cmGeneratorTarget: add a target-level query for "needs dyndep"

This can still be overridden per-source, but it indicates the state of
the target's default behavior.

Source/cmGeneratorTarget.cxx

@@ -9374,11 +9374,25 @@ bool cmGeneratorTarget::NeedDyndepForSource(std::string const& lang,
     return true;
   }
 
+  auto targetDyndep = this->NeedCxxDyndep(config);
+  if (targetDyndep == CxxModuleSupport::Unavailable) {
+    return false;
+  }
+  auto const sfProp = sf->GetProperty("CXX_SCAN_FOR_MODULES");
+  if (sfProp.IsSet()) {
+    return sfProp.IsOn();
+  }
+  return targetDyndep == CxxModuleSupport::Enabled;
+}
+
+cmGeneratorTarget::CxxModuleSupport cmGeneratorTarget::NeedCxxDyndep(
+  std::string const& config) const
+{
   bool haveRule = false;
   switch (this->HaveCxxModuleSupport(config)) {
     case Cxx20SupportLevel::MissingCxx:
     case Cxx20SupportLevel::NoCxx20:
-      return false;
+      return CxxModuleSupport::Unavailable;
     case Cxx20SupportLevel::MissingRule:
       break;
     case Cxx20SupportLevel::Supported:
@@ -9388,28 +9402,29 @@ bool cmGeneratorTarget::NeedDyndepForSource(std::string const& lang,
   bool haveGeneratorSupport =
     this->GetGlobalGenerator()->CheckCxxModuleSupport(
       cmGlobalGenerator::CxxModuleSupportQuery::Inspect);
-  auto const sfProp = sf->GetProperty("CXX_SCAN_FOR_MODULES");
-  if (sfProp.IsSet()) {
-    return sfProp.IsOn();
-  }
   auto const tgtProp = this->GetProperty("CXX_SCAN_FOR_MODULES");
   if (tgtProp.IsSet()) {
-    return tgtProp.IsOn();
+    return tgtProp.IsOn() ? CxxModuleSupport::Enabled
+                          : CxxModuleSupport::Disabled;
   }
 
-  bool policyAnswer = false;
+  CxxModuleSupport policyAnswer = CxxModuleSupport::Unavailable;
   switch (this->GetPolicyStatusCMP0155()) {
     case cmPolicies::WARN:
     case cmPolicies::OLD:
       // The OLD behavior is to not scan the source.
-      policyAnswer = false;
+      policyAnswer = CxxModuleSupport::Disabled;
       break;
     case cmPolicies::REQUIRED_ALWAYS:
     case cmPolicies::REQUIRED_IF_USED:
     case cmPolicies::NEW:
       // The NEW behavior is to scan the source if the compiler supports
       // scanning and the generator supports it.
-      policyAnswer = haveRule && haveGeneratorSupport;
+      if (haveRule && haveGeneratorSupport) {
+        policyAnswer = CxxModuleSupport::Enabled;
+      } else {
+        policyAnswer = CxxModuleSupport::Disabled;
+      }
       break;
   }
   return policyAnswer;

Source/cmGeneratorTarget.h

@@ -1327,6 +1327,13 @@ public:
                                        cmSourceFile const* sf) const;
   bool NeedDyndepForSource(std::string const& lang, std::string const& config,
                            cmSourceFile const* sf) const;
+  enum class CxxModuleSupport
+  {
+    Unavailable,
+    Enabled,
+    Disabled,
+  };
+  CxxModuleSupport NeedCxxDyndep(std::string const& config) const;
 
 private:
   void BuildFileSetInfoCache(std::string const& config) const;