enable_language: Include unset variables in what to propagate.

Fixes: #28200

Source/cmDefinitions.cxx

@@ -79,7 +79,8 @@ cmDefinitions cmDefinitions::MakeClosure(StackIter begin, StackIter end)
 }
 
 std::vector<std::string> cmDefinitions::ClosureKeys(StackIter begin,
-                                                    StackIter end)
+                                                    StackIter end,
+                                                    UnsetKeys unsetKeys)
 {
   std::vector<std::string> defined;
   std::unordered_set<cm::string_view> bound;
@@ -88,7 +89,8 @@ std::vector<std::string> cmDefinitions::ClosureKeys(StackIter begin,
     defined.reserve(defined.size() + it->Map.size());
     for (auto const& mi : it->Map) {
       // Use this key if it is not already set or unset.
-      if (bound.emplace(mi.first.view()).second && mi.second.Value) {
+      if (bound.emplace(mi.first.view()).second &&
+          (unsetKeys == UnsetKeys::Include || mi.second.Value)) {
         defined.push_back(*mi.first.str_if_stable());
       }
     }

Source/cmDefinitions.h

@@ -34,7 +34,14 @@ public:
 
   static bool HasKey(std::string const& key, StackIter begin, StackIter end);
 
-  static std::vector<std::string> ClosureKeys(StackIter begin, StackIter end);
+  enum class UnsetKeys
+  {
+    Exclude,
+    Include,
+  };
+
+  static std::vector<std::string> ClosureKeys(
+    StackIter begin, StackIter end, UnsetKeys unsetKeys = UnsetKeys::Exclude);
 
   static cmDefinitions MakeClosure(StackIter begin, StackIter end);
 

Source/cmGlobalGenerator.cxx

@@ -1159,7 +1159,6 @@ void cmGlobalGenerator::EnableLanguage(
     cmStateSnapshot snapshot = mf->GetStateSnapshot();
     bool warnCMP0220 = false;
     for (std::string const& key : snapshot.LocalKeys()) {
-      // Should never need to propagate unsets
       if (!key.empty()) {
         if (mf->RaiseToRoot(key, snapshot.GetDefinition(key).GetCStr()) ==
             cmStateSnapshot::WarnCMP0220::Yes) {

Source/cmStateSnapshot.cxx

@@ -417,7 +417,8 @@ std::vector<std::string> cmStateSnapshot::ClosureKeys() const
 std::vector<std::string> cmStateSnapshot::LocalKeys() const
 {
   std::vector<std::string> keys = cmDefinitions::ClosureKeys(
-    this->Position->Vars, this->Position->ScopeParent->Vars);
+    this->Position->Vars, this->Position->ScopeParent->Vars,
+    cmDefinitions::UnsetKeys::Include);
 
   // Remove keys that are the same in the parent scope to avoid propagating
   // logic meant to restore variables back to their original value.