From ac0ef82504747d3d8213f828736adf63bb304bc4 Mon Sep 17 00:00:00 2001
From: cinereal <cinereal@riseup.net>
Date: Sun, 15 Mar 2026 08:38:10 -0700
Subject: [PATCH] lib.modules: default to `emptyValue`

Signed-off-by: cinereal <cinereal@riseup.net>
---
 lib/modules.nix                |  2 ++
 lib/tests/modules.sh           | 11 +++++++++++
 lib/tests/modules/defaults.nix | 33 +++++++++++++++++++++++++++++++++
 3 files changed, 46 insertions(+)
 create mode 100644 lib/tests/modules/defaults.nix

diff --git a/lib/modules.nix b/lib/modules.nix
index c67ea2247ca7..ff286ad7c4b2 100644
--- a/lib/modules.nix
+++ b/lib/modules.nix
@@ -1246,6 +1246,8 @@ let
             allInvalid = filter (def: !type.check def.value) defsFinal;
           in
           throw "A definition for option `${showOption loc}' is not of type `${type.description}'. Definition values:${showDefs allInvalid}"
+      else if type.emptyValue ? value then
+        type.emptyValue.value
       else
         # (nixos-option detects this specific error message and gives it special
         # handling.  If changed here, please change it there too.)
diff --git a/lib/tests/modules.sh b/lib/tests/modules.sh
index aa9d3443fb6a..bc89d4c20851 100755
--- a/lib/tests/modules.sh
+++ b/lib/tests/modules.sh
@@ -674,6 +674,17 @@ checkConfigOutput "{}" config.submodule.a ./emptyValues.nix
 checkConfigError 'The option .int.a. was accessed but has no value defined. Try setting the option.' config.int.a ./emptyValues.nix
 checkConfigError 'The option .nonEmptyList.a. was accessed but has no value defined. Try setting the option.' config.nonEmptyList.a ./emptyValues.nix
 
+## defaults
+checkConfigOutput "\[\]" config.list ./defaults.nix
+checkConfigOutput "{}" config.attrs ./defaults.nix
+checkConfigOutput "{}" config.attrsOf ./defaults.nix
+checkConfigOutput "null" config.null ./defaults.nix
+checkConfigOutput "{}" config.submodule ./defaults.nix
+checkConfigOutput "\[\]" config.unique ./defaults.nix
+checkConfigOutput "\[\]" config.coercedTo ./defaults.nix
+# These types don't have empty values
+checkConfigError 'The option .int. was accessed but has no value defined. Try setting the option.' config.int ./defaults.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
diff --git a/lib/tests/modules/defaults.nix b/lib/tests/modules/defaults.nix
new file mode 100644
index 000000000000..56eee2c9c286
--- /dev/null
+++ b/lib/tests/modules/defaults.nix
@@ -0,0 +1,33 @@
+{ lib, ... }:
+let
+  inherit (lib) types;
+in
+{
+  options = {
+    list = lib.mkOption {
+      type = types.listOf types.int;
+    };
+    attrs = lib.mkOption {
+      type = types.attrs;
+    };
+    attrsOf = lib.mkOption {
+      type = types.attrsOf types.int;
+    };
+    null = lib.mkOption {
+      type = types.nullOr types.int;
+    };
+    submodule = lib.mkOption {
+      type = types.submodule { };
+    };
+    unique = lib.mkOption {
+      type = types.unique { message = "hi"; } (types.listOf types.int);
+    };
+    coercedTo = lib.mkOption {
+      type = types.coercedTo (types.attrsOf types.int) builtins.attrNames (types.listOf types.str);
+    };
+    # no empty value
+    int = lib.mkOption {
+      type = types.int;
+    };
+  };
+}