Index: cmPolicies.h
===================================================================
RCS file: /cvsroot/CMake/CMake/Source/cmPolicies.h,v
retrieving revision 1.24
diff -b -u -p -r1.24 cmPolicies.h
--- cmPolicies.h	17 Jun 2009 17:40:09 -0000	1.24
+++ cmPolicies.h	28 Jun 2009 20:41:04 -0000
@@ -54,6 +54,7 @@ public:
     CMP0011, // Strong policy scope for include and find_package
     CMP0012, // Strong handling of boolean constants
     CMP0013, // Duplicate binary directories not allowed
+    CMP0014, // set( CACHE ) sets variable to CACHE value if present
 
     // Always the last entry.  Useful mostly to avoid adding a comma
     // the last policy when adding a new one.
Index: cmPolicies.cxx
===================================================================
RCS file: /cvsroot/CMake/CMake/Source/cmPolicies.cxx,v
retrieving revision 1.40
diff -b -u -p -r1.40 cmPolicies.cxx
--- cmPolicies.cxx	17 Jun 2009 18:18:14 -0000	1.40
+++ cmPolicies.cxx	28 Jun 2009 20:41:04 -0000
@@ -387,6 +387,18 @@ cmPolicies::cmPolicies()
     "The NEW behavior for this policy is to disallow duplicate binary "
     "directories with an error.",
     2,6,5, cmPolicies::WARN);
+
+    this->DefinePolicy(
+    CMP0014, "CMP0014",
+    "SET(CACHE) gets the CACHE value if present and variable already set.",
+    "In CMake versions prior to 2.7.0 set(CACHE) and option() did nothing "
+    "if the variable was already in the CACHE and "
+    "additionally already set(), i.e. the value in the cache was ignored "
+    "and the variable kept the value from the previous set() command. "
+    "The NEW behaviour is that in this case the variable will be assigned "
+    "the value which is found in the cache. ",
+    2,7,0, cmPolicies::WARN);
+
 }
 
 cmPolicies::~cmPolicies()
Index: cmSetCommand.cxx
===================================================================
RCS file: /cvsroot/CMake/CMake/Source/cmSetCommand.cxx,v
retrieving revision 1.35
diff -b -u -p -r1.35 cmSetCommand.cxx
--- cmSetCommand.cxx	10 May 2009 20:07:34 -0000	1.35
+++ cmSetCommand.cxx	28 Jun 2009 20:41:04 -0000
@@ -160,6 +160,34 @@ bool cmSetCommand
     // or the makefile
     if(cache && type != cmCacheManager::INTERNAL && !force)
       {
+      // if the variable has already been set, and it is also already 
+      // stored in the cache, and if these two values differ, with the NEW
+      // behaviour, set the variable to the value from the cache, with OLD
+      // behaviour ignore the value in the cache.
+      if(strcmp(this->Makefile->GetSafeDefinition(variable), it.GetValue())!=0)
+        {
+        cmOStringStream e;
+        switch (this->Makefile->GetPolicyStatus(cmPolicies::CMP0014))
+          {
+          case cmPolicies::WARN:
+            // Print the warning.
+            e << this->Makefile->GetPolicies()->
+                                          GetPolicyWarning(cmPolicies::CMP0014)
+              << "\n"
+              << "Variable \"" << variable << "\" is set to \"" 
+              << it.GetValue() << "\""
+              << " from the CACHE, with the OLD behaviour it would have been "
+              << " set to \"" << this->Makefile->GetSafeDefinition(variable)
+              << "\".";
+            this->Makefile->IssueMessage(cmake::AUTHOR_WARNING, e.str());
+            // intentional fall through
+          case cmPolicies::NEW:
+            this->Makefile->RemoveDefinition(variable);
+            break;
+          default:   // do nothing
+            break;
+          }
+        }
       return true;
       }
     }
Index: cmOptionCommand.cxx
===================================================================
RCS file: /cvsroot/CMake/CMake/Source/cmOptionCommand.cxx,v
retrieving revision 1.23
diff -b -u -p -r1.23 cmOptionCommand.cxx
--- cmOptionCommand.cxx	23 Jan 2008 15:27:59 -0000	1.23
+++ cmOptionCommand.cxx	28 Jun 2009 20:41:04 -0000
@@ -3,7 +3,7 @@
   Program:   CMake - Cross-Platform Makefile Generator
   Module:    $RCSfile: cmOptionCommand.cxx,v $
   Language:  C++
-  Date:      $Date: 2008-01-23 15:27:59 $
+  Date:      $Date: 2008/01/23 15:27:59 $
   Version:   $Revision: 1.23 $
 
