diff --git a/lib/tests/modules.sh b/lib/tests/modules.sh
index e623c0fb55b86..0a18d092a9657 100755
--- a/lib/tests/modules.sh
+++ b/lib/tests/modules.sh
@@ -513,6 +513,10 @@ checkConfigOutput '^{}$' config.value.mkmerge ./types-anything/mk-mods.nix
 checkConfigOutput '^true$' config.value.mkbefore ./types-anything/mk-mods.nix
 checkConfigOutput '^1$' config.value.nested.foo ./types-anything/mk-mods.nix
 checkConfigOutput '^"baz"$' config.value.nested.bar.baz ./types-anything/mk-mods.nix
+# Function reflection works
+checkConfigOutput '^{"p":1,"q":2}$' 'config.applied.merging-args.value' ./types-anything/functions.nix
+checkConfigOutput '^{"a":false,"b":false}$' 'config.applied.merging-args.args' ./types-anything/functions.nix
+
 
 ## types.functionTo
 checkConfigOutput '^"input is input"$' config.result ./functionTo/trivial.nix
diff --git a/lib/tests/modules/types-anything/functions.nix b/lib/tests/modules/types-anything/functions.nix
index c29e3a1c5d8c1..7561a38301e04 100644
--- a/lib/tests/modules/types-anything/functions.nix
+++ b/lib/tests/modules/types-anything/functions.nix
@@ -10,7 +10,19 @@
   };
 
   options.applied = lib.mkOption {
-    default = lib.mapAttrs (name: fun: fun null) config.value;
+    default = lib.mapAttrs (name: fun:
+      if name == "merging-args"
+      then
+        {
+          args =
+            let x = lib.functionArgs fun;
+            in builtins.deepSeq x x;
+          value =
+            let x = fun { a = 1; b = 2; };
+            # can't pass --strict easily
+            in builtins.deepSeq x x;
+        }
+      else fun null) config.value;
   };
 
   config = lib.mkMerge [
@@ -18,10 +30,12 @@
       value.single-lambda = x: x;
       value.multiple-lambdas = x: { inherit x; };
       value.merging-lambdas = x: { inherit x; };
+      value.merging-args = args@{ a, ... }: { p = a; };
     }
     {
       value.multiple-lambdas = x: [ x ];
       value.merging-lambdas = y: { inherit y; };
+      value.merging-args = args@{ b, ... }: { q = b; };
     }
   ];
 
diff --git a/lib/types.nix b/lib/types.nix
index ee1410127b1ae..9cec764f4ba07 100644
--- a/lib/types.nix
+++ b/lib/types.nix
@@ -15,6 +15,8 @@ let
     isList
     isString
     isStorePath
+    functionArgs
+    setFunctionArgs
     throwIf
     toDerivation
     toList
@@ -303,12 +305,26 @@ rec {
             set = (attrsOf anything).merge;
             # This is the type of packages, only accept a single definition
             stringCoercibleSet = mergeOneOption;
-            lambda = loc: defs: arg: anything.merge
-              (loc ++ [ "<function body>" ])
-              (map (def: {
-                file = def.file;
-                value = def.value arg;
-              }) defs);
+            lambda = loc: defs:
+              let
+                args = zipAttrsWith
+                  (name:
+                    # All params must have a default for the combination
+                    # to have one. AND on a list is `all id`.
+                    all (b: b))
+                  (map
+                    (def: functionArgs def.value)
+                    defs);
+                mergedFun = arg: anything.merge
+                  (loc ++ [ "<function body>" ])
+                  (map (def: {
+                    file = def.file;
+                    value = def.value arg;
+                  }) defs);
+              in
+                if args == {}
+                then mergedFun
+                else setFunctionArgs mergedFun args;
             # Otherwise fall back to only allowing all equal definitions
           }.${commonType} or mergeEqualOption;
         in mergeFunction loc defs;