Show existential types as $a and name their constructor

Why:
    An existential type of a GADT constructor is an abstract type named
    `$Any_'a` internally, and that name was printed as is in errors and
    hover. It is now printed `$a`, as in OCaml 5, and the escape error
    says which constructor binds it instead of calling it a type
    constructor.

Diagnostic modified (existential escape):
Example:
    type rec any = Any('a): any

    let get = (Any(x)) => x
Before (diagnostic):
    This has type: $Any_'a
    But it's expected to have type: 'a
    The type constructor $Any_'a would escape its scope
After (diagnostic):
    This has type: $a
    But it's expected to have type: 'a
    $a is an existential type bound by the constructor Any.
    It is only known in the scope of the pattern that matches Any.

Review: Why decode the name instead of recording the constructor?
    The name already holds both parts, so printing needs no table that
    would have to be reset between compilations in one process.

Fixes: #9068
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Kbf3gLMawVgtmiKy3TMBc3
Signed-off-by: Christoph Knittel <ck@cca.io>

Author: cknitt

changes/existential-type-names.polish.md

@@ -0,0 +1 @@
+- Existential types of GADT constructors are shown as `$a` instead of `$Any_'a`, and an escaping one is explained by naming the constructor that binds it.

compiler/ml/printtyp.ml

@@ -41,8 +41,24 @@ let rec longident ppf = function
 let unique_names_key = Request_local.new_key (fun () -> ref Ident.empty)
 let unique_names () = Domain.DLS.get unique_names_key
 
+(* [Ctype.instance_constructor] names the existential type of variable ['a]
+   of constructor [C] [$C_'a], followed by a number when the name is taken.
+   It is printed [$a], and [C] is named in the escape explanation. *)
+let existential_of_name name =
+  match String.rindex_opt name '\'' with
+  | Some i when i >= 3 && name.[0] = '$' && name.[i - 1] = '_' ->
+    let constructor = String.sub name 1 (i - 2) in
+    let var = String.sub name (i + 1) (String.length name - i - 1) in
+    Some ("$" ^ var, constructor)
+  | _ -> None
+
 let ident_name id =
-  try Ident.find_same id !(unique_names ()) with Not_found -> Ident.name id
+  try Ident.find_same id !(unique_names ())
+  with Not_found -> (
+    let name = Ident.name id in
+    match existential_of_name name with
+    | Some (display, _) -> display
+    | None -> name)
 
 let add_unique id =
   try ignore (Ident.find_same id !(unique_names ()))
@@ -1299,16 +1315,31 @@ let rec mismatch = function
   | [] -> None
   | _ -> assert false
 
+let escape_explanation ppf p =
+  match p with
+  | Pident id -> (
+    match existential_of_name (Ident.name id) with
+    | Some (display, constructor) ->
+      fprintf ppf
+        "@,\
+         @[%s is an existential type bound by the constructor %s.@ It is only \
+         known in the scope of the pattern that matches %s.@]"
+        display constructor constructor
+    | None ->
+      fprintf ppf "@,@[The type constructor@;<1 2>%a@ would escape its scope@]"
+        path p)
+  | _ ->
+    fprintf ppf "@,@[The type constructor@;<1 2>%a@ would escape its scope@]"
+      path p
+
 let explanation unif t3 t4 ppf =
   match (t3.desc, t4.desc) with
   | Ttuple [], Tvar _ | Tvar _, Ttuple [] ->
     fprintf ppf "@,Self type cannot escape its class"
   | Tconstr (p, _, _), Tvar _ when unif && t4.level < Path.binding_time p ->
-    fprintf ppf "@,@[The type constructor@;<1 2>%a@ would escape its scope@]"
-      path p
+    escape_explanation ppf p
   | Tvar _, Tconstr (p, _, _) when unif && t3.level < Path.binding_time p ->
-    fprintf ppf "@,@[The type constructor@;<1 2>%a@ would escape its scope@]"
-      path p
+    escape_explanation ppf p
   | Tvar _, Tunivar _ | Tunivar _, Tvar _ ->
     fprintf ppf "@,The universal variable %a would escape its scope" type_expr
       (if is_Tunivar t3 then t3 else t4)

tests/analysis_tests/tests/src/HoverExistential.res

@@ -0,0 +1,4 @@
+type rec any = Any(array<'a>, 'a => string): any
+
+let showAll = (Any(xs, f)) => xs->Array.map(f)
+//                 ^hov

tests/analysis_tests/tests/src/expected/HoverExistential.res.txt

@@ -0,0 +1,5 @@
+Hover src/HoverExistential.res 2:19
+{
+  "contents": { "kind": "markdown", "value": "```rescript\narray<$a>\n```" }
+}
+

tests/build_tests/super_errors/expected/gadt_existential_escape.res.expected

@@ -0,0 +1,13 @@
+
+  We've found a bug for you!
+  /.../fixtures/gadt_existential_escape.res:3:23
+
+  1 │ type rec any = Any('a): any
+  2 │ 
+  3 │ let get = (Any(x)) => x
+  4 │ 
+
+  This has type: $a
+  But it's expected to have type: 'a
+  $a is an existential type bound by the constructor Any.
+  It is only known in the scope of the pattern that matches Any.

tests/build_tests/super_errors/fixtures/gadt_existential_escape.res

@@ -0,0 +1,3 @@
+type rec any = Any('a): any
+
+let get = (Any(x)) => x