Allow users to specify defaults for unset policies

Check CMAKE_POLICY_DEFAULT_CMP<NNNN> for a default when policy CMP<NNNN>
would otherwise be left unset.  This allows users to set policies on the
command line when the project does not set them.  One may do this to
quiet warnings or test whether a project will build with new behavior
without modifying code.  There may also be cases when users want to
build an existing project release using new behavior for policies
unknown to the project at the time of the release.

Source/cmCMakePolicyCommand.h

@@ -85,7 +85,8 @@ public:
       "given version of CMake.  "
       "All policies introduced in the specified version or earlier "
       "will be set to use NEW behavior.  "
-      "All policies introduced after the specified version will be unset.  "
+      "All policies introduced after the specified version will be unset "
+      "(unless variable CMAKE_POLICY_DEFAULT_CMP<NNNN> sets a default).  "
       "This effectively requests behavior preferred as of a given CMake "
       "version and tells newer CMake versions to warn about their new "
       "policies.  "

Source/cmDocumentVariables.cxx

@@ -464,6 +464,25 @@ void cmDocumentVariables::DefineVariables(cmake* cm)
 
   // Variables defined by cmake, that change the behavior
   // of cmake
+
+  cm->DefineProperty
+    ("CMAKE_POLICY_DEFAULT_CMP<NNNN>",  cmProperty::VARIABLE,
+     "Default for CMake Policy CMP<NNNN> when it is otherwise left unset.",
+     "Commands cmake_minimum_required(VERSION) and cmake_policy(VERSION) "
+     "by default leave policies introduced after the given version unset.  "
+     "Set CMAKE_POLICY_DEFAULT_CMP<NNNN> to OLD or NEW to specify the "
+     "default for policy CMP<NNNN>, where <NNNN> is the policy number."
+     "\n"
+     "This variable should not be set by a project in CMake code; "
+     "use cmake_policy(SET) instead.  "
+     "Users running CMake may set this variable in the cache "
+     "(e.g. -DCMAKE_POLICY_DEFAULT_CMP<NNNN>=<OLD|NEW>) "
+     "to set a policy not otherwise set by the project.  "
+     "Set to OLD to quiet a policy warning while using old behavior "
+     "or to NEW to try building the project with new behavior.",
+     false,
+     "Variables That Change Behavior");
+
     cm->DefineProperty
     ("CMAKE_FIND_LIBRARY_PREFIXES",  cmProperty::VARIABLE,
      "Prefixes to prepend when looking for libraries.",

Source/cmPolicies.cxx

@@ -565,9 +565,14 @@ bool cmPolicies::ApplyPolicyVersion(cmMakefile *mf,
         {
         ancientPolicies.push_back(i->first);
         }
-      else if (!mf->SetPolicy(i->second->ID, cmPolicies::WARN))
+      else
         {
-        return false;
+        cmPolicies::PolicyStatus status = cmPolicies::WARN;
+        if(!this->GetPolicyDefault(mf, i->second->IDString, &status) ||
+           !mf->SetPolicy(i->second->ID, status))
+          {
+          return false;
+          }
         }
       }
     else
@@ -591,6 +596,36 @@ bool cmPolicies::ApplyPolicyVersion(cmMakefile *mf,
   return true;
 }
 
+//----------------------------------------------------------------------------
+bool cmPolicies::GetPolicyDefault(cmMakefile* mf, std::string const& policy,
+                                  cmPolicies::PolicyStatus* defaultSetting)
+{
+  std::string defaultVar = "CMAKE_POLICY_DEFAULT_" + policy;
+  std::string defaultValue = mf->GetSafeDefinition(defaultVar.c_str());
+  if(defaultValue == "NEW")
+    {
+    *defaultSetting = cmPolicies::NEW;
+    }
+  else if(defaultValue == "OLD")
+    {
+    *defaultSetting = cmPolicies::OLD;
+    }
+  else if(defaultValue == "")
+    {
+    *defaultSetting = cmPolicies::WARN;
+    }
+  else
+    {
+    cmOStringStream e;
+    e << defaultVar << " has value \"" << defaultValue
+      << "\" but must be \"OLD\", \"NEW\", or \"\" (empty).";
+    mf->IssueMessage(cmake::FATAL_ERROR, e.str().c_str());
+    return false;
+    }
+
+  return true;
+}
+
 bool cmPolicies::GetPolicyID(const char *id, cmPolicies::PolicyID &pid)
 {
   if (!id || strlen(id) < 1)

Source/cmPolicies.h

@@ -102,6 +102,10 @@ public:
   void DiagnoseAncientPolicies(std::vector<PolicyID> const& ancient,
                                unsigned int majorVer, unsigned int minorVer,
                                unsigned int patchVer, cmMakefile* mf);
+
+  bool GetPolicyDefault(cmMakefile* mf, std::string const& policy,
+                        cmPolicies::PolicyStatus* defaultStatus);
+
 };
 
 #endif

Tests/PolicyScope/CMakeLists.txt

@@ -81,6 +81,24 @@ macro1()
 cmake_policy(GET CMP0002 cmp)
 check(CMP0002 "OLD" "${cmp}")
 
+#-----------------------------------------------------------------------------
+# Test CMAKE_POLICY_DEFAULT_CMP<NNNN> variable.
+cmake_policy(PUSH)
+  set(CMAKE_POLICY_DEFAULT_CMP0010 OLD) # ignored
+  set(CMAKE_POLICY_DEFAULT_CMP0012 OLD) # honored
+  set(CMAKE_POLICY_DEFAULT_CMP0013 NEW) # honored
+  set(CMAKE_POLICY_DEFAULT_CMP0014 "")  # noop
+  cmake_policy(VERSION 2.6.3)
+  cmake_policy(GET CMP0010 cmp)
+  check(CMP0010 "NEW" "${cmp}")
+  cmake_policy(GET CMP0012 cmp)
+  check(CMP0012 "OLD" "${cmp}")
+  cmake_policy(GET CMP0013 cmp)
+  check(CMP0013 "NEW" "${cmp}")
+  cmake_policy(GET CMP0014 cmp)
+  check(CMP0014 "" "${cmp}")
+cmake_policy(POP)
+
 #-----------------------------------------------------------------------------
 # Dummy executable so the project can build and run.
 add_executable(PolicyScope main.c)