diff --git a/lib/tests/modules/types-attrTag.nix b/lib/tests/modules/types-attrTag.nix
index d48badae391f..167f839c027c 100644
--- a/lib/tests/modules/types-attrTag.nix
+++ b/lib/tests/modules/types-attrTag.nix
@@ -120,7 +120,17 @@ in
       assert config.merged.positive == { yay = 100; };
       assert config.merged.extensi-foo == { extensible = "foo"; };
       assert config.merged.extensi-bar == { extensible = "bar"; };
+      assert config.docs."submodules.<name>.foo.bar".declarations == [ __curPos.file ];
       assert config.docs."submodules.<name>.foo.bar".type == "signed integer";
+      assert
+        lib.length
+          (options.submodules.type.nestedTypes.elemType.nestedTypes.foo.type.getSubOptions [ ])
+          .bar.declarationPositions == 1;
+      assert
+        (lib.head
+          (options.submodules.type.nestedTypes.elemType.nestedTypes.foo.type.getSubOptions [ ])
+          .bar.declarationPositions
+        ).file == __curPos.file;
       assert config.docs."submodules.<name>.qux".type == "string";
       assert config.docs."submodules.<name>.qux".declarations == [ __curPos.file ];
       assert
@@ -133,8 +143,11 @@ in
       assert config.docs."submodules.<name>.qux".description == "A qux for when you don't want a foo";
       assert config.docs."submodules.<name>.qux".readOnly == false;
       assert config.docs."submodules.<name>.qux".visible == true;
-      # Not available (yet?)
-      # assert config.docs."submodules.<name>.qux".declarationsWithPositions == [ ... ];
+      assert
+        lib.length options.submodules.type.nestedTypes.elemType.nestedTypes.qux.declarationPositions == 1;
+      assert
+        (lib.head options.submodules.type.nestedTypes.elemType.nestedTypes.qux.declarationPositions).file
+        == __curPos.file;
       assert options.submodules.declarations == [ __curPos.file ];
       assert lib.length options.submodules.declarationPositions == 1;
       assert (lib.head options.submodules.declarationPositions).file == __curPos.file;
diff --git a/lib/types.nix b/lib/types.nix
index e7a8763983a1..8ed7bec21713 100644
--- a/lib/types.nix
+++ b/lib/types.nix
@@ -1140,6 +1140,27 @@ rec {
         else
           throw "The option `${showOption loc}` is defined as ${lib.strings.escapeNixIdentifier choice}, but ${lib.strings.escapeNixIdentifier choice} is not among the valid choices (${choicesStr}). Value ${choice} was defined in ${showFiles (getFiles defs)}.";
       nestedTypes = tags;
+      getSubModules =
+        let
+          tagsWithSubModules = filterAttrs (_: mods: mods != null) (
+            mapAttrs (_: opt: opt.type.getSubModules) tags
+          );
+        in
+        if tagsWithSubModules == { } then null else [ tagsWithSubModules ];
+      substSubModules =
+        allWrappedModules:
+        let
+          tagsWithNewTypes = zipAttrsWith (tag: tags.${tag}.type.substSubModules) (
+            concatMap (
+              { _file, imports }: map (mapAttrs (_: imports: { inherit _file imports; })) imports
+            ) allWrappedModules
+          );
+        in
+        attrTag (
+          mapAttrs (
+            tag: opt: opt // (optionalAttrs (tagsWithNewTypes ? ${tag}) { type = tagsWithNewTypes.${tag}; })
+          ) tags
+        );
       functor = defaultFunctor "attrTag" // {
         type = { tags, ... }: lib.types.attrTag tags;
         payload = { inherit tags; };