   Copyright (c) 2002 Kitware, Inc., Insight Consortium.  All rights reserved.
@@ -58,6 +58,31 @@ bool cmOptionCommand
     if ( it.GetType() != cmCacheManager::UNINITIALIZED )
       {
       it.SetProperty("HELPSTRING", args[1].c_str());
+
+      if (strcmp(this->Makefile->GetSafeDefinition(args[0].c_str()),
+                                                           it.GetValue()) != 0)
+        {
+        cmOStringStream e;
+        switch (this->Makefile->GetPolicyStatus(cmPolicies::CMP0014))
+          {
+          case cmPolicies::WARN:
+            // Print the warning.
+            e << this->Makefile->GetPolicies()->
+                                          GetPolicyWarning(cmPolicies::CMP0014)
+              << "\n"
+              << "Variable \"" << args[0].c_str() << "\" is set to \"" 
+              << it.GetValue() << "\" from the CACHE, with the OLD behaviour "
+              << " it would have been set to \"" 
+              << this->Makefile->GetSafeDefinition(args[0].c_str()) << "\".";
+            this->Makefile->IssueMessage(cmake::AUTHOR_WARNING, e.str());
+            // intentional fall through
+          case cmPolicies::NEW:
+            this->Makefile->RemoveDefinition(args[0].c_str());
+            break;
+          default:       // do nothing
+            break;
+          }
+        }
       return true;
       }
     if ( it.GetValue() )

Index: cmOptionCommand.cxx
===================================================================
RCS file: /cvsroot/CMake/CMake/Source/cmOptionCommand.cxx,v
retrieving revision 1.23
diff -b -u -p -r1.23 cmOptionCommand.cxx
--- cmOptionCommand.cxx	23 Jan 2008 15:27:59 -0000	1.23
+++ cmOptionCommand.cxx	6 Sep 2009 08:43:22 -0000
@@ -3,7 +3,7 @@
   Program:   CMake - Cross-Platform Makefile Generator
   Module:    $RCSfile: cmOptionCommand.cxx,v $
   Language:  C++
-  Date:      $Date: 2008-01-23 15:27:59 $
+  Date:      $Date: 2008/01/23 15:27:59 $
   Version:   $Revision: 1.23 $
 
   Copyright (c) 2002 Kitware, Inc., Insight Consortium.  All rights reserved.
@@ -58,6 +58,31 @@ bool cmOptionCommand
     if ( it.GetType() != cmCacheManager::UNINITIALIZED )
       {
       it.SetProperty("HELPSTRING", args[1].c_str());
+
+      if (strcmp(this->Makefile->GetSafeDefinition(args[0].c_str()),
+                                                           it.GetValue()) != 0)
+        {
+        cmOStringStream e;
+        switch (this->Makefile->GetPolicyStatus(cmPolicies::CMP0015))
+          {
+          case cmPolicies::WARN:
+            // Print the warning.
+            e << this->Makefile->GetPolicies()->
+                                          GetPolicyWarning(cmPolicies::CMP0015)
+              << "\n"
+              << "Variable \"" << args[0].c_str() << "\" is set to \"" 
+              << it.GetValue() << "\" from the CACHE, with the OLD behaviour "
+              << " it would have been set to \"" 
+              << this->Makefile->GetSafeDefinition(args[0].c_str()) << "\".";
+            this->Makefile->IssueMessage(cmake::AUTHOR_WARNING, e.str());
+            // intentional fall through
+          case cmPolicies::NEW:
+            this->Makefile->RemoveDefinition(args[0].c_str());
+            break;
+          default:       // do nothing
+            break;
+          }
+        }
       return true;
       }
     if ( it.GetValue() )
Index: cmPolicies.cxx
===================================================================
RCS file: /cvsroot/CMake/CMake/Source/cmPolicies.cxx,v
retrieving revision 1.42
diff -b -u -p -r1.42 cmPolicies.cxx
--- cmPolicies.cxx	3 Sep 2009 12:27:12 -0000	1.42
+++ cmPolicies.cxx	6 Sep 2009 08:43:22 -0000
@@ -399,6 +399,18 @@ cmPolicies::cmPolicies()
     "The OLD behavior for this policy is to silently ignore the problem.  "
     "The NEW behavior for this policy is to report an error.",
     2,7,20090902, cmPolicies::WARN);
+
+    this->DefinePolicy(
+    CMP0015, "CMP0015",
+    "SET(CACHE) gets the CACHE value if present and variable already set.",
+    "In CMake versions prior to 2.7.0 set(CACHE) and option() did nothing "
+    "if the variable was already in the CACHE and "
+    "additionally already set(), i.e. the value in the cache was ignored "
+    "and the variable kept the value from the previous set() command. "
+    "The NEW behaviour is that in this case the variable will be assigned "
+    "the value which is found in the cache. ",
+    2,7,20090906, cmPolicies::WARN);
+
 }
 
