fix(syntax): allow private in with constraints

CHANGELOG.md

@@ -18,6 +18,7 @@
 - Add `rewatch` to the npm package as an alternative build tool. https://github.com/rescript-lang/rescript-compiler/pull/6762
 - Throws an instance of JavaScript's `new Error()` and adds the extension payload for `cause` option. https://github.com/rescript-lang/rescript-compiler/pull/6611
 - Allow free vars in types for type coercion `e :> t`. https://github.com/rescript-lang/rescript-compiler/pull/6828
+- Allow `private` in with constraints. https://github.com/rescript-lang/rescript-compiler/pull/6843
 
 #### :boom: Breaking Change
 

jscomp/syntax/src/res_core.ml

@@ -6269,12 +6269,16 @@ and parse_with_constraint p =
             (Location.mkloc (Longident.last type_constr.txt) type_constr.loc) )
     | Equal ->
       Parser.next p;
+      let private_flag =
+        if Parser.optional p Token.Private then Asttypes.Private
+        else Asttypes.Public
+      in
       let typ_expr = parse_typ_expr p in
       let type_constraints = parse_type_constraints p in
       Parsetree.Pwith_type
         ( type_constr,
-          Ast_helper.Type.mk ~loc:type_constr.loc ~params ~manifest:typ_expr
-            ~cstrs:type_constraints
+          Ast_helper.Type.mk ~loc:type_constr.loc ~priv:private_flag ~params
+            ~manifest:typ_expr ~cstrs:type_constraints
             (Location.mkloc (Longident.last type_constr.txt) type_constr.loc) )
     | token ->
       (* TODO: revisit *)

jscomp/syntax/tests/parsing/grammar/modtype/expected/with.res.txt

@@ -9,6 +9,7 @@ module type A  =
   (Foo with type  t =  'st constraint 'st = int constraint 'x = int and type
      t =  'st constraint 'st = int constraint 'x = int and type  t = 
     'st constraint 'st = int constraint 'x = int)
+module type A  = (Foo with type  t =  private string)
 module type A  = (Foo with type  t :=  string)
 module type A  = (Foo with type 'a t :=  string)
 module type A  = (Foo with type ('a,'b) t :=  string)

jscomp/syntax/tests/parsing/grammar/modtype/with.res

@@ -8,6 +8,7 @@ module type A = Foo
   with type t = 'st constraint 'st = int constraint 'x = int
    and type t = 'st constraint 'st = int constraint 'x = int
    and type t = 'st constraint 'st = int constraint 'x = int
+module type A = Foo with type t = private string
 
 module type A = Foo with type t := string
 module type A = Foo with type t<'a> := string