838](https://github.com/ocaml/ocaml/issues/14838): Improve ocamlc’s and ocamlopt’s manual pages and fix  
> small issues in the manual  
> (Samuel Hym, review by Florian Angeletti, Antonin Décimo, Gabriel Scherer and  
> Nicolás Ojeda Bär)
> 
> - [#14397](https://github.com/ocaml/ocaml/issues/14397): Improve documentation of type-directed disambiguation of array  
> literals  
> (Alicia Michael, review by Olivier Nicole and Florian Angeletti)
> 
> - [#14293](https://github.com/ocaml/ocaml/issues/14293): Improve documentation of Runtime\_events.Timestamp  
> (Raphaël Proust, review by Gabriel Scherer)
> 
> - [#13747](https://github.com/ocaml/ocaml/issues/13747): Document support for native debugging with GDB and LLDB.  
> (Tim McGilchrist, review by Daniel Bünzli, Samuel Hym, Olivier Nicole  
> and Antonin Décimo)
> 
> - (_breaking change_) [#13975](https://github.com/ocaml/ocaml/issues/13975): documented the `[@remove_aliases]` built-in attribute for signatures  
> (introduced by [#1652](https://github.com/ocaml/ocaml/issues/1652) in 2018). Small refactor of the code that fetches the  
> attribute.  
> (Clement Blaudeau, review by Gabriel Scherer)
> 
> - [#14002](https://github.com/ocaml/ocaml/issues/14002): Add anchors to items and headings of the web version of the API  
> documentation for easier linking.  
> (Nicolás Ojeda Bär, report by Louis Roché, review by Gabriel Scherer and  
> Florian Angeletti)
> 
> - [#14023](https://github.com/ocaml/ocaml/issues/14023): Add documentation for the [row\_more] function.  
> (Richard Eisenberg, review by Jacques Garrigue)
> 
> - [#14038](https://github.com/ocaml/ocaml/issues/14038): Fall back immediately to user-agent-defined fonts when web fonts  
> fail to load.  
> (toastal)
> 
> - [#14048](https://github.com/ocaml/ocaml/issues/14048): document modular explicits  
> (Gabriel Scherer, review by Samuel Vivien, Ali Caglayan,  
> Didier Remy and François Pottier)
> 
> - [#13994](https://github.com/ocaml/ocaml/issues/13994): document external types  
> (Gabriel Scherer, review by Jan Midtgaard, Jacques Garrigue  
> and Florian Angeletti)
> 
> - [#14077](https://github.com/ocaml/ocaml/issues/14077): Add missing `item-attribute` rule for `let-binding`s in documentation  
> for attributes.  
> (Shon Feder)
> 
> - [#14228](https://github.com/ocaml/ocaml/issues/14228): Trim leading spaces from first lines of LaTeX `ocamldoccode`  
> environments.  
> (Yukai Chou, review by Nicolás Ojeda Bär)
> 
> - [#14248](https://github.com/ocaml/ocaml/issues/14248): Added documentation about the way `Domain.join` triggers a  
> `Thread.join` on the domain’s systhreads.  
> (Raphaël Proust, review by Gabriel Scherer)
> 
> - [#14392](https://github.com/ocaml/ocaml/issues/14392): Fix AsciiDoc files, add more markup, xrefs, documentation.  
> (Antonin Décimo, review by Gabriel Scherer and David Allsopp)
> 
> - [#13590](https://github.com/ocaml/ocaml/issues/13590): Document automatic command-line expansion of `*` and `?` wildcards by  
> the runtime under Windows.  
> (Benjamin Sigonneau, review by Nicolás Ojeda Bär)

### Internal/compiler-libs changes:

- [#13913](https://github.com/ocaml/ocaml/issues/13913): Use Blake128 as the hash function for the compiler’s CRCs  
(Vincent Laviron, review by Xavier Leroy and Gabriel Scherer)

- [#13911](https://github.com/ocaml/ocaml/issues/13911), [#14117](https://github.com/ocaml/ocaml/issues/14117), [#14127](https://github.com/ocaml/ocaml/issues/14127): Refactor the merging of signature constraints, by splitting the monolithic merge function into separate, specialized functions (for merging types, modules and module types) - sharing only the recursive part for handling deep constraints.  
(Clement Blaudeau, review by Florian Angeletti and Samuel Vivien, fix by Ryan  
Tjoa)

