lib/modules: Throw better error when definitions assign to an option set

lib/modules.nix

@@ -299,7 +299,9 @@ rec {
             in
               throw "The option `${showOption loc}' in `${firstOption._file}' is a prefix of options in `${firstNonOption._file}'."
           else
-            mergeModules' loc decls defns
+            if all (def: isAttrs def.value) defns' then mergeModules' loc decls defns
+            else let firstInvalid = findFirst (def: ! isAttrs def.value) null defns';
+            in throw "The option path `${showOption loc}' is an attribute set of options, but it is defined to not be an attribute set in `${firstInvalid.file}'. Did you define its value at the correct and complete path?"
       ))
     // { _definedNames = map (m: { inherit (m) file; names = attrNames m.config; }) configs; };
 

lib/tests/modules.sh

@@ -194,6 +194,10 @@ checkConfigOutput "true" config.conditionalWorks ./declare-attrsOf.nix ./attrsOf
 checkConfigOutput "false" config.conditionalWorks ./declare-lazyAttrsOf.nix ./attrsOf-conditional-check.nix
 checkConfigOutput "empty" config.value.foo ./declare-lazyAttrsOf.nix ./attrsOf-conditional-check.nix
 
+# Check error for when an option set is defined to be a non-attribute set value
+checkConfigError 'The option path .* is an attribute set of options, but it is defined to not be an attribute set in' \
+  config.value ./declare-option-set.nix ./define-value-int-zero.nix
+
 cat <<EOF
 ====== module tests ======
 $pass Pass

lib/tests/modules/declare-option-set.nix

@@ -0,0 +1,3 @@
+{
+  options.value = {};
+}