diff --git a/lib/modules.nix b/lib/modules.nix
index 9fe26083cfd4..1d428311cd19 100644
--- a/lib/modules.nix
+++ b/lib/modules.nix
@@ -132,20 +132,44 @@ rec {
      The exception is the ‘options’ attribute, which specifies
      sub-options.  These can be specified multiple times to allow one
      module to add sub-options to an option declared somewhere else
-     (e.g. multiple modules define sub-options for ‘fileSystems’). */
+     (e.g. multiple modules define sub-options for ‘fileSystems’).
+
+     'loc' is the list of attribute names where the option is located.
+
+     'opts' is a list of modules.  Each module has an options attribute which
+     correspond to the definition of 'loc' in 'opt.file'. */
   mergeOptionDecls = loc: opts:
     fold (opt: res:
       if opt.options ? default && res ? default ||
          opt.options ? example && res ? example ||
          opt.options ? description && res ? description ||
          opt.options ? apply && res ? apply ||
-         opt.options ? type && res ? type
+         # Accept to merge options which have identical types.
+         opt.options ? type && res ? type && opt.options.type.name != res.type.name
       then
         throw "The option `${showOption loc}' in `${opt.file}' is already declared in ${showFiles res.declarations}."
       else
-        opt.options // res //
+        let
+          /* Add the modules of the current option to the list of modules
+             already collected.  The options attribute except either a list of
+             submodules or a submodule. For each submodule, we add the file of the
+             current option declaration as the file use for the submodule.  If the
+             submodule defines any filename, then we ignore the enclosing option file. */
+          options' = toList opt.options.options;
+          addModuleFile = m:
+            if isFunction m then args: { _file = opt.file; } // (m args)
+            else { _file = opt.file; } // m;
+          coerceOption = file: opt:
+            if isFunction opt then args: { _file = file; } // (opt args)
+            else { _file = file; options = opt; };
+          getSubModules = opt.options.type.getSubModules or null;
+          submodules =
+            if getSubModules != null then map addModuleFile getSubModules ++ res.options
+            else if opt.options ? options then map (coerceOption opt.file) options' ++ res.options
+            else res.options;
+        in opt.options // res //
           { declarations = [opt.file] ++ res.declarations;
-            options = if opt.options ? options then [(toList opt.options.options ++ res.options)] else [];
+            options = submodules;
           }
     ) { inherit loc; declarations = []; options = []; } opts;
 
@@ -273,15 +297,12 @@ rec {
     in sort compare defs';
 
   /* Hack for backward compatibility: convert options of type
-     optionSet to configOf.  FIXME: remove eventually. */
+     optionSet to options of type submodule.  FIXME: remove
+     eventually. */
   fixupOptionType = loc: opt:
     let
-      options' = opt.options or
+      options = opt.options or
         (throw "Option `${showOption loc'}' has type optionSet but has no option attribute, in ${showFiles opt.declarations}.");
-      coerce = x:
-        if isFunction x then x
-        else { config, ... }: { options = x; };
-      options = map coerce (flatten options');
       f = tp:
         if tp.name == "option set" || tp.name == "submodule" then
           throw "The option ${showOption loc} uses submodules without a wrapping type, in ${showFiles opt.declarations}."
@@ -290,7 +311,10 @@ rec {
         else if tp.name == "list of option sets" then types.listOf (types.submodule options)
         else if tp.name == "null or option set" then types.nullOr (types.submodule options)
         else tp;
-    in opt // { type = f (opt.type or types.unspecified); };
+    in
+      if opt.type.getSubModules or null == null
+      then opt // { type = f (opt.type or types.unspecified); }
+      else opt // { type = opt.type.substSubModules opt.options; options = []; };
 
 
   /* Properties. */
diff --git a/lib/types.nix b/lib/types.nix
index 0e2b6515e165..783e07cdc721 100644
--- a/lib/types.nix
+++ b/lib/types.nix
@@ -33,9 +33,14 @@ rec {
     , # Return a flat list of sub-options.  Used to generate
       # documentation.
       getSubOptions ? prefix: {}
+    , # List of modules if any, or null if none.
+      getSubModules ? null
+    , # Function for building the same option type  with a different list of
+      # modules.
+      substSubModules ? m: null
     }:
     { _type = "option-type";
-      inherit name check merge getSubOptions;
+      inherit name check merge getSubOptions getSubModules substSubModules;
     };
 
 
@@ -110,6 +115,8 @@ rec {
           elemType.merge (loc ++ ["[${toString n}-${toString m}]"])
             [{ inherit (def) file; value = def'; }]) def.value) defs);
       getSubOptions = prefix: elemType.getSubOptions (prefix ++ ["*"]);
+      getSubModules = elemType.getSubModules;
+      substSubModules = m: listOf (elemType.substSubModules m);
     };
 
     attrsOf = elemType: mkOptionType {
@@ -121,6 +128,8 @@ rec {
           (map (def: listToAttrs (mapAttrsToList (n: def':
             { name = n; value = { inherit (def) file; value = def'; }; }) def.value)) defs);
       getSubOptions = prefix: elemType.getSubOptions (prefix ++ ["<name>"]);
+      getSubModules = elemType.getSubModules;
+      substSubModules = m: attrsOf (elemType.substSubModules m);
     };
 
     # List or attribute set of ...
@@ -147,12 +156,16 @@ rec {
           else false;
         merge = loc: defs: attrOnly.merge loc (imap convertIfList defs);
         getSubOptions = prefix: elemType.getSubOptions (prefix ++ ["<name?>"]);
+        getSubModules = elemType.getSubModules;
+        substSubModules = m: loaOf (elemType.substSubModules m);
       };
 
     uniq = elemType: mkOptionType {
       inherit (elemType) name check;
       merge = mergeOneOption;
       getSubOptions = elemType.getSubOptions;
+      getSubModules = elemType.getSubModules;
+      substSubModules = m: uniq (elemType.substSubModules m);
     };
 
     nullOr = elemType: mkOptionType {
@@ -165,6 +178,8 @@ rec {
           throw "The option `${showOption loc}' is defined both null and not null, in ${showFiles (getFiles defs)}."
         else elemType.merge loc defs;
       getSubOptions = elemType.getSubOptions;
+      getSubModules = elemType.getSubModules;
+      substSubModules = m: nullOr (elemType.substSubModules m);
     };
 
     functionTo = elemType: mkOptionType {
@@ -173,6 +188,8 @@ rec {
       merge = loc: defs:
         fnArgs: elemType.merge loc (map (fn: { inherit (fn) file; value = fn.value fnArgs; }) defs);
       getSubOptions = elemType.getSubOptions;
+      getSubModules = elemType.getSubModules;
+      substSubModules = m: functionTo (elemType.substSubModules m);
     };
 
     submodule = opts:
@@ -192,6 +209,8 @@ rec {
           { modules = opts'; inherit prefix;
             # FIXME: hack to get shit to evaluate.
             args = { name = ""; }; }).options;
+        getSubModules = opts';
+        substSubModules = m: submodule m;
       };
 
     enum = values: mkOptionType {