lib/types: refactor elemTypeFunctor for types.attrsWith

lib/types.nix

@@ -71,6 +71,26 @@ let
     let pos = builtins.unsafeGetAttrPos name v; in
     if pos == null then "" else " at ${pos.file}:${toString pos.line}:${toString pos.column}";
 
+  # Internal functor to help for migrating functor.wrapped to functor.payload.elemType
+  # Note that individual attributes can be overriden if needed.
+  elemTypeFunctor = name: { elemType, ... }@payload: {
+    inherit name payload;
+    type    = outer_types.types.${name};
+    binOp   = a: b:
+      let
+        merged = a.elemType.typeMerge b.elemType.functor;
+      in
+        if merged == null
+        then
+          null
+        else
+          { elemType = merged; };
+    wrappedDeprecationMessage = { loc }: lib.warn ''
+      The deprecated `type.functor.wrapped` attribute of the option `${showOption loc}` is accessed, use `type.nestedTypes.elemType` instead.
+    '' payload.elemType;
+  };
+
+
   outer_types =
 rec {
   isType = type: x: (x._type or "") == type;
@@ -664,14 +684,10 @@ rec {
       getSubOptions = prefix: elemType.getSubOptions (prefix ++ ["<${placeholder}>"]);
       getSubModules = elemType.getSubModules;
       substSubModules = m: attrsWith { elemType = elemType.substSubModules m; inherit lazy placeholder; };
-      functor = defaultFunctor "attrsWith" // {
+      functor = (elemTypeFunctor "attrsWith" {
-        wrappedDeprecationMessage = { loc }: lib.warn ''
-          The deprecated `type.functor.wrapped` attribute of the option `${showOption loc}` is accessed, use `type.nestedTypes.elemType` instead.
-        '' elemType;
-        payload = {
-          # Important!: Add new function attributes here in case of future changes
           inherit elemType lazy placeholder;
-        };
+      }) // {
+        # Custom type merging required because of the "placeholder" attribute
         inherit binOp;
       };
       nestedTypes.elemType = elemType;