 cmPolicies::~cmPolicies()
Index: cmPolicies.h
===================================================================
RCS file: /cvsroot/CMake/CMake/Source/cmPolicies.h,v
retrieving revision 1.25
diff -b -u -p -r1.25 cmPolicies.h
--- cmPolicies.h	3 Sep 2009 12:27:12 -0000	1.25
+++ cmPolicies.h	6 Sep 2009 08:43:22 -0000
@@ -55,6 +55,7 @@ public:
     CMP0012, // Strong handling of boolean constants
     CMP0013, // Duplicate binary directories not allowed
     CMP0014, // Input directories must have CMakeLists.txt
+    CMP0015, // set( CACHE ) sets variable to CACHE value if present
 
     // Always the last entry.  Useful mostly to avoid adding a comma
     // the last policy when adding a new one.
Index: cmSetCommand.cxx
===================================================================
RCS file: /cvsroot/CMake/CMake/Source/cmSetCommand.cxx,v
retrieving revision 1.35
diff -b -u -p -r1.35 cmSetCommand.cxx
--- cmSetCommand.cxx	10 May 2009 20:07:34 -0000	1.35
+++ cmSetCommand.cxx	6 Sep 2009 08:43:22 -0000
@@ -160,6 +160,34 @@ bool cmSetCommand
     // or the makefile
     if(cache && type != cmCacheManager::INTERNAL && !force)
       {
+      // if the variable has already been set, and it is also already 
+      // stored in the cache, and if these two values differ, with the NEW
+      // behaviour, set the variable to the value from the cache, with OLD
+      // behaviour ignore the value in the cache.
+      if(strcmp(this->Makefile->GetSafeDefinition(variable), it.GetValue())!=0)
+        {
+        cmOStringStream e;
+        switch (this->Makefile->GetPolicyStatus(cmPolicies::CMP0015))
+          {
+          case cmPolicies::WARN:
+            // Print the warning.
+            e << this->Makefile->GetPolicies()->
+                                          GetPolicyWarning(cmPolicies::CMP0015)
+              << "\n"
+              << "Variable \"" << variable << "\" is set to \"" 
+              << it.GetValue() << "\""
+              << " from the CACHE, with the OLD behaviour it would have been "
+              << " set to \"" << this->Makefile->GetSafeDefinition(variable)
+              << "\".";
+            this->Makefile->IssueMessage(cmake::AUTHOR_WARNING, e.str());
+            // intentional fall through
+          case cmPolicies::NEW:
+            this->Makefile->RemoveDefinition(variable);
+            break;
+          default:   // do nothing
+            break;
+          }
+        }
       return true;
       }
     }

Index: cmSetCommand.h
===================================================================
RCS file: /cvsroot/CMake/CMake/Source/cmSetCommand.h,v
retrieving revision 1.22
diff -b -u -p -r1.22 cmSetCommand.h
--- cmSetCommand.h	3 Sep 2009 19:29:29 -0000	1.22
+++ cmSetCommand.h	6 Sep 2009 13:07:27 -0000
@@ -69,8 +69,10 @@ public:
       "  set(<variable> <value>\n"
       "      [[CACHE <type> <docstring> [FORCE]] | PARENT_SCOPE])\n"
       "Within CMake sets <variable> to the value <value>.  <value> is expanded"
-      "  before <variable> is set to it.  If CACHE is present, then the "
-      "<variable> is put in the cache. <type> and <docstring> are then "
+      "  before <variable> is set to it.  If CACHE is present and <variable> "
+      "is not yet in the cache, then <variable> is put in the cache. If it is "
+      "already in the cache, <variable> is assigned the value stored in the "
+      "cache.  If CACHE is present, also <type> and <docstring> are "
       "required. <type> is used by the CMake GUI to choose a widget with "
       "which the user sets a value.  The value for <type> may be one of\n"
       "  FILEPATH = File chooser dialog.\n"

cmake_minimum_required(VERSION 2.6)

macro(print var)
   message(STATUS "${var} = \"${${var}}\"")
endmacro(print)

set(FOO foo)
print(FOO)              # gives "foo"

find_program(FOO ls)    # this line doesn't affect the result at all
print(FOO)

mark_as_advanced(FOO)   # without this line FOO is still "foo" after the next find_program()
                        # with this line it is empty

print(FOO)              # gives "foo"

find_program(FOO ls)    # does nothing, since FOO is already valid
print(FOO)              # gives ""