- [#14120](https://github.com/ocaml/ocaml/issues/14120), [#14474](https://github.com/ocaml/ocaml/issues/14474), [#14476](https://github.com/ocaml/ocaml/issues/14476): Associate Uids to items that don’t have a concrete definition; improves Merlin’s renaming for functors.  
(Ulysse Gérard, review by Florian Angeletti and Gabriel Scherer)

> **more internal changes**
>
> - [#13839](https://github.com/ocaml/ocaml/issues/13839), [#14008](https://github.com/ocaml/ocaml/issues/14008): Reimplement `let open`, `let module` and `let exception` in  
> terms of a single construct.  
> (Nicolás Ojeda Bär, review by Gabriel Scherer, Samuel Vivien, Ulysse Gérard  
> and Vincent Laviron, temporary regression reported by Antonio Monteiro)
> 
> - [#13980](https://github.com/ocaml/ocaml/issues/13980) Refactor `type-approx` and improve some errors’ locations.  
> (Leo White, Ulysse Gérard, review by Samuel Vivien and Florian Angeletti)
> 
> - [#14024](https://github.com/ocaml/ocaml/issues/14024): Fix unterminated-string-initialization warnings from the C compiler.  
> (Antonin Décimo, review by David Allsopp and Miod Vallat)
> 
> - [#14094](https://github.com/ocaml/ocaml/issues/14094): toplevel, simplify check on installed printer types  
> (Florian Angeletti, review by Gabriel Scherer)
> 
> - [#13656](https://github.com/ocaml/ocaml/issues/13656), [#14114](https://github.com/ocaml/ocaml/issues/14114), [#14308](https://github.com/ocaml/ocaml/issues/14308): Use C99 stdint.h/inttypes.h fixed-width  
> integer types and macros to define OCaml integers.  
> (Antonin Décimo, review by Nick Barnes and David Allsopp)
> 
> - [#14148](https://github.com/ocaml/ocaml/issues/14148): Remove an unused field from package\_type in typedtree  
> (Samuel Vivien, review by Gabriel Scherer)
> 
> - [#14141](https://github.com/ocaml/ocaml/issues/14141): Rename cstrs to constraints when it refers to constraints to avoid  
> confusing it with constructors.  
> (Stefan Muenzel, review by Nicolás Ojeda Bär)
> 
> - [#14149](https://github.com/ocaml/ocaml/issues/14149): Distinguish `(module M : S)` from `(module M) : (module S)` in  
> patterns.  
> (Samuel Vivien, review by Gabriel Scherer)
> 
> - [#14161](https://github.com/ocaml/ocaml/issues/14161): refactor the STW-participants machinery to add an intermediate  
> category of ‘parked’ domain structures.  
> (Gabriel Scherer, review by Sivaramakrishnan)
> 
> - [#14198](https://github.com/ocaml/ocaml/issues/14198) Constraints on module unpacking are not ghost  
> (Thomas Refis, review by Nicolás Ojeda Bär)
> 
> - [#14243](https://github.com/ocaml/ocaml/issues/14243): ocamlc now uses the same code as the runtime to parse ld.conf (via a  
> C primitive), eliminating some highly obscure corner cases.  
> (David Allsopp, review by Jonah Beckford, Damien Doligez and Hugo Heuzard)
> 
> - [#14260](https://github.com/ocaml/ocaml/issues/14260): Refactor Lambda.structured\_constant to avoid duplicate  
> representations for string constants  
> (Vincent Laviron, review by Nicolás Ojeda Bär and Gabriel Scherer)
> 
> - [#14297](https://github.com/ocaml/ocaml/issues/14297), [#14299](https://github.com/ocaml/ocaml/issues/14299): Avoid iterating on hash tables to produce types or terms  
> (Vincent Laviron, review by Nicolás Ojeda Bär and Gabriel Scherer)
> 
> - (_breaking change_) [#14322](https://github.com/ocaml/ocaml/issues/14322): Remove leftover hacks for handling pattern constraints  
> As a side effect, `let rec (_ as x) = ...` is now always rejected instead of  
> being treated as equivalent to `let rec x = ...`  
> (Vincent Laviron, review by Alistair O’Brien and Florian Angeletti)
> 
> - [#14331](https://github.com/ocaml/ocaml/issues/14331): Enforce current\_level \<= generic\_level, and explain create\_scope  
> (Jacques Garrigue and Takafumi Saikawa, review by Gabriel Scherer)
> 
> - (_breaking change_) [#14388](https://github.com/ocaml/ocaml/issues/14388): Remove support for `let rec (module M : S) = e1 in e2`.  
> (Alistair O’Brien, review by Vincent Laviron and Gabriel Scherer)
> 
> - [#14422](https://github.com/ocaml/ocaml/issues/14422): Refactoring an `if match e with p1 -> true | p2 -> false then ...`  
> into a match in typetexp.  
> (Samuel Vivien, review by Gabriel Scherer)
> 
> - [#13224](https://github.com/ocaml/ocaml/issues/13224): Clarify barriers and spin macros with delayed expansion.  
> (Antonin Décimo, review by David Allsopp and Gabriel Scherer)
> 
> - [#14435](https://github.com/ocaml/ocaml/issues/14435), [#14455](https://github.com/ocaml/ocaml/issues/14455), [#14550](https://github.com/ocaml/ocaml/issues/14550): Add the not-root builtin ocamltest action. This  
> allows to skip tests that fail if the current user is root (superuser).  
> (Kate Deplaix, review by Gabriel Scherer, Nicolás Ojeda Bär, and  
> Antonin Décimo)
> 
> - [#14457](https://github.com/ocaml/ocaml/issues/14457): Handle qualified `M.{ x }` patterns in untypeast  
> (Basile Clément, review by Florian Angeletti)

