diff --git a/Source/cmTarget.cxx b/Source/cmTarget.cxx
index c9905b643..26b1e7614 100644
--- a/Source/cmTarget.cxx
+++ b/Source/cmTarget.cxx
@@ -91,8 +91,8 @@ public:
     }
   ~cmTargetInternals();
   typedef cmTarget::SourceFileFlags SourceFileFlags;
-  std::map<cmSourceFile const*, SourceFileFlags> SourceFlagsMap;
-  bool SourceFileFlagsConstructed;
+  mutable std::map<cmSourceFile const*, SourceFileFlags> SourceFlagsMap;
+  mutable bool SourceFileFlagsConstructed;
 
   // The backtrace when the target was created.
   cmListFileBacktrace Backtrace;
@@ -577,7 +577,7 @@ void cmTarget::ProcessSourceExpression(std::string const& expr)
 
 //----------------------------------------------------------------------------
 struct cmTarget::SourceFileFlags
-cmTarget::GetTargetSourceFileFlags(const cmSourceFile* sf)
+cmTarget::GetTargetSourceFileFlags(const cmSourceFile* sf) const
 {
   struct SourceFileFlags flags;
   this->ConstructSourceFileFlags();
@@ -591,7 +591,7 @@ cmTarget::GetTargetSourceFileFlags(const cmSourceFile* sf)
 }
 
 //----------------------------------------------------------------------------
-void cmTarget::ConstructSourceFileFlags()
+void cmTarget::ConstructSourceFileFlags() const
 {
   if(this->Internal->SourceFileFlagsConstructed)
     {
diff --git a/Source/cmTarget.h b/Source/cmTarget.h
index bafcb15ea..e67edd745 100644
--- a/Source/cmTarget.h
+++ b/Source/cmTarget.h
@@ -162,7 +162,8 @@ public:
   /**
    * Get the flags for a given source file as used in this target
    */
-  struct SourceFileFlags GetTargetSourceFileFlags(const cmSourceFile* sf);
+  struct SourceFileFlags
+  GetTargetSourceFileFlags(const cmSourceFile* sf) const;
 
   /**
    * Add sources to the target.
@@ -742,7 +743,7 @@ private:
   friend class cmTargetTraceDependencies;
   cmTargetInternalPointer Internal;
 
-  void ConstructSourceFileFlags();
+  void ConstructSourceFileFlags() const;
   void ComputeVersionedName(std::string& vName,
                             std::string const& prefix,
                             std::string const& base,