lib/tests/modules: add a test for the functionTo type

lib/tests/modules.sh

@@ -149,6 +149,9 @@ checkConfigOutput "1 2 3 4 5 6 7 8 9 10" config.result ./loaOf-with-long-list.ni
 # Check loaOf with many merges of lists.
 checkConfigOutput "1 2 3 4 5 6 7 8 9 10" config.result ./loaOf-with-many-list-merges.nix
 
+# Check the merge behaviour of the functionTo type.
+checkConfigOutput "a b" config.result ./functionTo.nix
+
 cat <<EOF
 ====== module tests ======
 $pass Pass

lib/tests/modules/functionTo.nix

@@ -0,0 +1,29 @@
+{ lib, config, ... }:
+
+with lib;
+
+{
+  options = {
+    selector = mkOption {
+      default = _pkgs : [];
+      type = with types; functionTo (listOf str);
+      description = ''
+        Some descriptive text
+      '';
+    };
+
+    result = mkOption {
+      type = types.str;
+      default = toString (config.selector {
+        a = "a";
+        b = "b";
+        c = "c";
+      });
+    };
+  };
+
+  config = lib.mkMerge [
+    { selector = pkgs: [ pkgs.a ]; }
+    { selector = pkgs: [ pkgs.b ]; }
+  ];
+}