### Build system:

> **see build system changes**
>
> - [#13705](https://github.com/ocaml/ocaml/issues/13705), [#14444](https://github.com/ocaml/ocaml/issues/14444): Cache test results of custom Autoconf tests from aclocal.m4.  
> (Antonin Décimo, review by David Allsopp and Miod Vallat)
> 
> - [#13810](https://github.com/ocaml/ocaml/issues/13810): Support build of cross compilers to native freestanding targets  
> (Samuel Hym, review by Antonin Décimo and Romain Calascibetta)
> 
> - [#14243](https://github.com/ocaml/ocaml/issues/14243): New configure option --with-additional-stublibsdir allows an additional directory to be added to the start of ld.conf. Additionally, the stublibs subdirectory is no longer created, nor added to ld.conf, when building OCaml with `--disable-shared`.  
> (David Allsopp, review by Jonah Beckford, Damien Doligez and Hugo Heuzard)
> 
> - [#14244](https://github.com/ocaml/ocaml/issues/14244): When targeting native Windows on Cygwin or MSYS2, preserve  
> backslashes in the supplied `--prefix` (in particular, backslashes instead of  
> slashes will then be displayed by `ocamlopt -config-var standard_library`).  
> If the supplied prefix contains a slash, then it is normalised, as  
> previously.  
> (David Allsopp, review by Jonah Beckford, Antonin Décimo, Damien Doligez and  
> Samuel Hym)
> 
> - [#14245](https://github.com/ocaml/ocaml/issues/14245): New --enable-runtime-search configure option controls the  
> -runtime-search option used to build the bytecode binaries in the compiler  
> distribution. --enable-runtime-search-target controls the default value of  
> -runtime-search used for bytecode executables produced by the compiler.  
> (David Allsopp, review by Damien Doligez and Samuel Hym)
> 
> - [#14484](https://github.com/ocaml/ocaml/issues/14484): Set `_WIN32_WINNT` to require Windows 8/Server 2012 Windows header SDK  
> support.  
> (Antonin Décimo, review by David Allsopp)

