Comment and test for destructive substitution

Author: Roman Leshchinskiy

ocaml/testsuite/tests/typing-modules/strengthening.ml

@@ -168,3 +168,34 @@ module Ignore_strengthening :
     module M : S
   end
 |}]
+
+module Expand_destructive_with = struct
+  module type S = sig
+    module type T = sig type t val foo : t -> t end
+    module M : T
+    module N : T with M
+  end
+
+  module F(X:S with type M.t := int) = struct
+    let bar = X.N.foo 1
+  end
+end
+[%%expect{|
+module Expand_destructive_with :
+  sig
+    module type S =
+      sig
+        module type T = sig type t val foo : t -> t end
+        module M : T
+        module N : sig type t = M.t val foo : t -> t end
+      end
+    module F :
+      functor
+        (X : sig
+               module type T = sig type t val foo : t -> t end
+               module M : sig val foo : int -> int end
+               module N : sig type t = int val foo : t -> t end
+             end)
+        -> sig val bar : X.N.t end
+  end
+|}]

ocaml/typing/typemod.ml

@@ -787,11 +787,15 @@ let merge_constraint initial_env loc sg lid constr =
     in
     let sg = match sub with
       | Some sub ->
+          (* Since destructive with is implemented via substitution, we need to
+            expand any type abbreviations (like strengthening) where the expanded
+            form might contain the thing we need to substitute. See corresponding
+            test in strengthening.ml.  *)
+          let sg = Mtype.expand_to initial_env sg !real_ids in
           (* This signature will not be used directly, it will always be freshened
             by the caller. So what we do with the scope doesn't really matter. But
             making it local makes it unlikely that we will ever use the result of
             this function unfreshened without issue. *)
-          let sg = Mtype.expand_to initial_env sg !real_ids in
           Subst.signature Make_local sub sg
       | None -> sg
     in