2 files changed, 37 insertions, 0 deletions
diff --git a/lib/tests/modules.sh b/lib/tests/modules.sh
index a90ff4ad9a2f..1221ba7143f6 100755
--- a/lib/tests/modules.sh
+++ b/lib/tests/modules.sh
@@ -406,6 +406,16 @@ checkConfigOutput "{}" config.submodule.a ./emptyValues.nix
 checkConfigError 'The option .int.a. is used but not defined' config.int.a ./emptyValues.nix
 checkConfigError 'The option .nonEmptyList.a. is used but not defined' config.nonEmptyList.a ./emptyValues.nix
 
+# types.unique
+#   requires a single definition
+checkConfigError 'The option .examples\.merged. is defined multiple times while it.s expected to be unique' config.examples.merged.a ./types-unique.nix
+#   user message is printed
+checkConfigError 'We require a single definition, because seeing the whole value at once helps us maintain critical invariants of our system.' config.examples.merged.a ./types-unique.nix
+#   let the inner merge function check the values (on demand)
+checkConfigError 'A definition for option .examples\.badLazyType\.a. is not of type .string.' config.examples.badLazyType.a ./types-unique.nix
+#   overriding still works (unlike option uniqueness)
+checkConfigOutput '^"bee"$' config.examples.override.b ./types-unique.nix
+
 ## types.raw
 checkConfigOutput '^true$' config.unprocessedNestingEvaluates.success ./raw.nix
 checkConfigOutput "10" config.processedToplevel ./raw.nix
diff --git a/lib/tests/modules/types-unique.nix b/lib/tests/modules/types-unique.nix
new file mode 100644
index 000000000000..115be0126975
--- /dev/null
+++ b/lib/tests/modules/types-unique.nix
@@ -0,0 +1,27 @@
+{ lib, ... }:
+let
+  inherit (lib) mkOption types;
+in
+{
+  options.examples = mkOption {
+    type = types.lazyAttrsOf
+      (types.unique
+        { message = "We require a single definition, because seeing the whole value at once helps us maintain critical invariants of our system."; }
+        (types.attrsOf types.str));
+  };
+  imports = [
+    { examples.merged = { b = "bee"; }; }
+    { examples.override = lib.mkForce { b = "bee"; }; }
+  ];
+  config.examples = {
+    merged = {
+      a = "aye";
+    };
+    override = {
+      a = "aye";
+    };
+    badLazyType = {
+      a = true;
+    };
+  };
+}