### Bug fixes:

> **see bug fixes**
>
> - [#14123](https://github.com/ocaml/ocaml/issues/14123): Fix integer-overflow problems in heap compaction.  
> (Nick Barnes, review by Antonin Décimo).
> 
> - [#14035](https://github.com/ocaml/ocaml/issues/14035): Fix the alignment of `_Atomic long long unsigned int` fields  
> before GCC 11.1 on i686. Silence GCC note on newer versions.  
> (Antonin Décimo, review by Sadiq Jaffer)
> 
> - [#14010](https://github.com/ocaml/ocaml/issues/14010): Fix miscompilation / liveness errors for string operations  
> (Mark Shinwell, Xavier Clerc, review by Xavier Leroy and Gabriel Scherer)
> 
> - [#14036](https://github.com/ocaml/ocaml/issues/14036): Fix nontermination of cycle printing in recursive modules with `-short-paths`. Add error message for types considered abstract while checking recursive modules.  
> (Brandon Stride, review by Florian Angeletti)
> 
> - [#14065](https://github.com/ocaml/ocaml/issues/14065): Fix function signature mismatch of `__tsan_func_exit` with GCC 15.  
> Check in the configure step if the TSan provided internal builtins are the  
> same as what we expect, introduce `caml_tsan_*` wrappers for the `__tsan_*`  
> functions we use.  
> (Hari Hara Naveen S, report by Hari Hara Naveen S,  
> review by Gabriel Scherer, Antonin Décimo, Olivier Nicole)
> 
> - [#14071](https://github.com/ocaml/ocaml/issues/14071): Fix exception name in Dynlink.Error printer.  
> (Etienne Millon, review by Nicolás Ojeda Bär)
> 
> - [#13853](https://github.com/ocaml/ocaml/issues/13853): Format breaks some line too early when there is a break hint at the end.  
> (Florian Angeletti, review by Gabriel Scherer)
> 
> - [#10570](https://github.com/ocaml/ocaml/issues/10570): Fix handling of `caml_sys_argv` when exposed directly as an external  
> (Keryan Didier, review by Vincent Laviron and Gabriel Scherer)
> 
> - [#14155](https://github.com/ocaml/ocaml/issues/14155): Audit unexecuted phrases in compiler expect tests and fix all occurrences  
> (Stefan Muenzel, review by Gabriel Scherer)
> 
> - [#14163](https://github.com/ocaml/ocaml/issues/14163), [#14176](https://github.com/ocaml/ocaml/issues/14176): add a filename location to the deprecation alert for implicit  
> uses of libraries bundled with the compiler (unix,re,threads,dynlink)  
> (Florian Angeletti, report by Ali Caglayan, review by Gabriel Scherer)
> 
> - [#14210](https://github.com/ocaml/ocaml/issues/14210): fix TSan-reported data race in weak pointers runtime  
> (Gabriel Scherer and Damien Doligez, report by Olivier Nicole,  
> review by KC Sivaramakrishnan)
> 
> - [#14213](https://github.com/ocaml/ocaml/issues/14213): Fix shadow-stack-related crashes with TSan  
> (Olivier Nicole, report by Nathan Taylor, review by Gabriel Scherer and  
> Stefan Muenzel)
> 
> - [#13658](https://github.com/ocaml/ocaml/issues/13658), [#14181](https://github.com/ocaml/ocaml/issues/14181): Fix handling of recursive function types that can result in an unbounded number of labeled or optional arguments.  
> (Stefan Muenzel, report by Samuel Vivien, review by Florian Angeletti)
> 
> - [#14255](https://github.com/ocaml/ocaml/issues/14255): Fix TSan bug with C calls that take many arguments  
> (Olivier Nicole and Miod Vallat, report by Nathan Taylor, review by Gabriel  
> Scherer)
> 
> - [#14230](https://github.com/ocaml/ocaml/issues/14230) : ocamltest fails to link test program with -custom in some cases  
> (Damien Doligez, review by David Allsopp)
> 
> - [#14279](https://github.com/ocaml/ocaml/issues/14279): -dsource, preserve (mod) and other escaped infix keyword operators in  
> the printed source wherever possible.  
> (Florian Angeletti, review by Gabriel Scherer)
> 
> - [#14300](https://github.com/ocaml/ocaml/issues/14300), [#14304](https://github.com/ocaml/ocaml/issues/14304): fix a race between memprof and the minor GC, detected by TSan  
> (Gabriel Scherer, review by Stephen Dolan and Nick Barnes,  
> report by Olivier Nicole)
> 
> - [#14332](https://github.com/ocaml/ocaml/issues/14332): Fix missing TSan instrumentation in subexpressions  
> (Vincent Laviron, review by Gabriel Scherer and Olivier Nicole)
> 
> - [#14370](https://github.com/ocaml/ocaml/issues/14370), [#14429](https://github.com/ocaml/ocaml/issues/14429): Fix headers for C++ inclusion.  
> (Antonin Décimo, review by Gabriel Scherer)
> 
> - [#14417](https://github.com/ocaml/ocaml/issues/14417): Fix issue with nested packs on macOS.  
> (Vincent Laviron, report by Kate Deplaix, review by Gabriel Scherer)
> 
> - [#14423](https://github.com/ocaml/ocaml/issues/14423): Fix detection of `SetThreadDescription` on 32-bit builds, meaning that `Thread.set_current_thread_name` now works on 32-bit MSVC and uses the correct mechanism on 32-bit mingw-w64.  
> (David Allsopp, review by Antonin Décimo)
> 
> - [#14431](https://github.com/ocaml/ocaml/issues/14431): Enable native backend for DragonFly BSD. This builds ocamlopt on DragonFly.  
