Create CMake Policy CMP0015 to fix set(CACHE)

The set(CACHE) and option() commands should always expose the cache
value.  Previously we failed to expose the value when it was already set
if a local variable definition hid it.  When set to NEW, this policy
tells the commands to always remove the local variable definition to
expose the cache value.  See issue #9008.

Source/cmMakefile.cxx

@@ -1662,6 +1662,57 @@ void cmMakefile::AddDefinition(const char* name, const char* value)
 #endif
 }
 
+//----------------------------------------------------------------------------
+void cmMakefile::UseCacheDefinition(cmCacheManager::CacheIterator const& it)
+{
+  // Check for a local definition that might hide the cache value.
+  const char* name = it.GetName();
+  const char* def = this->Internal->VarStack.top().Get(name);
+  if(!def)
+    {
+    return;
+    }
+
+  // If the visible value will change then check policy CMP0015.
+  const char* cache = it.GetValue();
+  if(strcmp(def, cache) != 0)
+    {
+    cmOStringStream e;
+    switch (this->GetPolicyStatus(cmPolicies::CMP0015))
+      {
+      case cmPolicies::WARN:
+        e << "Local variable \"" << name << "\" is set to\n"
+          << "  " << def << "\n"
+          << "but the CACHE entry of the same name is set to\n"
+          << "  " << cache << "\n"
+          << "The local variable is hiding the cache value."
+          << "\n"
+          << this->GetPolicies()->GetPolicyWarning(cmPolicies::CMP0015);
+        this->IssueMessage(cmake::AUTHOR_WARNING, e.str());
+      case cmPolicies::OLD:
+        // OLD behavior is to leave local definition.
+        return;
+      case cmPolicies::REQUIRED_IF_USED:
+      case cmPolicies::REQUIRED_ALWAYS:
+        e << "Local variable \"" << name << "\" is set to\n"
+          << "  " << def << "\n"
+          << "but the CACHE entry of the same name is set to\n"
+          << "  " << cache << "\n"
+          << "This command is removing the local variable to expose "
+          << "the cache value."
+          << "\n"
+          << this->GetPolicies()->GetRequiredPolicyError(cmPolicies::CMP0015);
+        this->IssueMessage(cmake::FATAL_ERROR, e.str());
+      case cmPolicies::NEW:
+        // NEW behavior is to remove local definition (done below).
+        break;
+      }
+    }
+
+  // Remove the local definition to make the cache value visible.
+  this->RemoveDefinition(name);
+}
+
 
 void cmMakefile::AddCacheDefinition(const char* name, const char* value,
                                     const char* doc,

Source/cmMakefile.h

@@ -290,6 +290,11 @@ public:
                           cmCacheManager::CacheEntryType type,
                           bool force = false);
 
+  /**
+   * Update the variable scope to make the cache definition visible.
+   */
+  void UseCacheDefinition(cmCacheManager::CacheIterator const& it);
+
   /**
    * Add bool variable definition to the build. 
    */

Source/cmOptionCommand.cxx

@@ -58,6 +58,7 @@ bool cmOptionCommand
     if ( it.GetType() != cmCacheManager::UNINITIALIZED )
       {
       it.SetProperty("HELPSTRING", args[1].c_str());
+      this->Makefile->UseCacheDefinition(it);
       return true;
       }
     if ( it.GetValue() )

Source/cmPolicies.cxx

@@ -399,6 +399,37 @@ 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",
+    "The set() CACHE mode and option() command make the cache value visible.",
+    "In CMake 2.6 and below the CACHE mode of the set() command and the "
+    "option() command did not expose the value from the named cache entry "
+    "if it was already set both in the cache and as a local variable.  "
+    "This led to subtle differences between first and later configurations "
+    "because a conflicting local variable would be overridden only when the "
+    "cache value was first created.  "
+    "For example, the code\n"
+    "  set(x 1)\n"
+    "  set(before ${x})\n"
+    "  set(x 2 CACHE STRING \"X\")\n"
+    "  set(after ${x})\n"
+    "  message(STATUS \"${before},${after}\")\n"
+    "would print \"1,2\" on the first run and \"1,1\" on future runs."
+    "\n"
+    "CMake 2.8.0 and above prefer to expose the cache value in all cases by "
+    "removing the local variable definition, but this changes behavior in "
+    "subtle cases when the local variable has a different value than that "
+    "exposed from the cache.  "
+    "The example above will always print \"1,2\"."
+    "\n"
+    "This policy determines whether the commands should always expose the "
+    "cache value.  "
+    "The OLD behavior for this policy is to leave conflicting local "
+    "variable values untouched and hide the true cache value.  "
+    "The NEW behavior for this policy is to always expose the cache value.",
+    2,7,20090910, cmPolicies::WARN);
+
 }
 
 cmPolicies::~cmPolicies()

Source/cmPolicies.h

@@ -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) and option() make CACHE value visible
 
     // Always the last entry.  Useful mostly to avoid adding a comma
     // the last policy when adding a new one.

Source/cmSetCommand.cxx

@@ -160,6 +160,7 @@ bool cmSetCommand
     // or the makefile
     if(cache && type != cmCacheManager::INTERNAL && !force)
       {
+      this->Makefile->UseCacheDefinition(it);
       return true;
       }
     }

Source/cmSetCommand.h

@@ -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"