diff --git a/lib/tests/modules.sh b/lib/tests/modules.sh
index 58231a35636..66c6f560fbe 100755
--- a/lib/tests/modules.sh
+++ b/lib/tests/modules.sh
@@ -57,13 +57,17 @@ checkConfigError() {
     fi
 }
 
+# Check boolean option.
 checkConfigOutput "false" config.enable ./declare-enable.nix
 checkConfigError 'The option .* defined in .* does not exist.' config.enable ./define-enable.nix
+
+# Check mkForce without submodules.
 set -- config.enable ./declare-enable.nix ./define-enable.nix
 checkConfigOutput "true" "$@"
 checkConfigOutput "false" "$@" ./define-force-enable.nix
 checkConfigOutput "false" "$@" ./define-enable-force.nix
 
+# Check mkForce with option and submodules.
 checkConfigError 'attribute .*foo.* .* not found' config.loaOfSub.foo.enable ./declare-loaOfSub-any-enable.nix
 checkConfigOutput 'false' config.loaOfSub.foo.enable ./declare-loaOfSub-any-enable.nix ./define-loaOfSub-foo.nix
 set -- config.loaOfSub.foo.enable ./declare-loaOfSub-any-enable.nix ./define-loaOfSub-foo-enable.nix
@@ -73,6 +77,7 @@ checkConfigOutput 'false' "$@" ./define-loaOfSub-force-foo-enable.nix
 checkConfigOutput 'false' "$@" ./define-loaOfSub-foo-force-enable.nix
 checkConfigOutput 'false' "$@" ./define-loaOfSub-foo-enable-force.nix
 
+# Check overriding effect of mkForce on submodule definitions.
 checkConfigError 'attribute .*bar.* .* not found' config.loaOfSub.bar.enable ./declare-loaOfSub-any-enable.nix ./define-loaOfSub-foo.nix
 checkConfigOutput 'false' config.loaOfSub.bar.enable ./declare-loaOfSub-any-enable.nix ./define-loaOfSub-foo.nix ./define-loaOfSub-bar.nix
 set -- config.loaOfSub.bar.enable ./declare-loaOfSub-any-enable.nix ./define-loaOfSub-foo.nix ./define-loaOfSub-bar-enable.nix
@@ -82,6 +87,26 @@ checkConfigError 'attribute .*bar.* .* not found' "$@" ./define-loaOfSub-force-f
 checkConfigOutput 'true' "$@" ./define-loaOfSub-foo-force-enable.nix
 checkConfigOutput 'true' "$@" ./define-loaOfSub-foo-enable-force.nix
 
+# Check mkIf with submodules.
+checkConfigError 'attribute .*foo.* .* not found' config.loaOfSub.foo.enable ./declare-enable.nix ./declare-loaOfSub-any-enable.nix
+set -- config.loaOfSub.foo.enable ./declare-enable.nix ./declare-loaOfSub-any-enable.nix
+checkConfigError 'attribute .*foo.* .* not found' "$@" ./define-if-loaOfSub-foo-enable.nix
+checkConfigError 'attribute .*foo.* .* not found' "$@" ./define-loaOfSub-if-foo-enable.nix
+checkConfigError 'attribute .*foo.* .* not found' "$@" ./define-loaOfSub-foo-if-enable.nix
+checkConfigOutput 'false' "$@" ./define-loaOfSub-foo-enable-if.nix
+checkConfigOutput 'true' "$@" ./define-enable.nix ./define-if-loaOfSub-foo-enable.nix
+checkConfigOutput 'true' "$@" ./define-enable.nix ./define-loaOfSub-if-foo-enable.nix
+checkConfigOutput 'true' "$@" ./define-enable.nix ./define-loaOfSub-foo-if-enable.nix
+checkConfigOutput 'true' "$@" ./define-enable.nix ./define-loaOfSub-foo-enable-if.nix
+
+# Check _module.args.
+checkConfigOutput "true" config.enable ./declare-enable.nix ./custom-arg-define-enable.nix
+
+# Check _module.check.
+set -- config.enable ./declare-enable.nix ./define-enable.nix ./define-loaOfSub-foo.nix
+checkConfigError 'The option .* defined in .* does not exist.' "$@"
+checkConfigOutput "true" "$@" ./define-module-check.nix
+
 cat <<EOF
 ====== module tests ======
 $pass Pass
diff --git a/lib/tests/modules/custom-arg-define-enable.nix b/lib/tests/modules/custom-arg-define-enable.nix
new file mode 100644
index 00000000000..f04d30dd9b9
--- /dev/null
+++ b/lib/tests/modules/custom-arg-define-enable.nix
@@ -0,0 +1,8 @@
+{ lib, custom, ... }:
+
+{
+  config = {
+    _module.args.custom = true;
+    enable = custom;
+  };
+}
diff --git a/lib/tests/modules/define-if-loaOfSub-foo-enable.nix b/lib/tests/modules/define-if-loaOfSub-foo-enable.nix
new file mode 100644
index 00000000000..4288d74dec0
--- /dev/null
+++ b/lib/tests/modules/define-if-loaOfSub-foo-enable.nix
@@ -0,0 +1,5 @@
+{ config, lib, ... }:
+
+lib.mkIf config.enable {
+  loaOfSub.foo.enable = true;
+}
diff --git a/lib/tests/modules/define-loaOfSub-foo-enable-if.nix b/lib/tests/modules/define-loaOfSub-foo-enable-if.nix
new file mode 100644
index 00000000000..44b2c96cd02
--- /dev/null
+++ b/lib/tests/modules/define-loaOfSub-foo-enable-if.nix
@@ -0,0 +1,5 @@
+{ config, lib, ... }:
+
+{
+  loaOfSub.foo.enable = lib.mkIf config.enable true;
+}
diff --git a/lib/tests/modules/define-loaOfSub-foo-if-enable.nix b/lib/tests/modules/define-loaOfSub-foo-if-enable.nix
new file mode 100644
index 00000000000..236b2840ee5
--- /dev/null
+++ b/lib/tests/modules/define-loaOfSub-foo-if-enable.nix
@@ -0,0 +1,7 @@
+{ config, lib, ... }:
+
+{
+  loaOfSub.foo = lib.mkIf config.enable {
+    enable = true;
+  };
+}
diff --git a/lib/tests/modules/define-loaOfSub-if-foo-enable.nix b/lib/tests/modules/define-loaOfSub-if-foo-enable.nix
new file mode 100644
index 00000000000..bd2d068d31a
--- /dev/null
+++ b/lib/tests/modules/define-loaOfSub-if-foo-enable.nix
@@ -0,0 +1,7 @@
+{ config, lib, ... }:
+
+{
+  loaOfSub = lib.mkIf config.enable {
+    foo.enable = true;
+  };
+}
diff --git a/lib/tests/modules/define-module-check.nix b/lib/tests/modules/define-module-check.nix
new file mode 100644
index 00000000000..5a0707c975f
--- /dev/null
+++ b/lib/tests/modules/define-module-check.nix
@@ -0,0 +1,3 @@
+{
+  _module.check = false;
+}