> (Michael Neumann, review by Gabriel Scherer and Kate Deplaix)
> 
> - [#14495](https://github.com/ocaml/ocaml/issues/14495): Fix infix-tag bug in the minor collector which could cause SEGVs in multi-domain programs.  
> (Nick Barnes, review by Gabriel Scherer)
> 
> - [#14519](https://github.com/ocaml/ocaml/issues/14519): Fix segfault when using `Runtime_events` under certain circumstances due to bad error checking when calling `mmap()`.  
> (Mark Elvers, review by Nicolás Ojeda Bär)
> 
> - private, CVE-2026-28364, OSEC-2026-01: robustify intern.c  
> (Xavier Leroy and Nicolás Ojeda Bär, review by Olivier Nicole, Mindy Preston,  
> Edwin Török, and Gabriel Scherer)
> 
> - [#13693](https://github.com/ocaml/ocaml/issues/13693), [#14514](https://github.com/ocaml/ocaml/issues/14514): s390x: fix heap corruption with libasmrun\_shared.so caused  
> by PLT lazy binding trampoline saving FPRs into OCaml’s fiber stack.  
> Replace @PLT calls with GOT-indirect calls in the s390x code emitter.  
> (Zane Hambly, review by David Allsopp and Xavier Leroy)
> 
> - [#14557](https://github.com/ocaml/ocaml/issues/14557), [#12150](https://github.com/ocaml/ocaml/issues/12150), [#14696](https://github.com/ocaml/ocaml/issues/14696): ensure that the self type of class cannot escape  
> through type constraints.  
> (Leo White, review by Florian Angeletti)
> 
> - [#14603](https://github.com/ocaml/ocaml/issues/14603), [#14604](https://github.com/ocaml/ocaml/issues/14604): avoid Ctype.apply failures when mixing polymorphic types and unboxed constructors.  
> (Gabriel Scherer and Stefan Muenzel, report by Brandon Stride,  
> review by Florian Angeletti)
> 
> - [#14626](https://github.com/ocaml/ocaml/issues/14626), [#14675](https://github.com/ocaml/ocaml/issues/14675): take in account module-dependent functions when determining if an optional argument is non-erasable.  
> (Alistair O’Brien and Florian Angeletti, review by Gabriel Scherer)
> 
> - [#14635](https://github.com/ocaml/ocaml/issues/14635): Fix a bug in `caml_floatarray_gather` that would cause the result of `Float.Array.sub`, `Float.Array.append`, `Float.Array.concat` (when empty) not to be equal to `[||]`.  
> (Marc Lasson, review by Gabriel Scherer)
> 
> - [#14644](https://github.com/ocaml/ocaml/issues/14644), [#14647](https://github.com/ocaml/ocaml/issues/14647): Fix a bug related to unhandled effects in bytecode.  
> (Vincent Laviron, report by Thibaut Mattio,  
> review by Nicolás Ojeda Bär, Stephen Dolan and Olivier Nicole)
> 
> - [#14655](https://github.com/ocaml/ocaml/issues/14655), [#14691](https://github.com/ocaml/ocaml/issues/14691): check for size overflow in caml\_ba\_reshape  
> (Stephen Dolan, review by Xavier Leroy)
> 
> - [#14667](https://github.com/ocaml/ocaml/issues/14667): enable application related warnings for module-dependent functions  
> (Florian Angeletti, review by Gabriel Scherer)
> 
> - [#14690](https://github.com/ocaml/ocaml/issues/14690): Fix `Name_type_mismatch` error message when the expected type is an alias: print the expanded path on the right-hand side of the equality, not the alias twice.  
> (Weixie Cui, review by Florian Angeletti)
> 
> - [#14349](https://github.com/ocaml/ocaml/issues/14349), [#14718](https://github.com/ocaml/ocaml/issues/14718), [#14722](https://github.com/ocaml/ocaml/issues/14722): runtime, fix in the orphaning of ephemerons  
> (Gabriel Scherer, review by Olivier Nicole and Damien Doligez,  
> report by Jan Midtgaard)
> 
> - [#14702](https://github.com/ocaml/ocaml/issues/14702): Fix hidden directory files leaking into the visible load path table.  
> When a hidden directory contained a file whose basename was already present,  
> the file could be incorrectly added to the visible table.  
> (Hugo Heuzard, review by Florian Angeletti)
> 
> - [#14719](https://github.com/ocaml/ocaml/issues/14719), [#14721](https://github.com/ocaml/ocaml/issues/14721): compute arity correctly for module-dependent function  
> (Florian Angeletti, report by Jeremy Yallop, review by Stefan Muenzel)
> 
> - [#14760](https://github.com/ocaml/ocaml/issues/14760), [#14802](https://github.com/ocaml/ocaml/issues/14802), [#14846](https://github.com/ocaml/ocaml/issues/14846): Correct the detection of argument defaults in  
> configure, fixing an incorrect error message when installing OCaml through  
> opam on OpenSUSE with the site-config package installed.  
> (David Allsopp, report and review by Edwin Török)
> 
> - [#14797](https://github.com/ocaml/ocaml/issues/14797): avoid dropping attributes attached to package types when pretty  
> printing in surface syntax.  
> (Chet Murthy, review by Nicolás Ojeda Bär)
> 
> - [#14853](https://github.com/ocaml/ocaml/issues/14853), CVE-2026-41083, OSEC-2026-05: fix quoting of filenames  
> passed to Filename.quote\_command on Windows.  
> (David Allsopp, report by Andrew Nesbitt, review by Florian Angeletti)

