diff --git a/lib/tests/misc.nix b/lib/tests/misc.nix
index 498f9064d2ff..e86c366d4ce4 100644
--- a/lib/tests/misc.nix
+++ b/lib/tests/misc.nix
@@ -2521,6 +2521,21 @@ runTests {
     expr = (with types; either int (listOf (either bool str))).description;
     expected = "signed integer or list of (boolean or string)";
   };
+  testTypeFunctionToPropagateFunctionArgs = {
+    expr = lib.functionArgs ((types.functionTo types.null).merge [] [
+      {
+        value = {a, b ? false, ... }: null;
+      }
+      {
+        value = {b, c ? false, ... }: null;
+      }
+    ]);
+    expected = {
+      a = false;
+      b = false;
+      c = true;
+    };
+  };
 
 # Meta
   testGetExe'Output = {
diff --git a/lib/types.nix b/lib/types.nix
index cda048fd10ca..cdba500ae07f 100644
--- a/lib/types.nix
+++ b/lib/types.nix
@@ -873,8 +873,13 @@ rec {
       description = "function that evaluates to a(n) ${optionDescriptionPhrase (class: class == "noun" || class == "composite") elemType}";
       descriptionClass = "composite";
       check = isFunction;
-      merge = loc: defs:
-        fnArgs: (mergeDefinitions (loc ++ [ "<function body>" ]) elemType (map (fn: { inherit (fn) file; value = fn.value fnArgs; }) defs)).mergedValue;
+      merge = loc: defs: {
+        # An argument attribute has a default when it has a default in all definitions
+        __functionArgs = lib.zipAttrsWith (_: lib.all (x: x)) (
+          lib.map (fn: lib.functionArgs fn.value) defs
+        );
+        __functor = _: callerArgs: (mergeDefinitions (loc ++ [ "<function body>" ]) elemType (map (fn: { inherit (fn) file; value = fn.value callerArgs; }) defs)).mergedValue;
+      };
       getSubOptions = prefix: elemType.getSubOptions (prefix ++ [ "<function body>" ]);
       getSubModules = elemType.getSubModules;
       substSubModules = m: functionTo (elemType.substSubModules m);