Fix PR#6981

Changes

@@ -294,6 +294,7 @@ Bug fixes:
 - PR#6972, GPR#276: 4.02.3 regression on documentation comments in .cmt files
   (Leo White, report by Olivier Andrieu)
 - PR#6980: Assert failure from polymorphic variants and existentials
+- PR#6981: Ctype.Unify(_) with associated functor arg refering to previous one
 - PR#6982: unexpected type error when packing a module alias
 - PR#6985: `module type of struct include Bar end exposes
            %s#row when Bar contains private row types

testsuite/tests/typing-modules-bugs/pr6981_ok.ml

@@ -0,0 +1,10 @@
+module type S =
+sig
+  type a
+  type b
+end
+module Foo
+    (Bar : S with type a = private [> `A])
+    (Baz : S with type b = private < b : Bar.b ; .. >) =
+struct
+end

typing/typemod.ml

@@ -1104,6 +1104,7 @@ let rec type_module ?(alias=false) sttn funct_body anchor env smod =
       let (id, newenv), funct_body =
         match ty_arg with None -> (Ident.create "*", env), false
         | Some mty -> Env.enter_module ~arg:true name.txt mty env, true in
+      Ctype.init_def(Ident.current_time()); (* PR#6981 *)
       let body = type_module sttn funct_body None newenv sbody in
       rm { mod_desc = Tmod_functor(id, name, mty, body);
            mod_type = Mty_functor(id, ty_arg, body.mod_type);