---

<div class="post-metadata">

**Author:** ![octachron](https://avatars.discourse-cdn.com/v4/letter/o/49beb7/32.png) [@octachron](https://discuss.ocaml.org/u/octachron)\
**Post date:** [June 19, 2026, 9:35pm UTC](https://discuss.ocaml.org/t/ocaml-5-5-0-released/18265/2 "2026-06-19T21:35:59Z")

</div>



---

<div class="post-metadata">

**Author:** ![hyphenrf](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/hyphenrf/32/2733_2.png) [@hyphenrf](https://discuss.ocaml.org/u/hyphenrf)\
**Post date:** [June 19, 2026, 10:10pm UTC](https://discuss.ocaml.org/t/ocaml-5-5-0-released/18265/3 "2026-06-19T22:10:14Z")

</div>

Incredible release! The list of new features is impressive. Some of these could’ve easily carried its own release :)  
Great work to everyone involved

---

<div class="post-metadata">

**Author:** ![aguluman](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/aguluman/32/5709_2.png) [@aguluman](https://discuss.ocaml.org/u/aguluman)\
**Post date:** [June 20, 2026, 11:42am UTC](https://discuss.ocaml.org/t/ocaml-5-5-0-released/18265/4 "2026-06-20T11:42:29Z")

</div>

Thank you OCaml Compiler maintainers.

---

<div class="post-metadata">

**Author:** ![SpiceGuid](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/spiceguid/32/2752_2.png) [@SpiceGuid](https://discuss.ocaml.org/u/SpiceGuid)\
**Post date:** [June 20, 2026, 12:53pm UTC](https://discuss.ocaml.org/t/ocaml-5-5-0-released/18265/5 "2026-06-20T12:53:53Z")

</div>

[https://ocaml.org/](https://ocaml.org/) & [OCaml - The OCaml Manual](https://ocaml.org/manual/) have been updated 🙂

---

<div class="post-metadata">

**Author:** ![jhw](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/jhw/32/1639_2.png) [@jhw](https://discuss.ocaml.org/u/jhw)\
**Post date:** [June 20, 2026, 9:16pm UTC](https://discuss.ocaml.org/t/ocaml-5-5-0-released/18265/6 "2026-06-20T21:16:32Z")

</div>

Smoothest compiler upgrade I can remember ever experiencing. Bravissimo!

---

<div class="post-metadata">

**Author:** ![hanin\_mp](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/hanin_mp/32/6560_2.png) [@hanin\_mp](https://discuss.ocaml.org/u/hanin_mp)\
**Post date:** [June 22, 2026, 10:56am UTC](https://discuss.ocaml.org/t/ocaml-5-5-0-released/18265/7 "2026-06-22T10:56:47Z")

</div>

OCaml 5.5.0 looks excellent. module-dependent functions and relocatable compiler are big wins. Congrats 😀

---

<div class="post-metadata">

**Author:** ![pguyot](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/pguyot/32/6575_2.png) [@pguyot](https://discuss.ocaml.org/u/pguyot)\
**Post date:** [June 29, 2026, 8:33pm UTC](https://discuss.ocaml.org/t/ocaml-5-5-0-released/18265/8 "2026-06-29T20:33:27Z")

</div>

Release page [OCaml 5.5.0 Release Notes](https://ocaml.org/releases/5.5.0) links to two distinct ocaml-5.5.0.tar.gz files, one on GitHub and one on caml.inria fr (which was down earlier today), with distinct sizes and contents.

Also, caml.inria fr has a smaller .xz archive and also SHA512 sums which are welcome.

Should the page be updated and link to a single .gz file? Or should the caml.inria fr file be uploaded on GitHub as a release artifact? Should a link to the .xz be also published?

---

<div class="post-metadata">

**Author:** ![octachron](https://avatars.discourse-cdn.com/v4/letter/o/49beb7/32.png) [@octachron](https://discuss.ocaml.org/u/octachron)\
**Post date:** [June 29, 2026, 8:46pm UTC](https://discuss.ocaml.org/t/ocaml-5-5-0-released/18265/9 "2026-06-29T20:46:04Z")

</div>

Compared to the Inria archive, the GitHub archive has been purposefully expunged of compiler-developer-only subrepositories (like the compiler test suite). For the next release, I will rename the Inria archive to `ocaml-$version-full.tar.gz` to avoid this confusion.

It could also make sense to link only the xz version for the Inria archive. However, I would rather avoid multiplying the number of links to the Inria archive, which is nowadays more intended as a last resort archive.

Similarly, I am not sure what is the preferred archive format for opam in term of compression nowadays.

---

<div class="post-metadata">

**Author:** ![patrick-nicodemus](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/patrick-nicodemus/32/4704_2.png) [@patrick-nicodemus](https://discuss.ocaml.org/u/patrick-nicodemus)\
**Post date:** [June 30, 2026, 4:58am UTC](https://discuss.ocaml.org/t/ocaml-5-5-0-released/18265/10 "2026-06-30T04:58:56Z")

</div>

```auto
type 'a nat = ('a -> 'a) -> 'a -> 'a;;

let exp (type c) (n : 'a. 'a nat) : c nat = n two;;

let five = fun f x -> (f (f (f (f (f x)))));;

exp five ((+) 1) 0;;

> 32

```

Cool! What is the fastest-growing class of functions you can define on church numerals under this new type system?

---

<div class="post-metadata">

**Author:** ![aguluman](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/aguluman/32/5709_2.png) [@aguluman](https://discuss.ocaml.org/u/aguluman)\
**Post date:** [July 2, 2026, 8:10pm UTC](https://discuss.ocaml.org/t/ocaml-5-5-0-released/18265/11 "2026-07-02T20:10:39Z")

</div>

I attempted to apply OCaml 5.5.0 new features in a program that addresses this challenge question from [everybody codes](https://everybody.codes/story/3/quests/3)

Here is my solution.

```ocaml
module type Enum = sig
  type t
  val of_string : string -> t option
end

module Color = struct
  type t = Blue | Green | Red | Black | White
  let of_string = function
    | "BLUE" -> Some Blue
    | "GREEN" -> Some Green
    | "RED" -> Some Red
    | "BLACK" -> Some Black
    | "WHITE" -> Some White
    | _ -> None
end

module Shape = struct
  type t = Circle | Pentagon | Hexagon | Triangle | Square
  let of_string = function
    | "CIRCLE" -> Some Circle
    | "PENTAGON" -> Some Pentagon
    | "HEXAGON" -> Some Hexagon
    | "TRIANGLE" -> Some Triangle
    | "SQUARE" -> Some Square
    | _ -> None
end

type connector = { color : Color.t; shape : Shape.t }

type spec = { id : int; plug : connector; lsock : connector; rsock : connector }

type tree = { id : int; left : socket; right : socket }
and socket = Free of connector | Plugged of tree

let plant (s : spec) : tree =
  { id = s.id; left = Free s.lsock; right = Free s.rsock }

let required (module E : Enum) ~what s : E.t =
  match E.of_string s with
  | Some v -> v
  | None -> invalid_arg (Printf.sprintf "unknown %s %S" what s)

let connector_of_string s =
  match String.split_first ~sep:" " s with
  | None -> invalid_arg ("connector: " ^ s)
  | Some (c, sh) ->
      { color = required (module Color) ~what:"color" c;
        shape = required (module Shape) ~what:"shape" sh }

let spec_of_line line =
  let open Option.Syntax in
  let get key s =
    let* k, v = String.split_first ~sep:"=" s in
    if String.equal k key then Some v else None
  in
  let parsed =
    match String.split_all ~sep:", " ~drop:String.is_empty line with
    | id :: plug :: lsock :: rsock :: _ ->
        let* id = get "id" id in
        let* id = int_of_string_opt id in
        let* plug = get "plug" plug in
        let* lsock = get "leftSocket" lsock in
        let+ rsock = get "rightSocket" rsock in
        { id;
          plug = connector_of_string plug;
          lsock = connector_of_string lsock;
          rsock = connector_of_string rsock }
    | _ -> None
  in
  match parsed with
  | Some s -> s
  | None -> invalid_arg ("node line: " ^ line)

let insert tree spec =
  let type verdict = Placed of tree | No_match in
  let rec go t =
    let into sock rebuild =
      match sock with
      | Free c when c = spec.plug -> Placed (rebuild (Plugged (plant spec)))
      | Free _ -> No_match
      | Plugged child -> (
          match go child with
          | Placed child' -> Placed (rebuild (Plugged child'))
          | No_match -> No_match)
    in
    match into t.left (fun l -> { t with left = l }) with
    | Placed _ as placed -> placed
    | No_match -> into t.right (fun r -> { t with right = r })
  in
  match go tree with
  | Placed t -> t
  | No_match -> invalid_arg (Printf.sprintf "node %d fits no socket" spec.id)

let rec in_order t =
  Seq.delay (fun () ->
      let sub = function Free _ -> Seq.empty | Plugged c -> in_order c in
      Seq.append (sub t.left) (Seq.cons t.id (sub t.right)))

let checksum tree =
  Seq.fold_lefti (fun acc i id -> acc + (i + 1) * id) 0 (in_order tree)

let () =
  match
    In_channel.input_lines stdin
    |> List.filter (Fun.negate String.is_empty)
    |> List.map spec_of_line
  with
  | [] -> ()
  | root :: rest ->
      let timer_start = Sys.time () in
      List.fold_left insert (plant root) rest
      |> checksum
      |> Printf.printf "%d\n%!";
      (Sys.time () -. timer_start) *. 1e6
      |> Printf.printf "Elapsed time: %.1f microseconds\n%!"

```

I would like to have a conversation about my conversation regarding what I could have done better.  
Thank you for your time.