ENH: Create notion of a 'weak' policy stack entry

A 'weak' poilcy stack entry responds normally to queries.  However,
setting a policy in a weak entry will recursively set the policy in the
next entry too.  This also gives the internal interface to create a weak
entry the option to provide an initial PolicyMap for it.

Source/cmMakefile.cxx

@@ -3666,8 +3666,14 @@ bool cmMakefile::SetPolicy(cmPolicies::PolicyID id,
     return false;
     }
 
-  // Store the setting.
-  this->PolicyStack.back()[id] = status;
+  // Update the policy stack from the top to the top-most strong entry.
+  bool previous_was_weak = true;
+  for(PolicyStackType::reverse_iterator psi = this->PolicyStack.rbegin();
+      previous_was_weak && psi != this->PolicyStack.rend(); ++psi)
+    {
+    (*psi)[id] = status;
+    previous_was_weak = psi->Weak;
+    }
 
   // Special hook for presenting compatibility variable as soon as
   // the user requests it.
@@ -3692,10 +3698,11 @@ bool cmMakefile::SetPolicy(cmPolicies::PolicyID id,
 }
 
 //----------------------------------------------------------------------------
-cmMakefile::PolicyPushPop::PolicyPushPop(cmMakefile* m):
+cmMakefile::PolicyPushPop::PolicyPushPop(cmMakefile* m, bool weak,
+                                         cmPolicies::PolicyMap const& pm):
   Makefile(m), ReportError(true)
 {
-  this->Makefile->PushPolicy();
+  this->Makefile->PushPolicy(weak, pm);
   this->Makefile->PushPolicyBarrier();
 }
 
@@ -3707,10 +3714,10 @@ cmMakefile::PolicyPushPop::~PolicyPushPop()
 }
 
 //----------------------------------------------------------------------------
-void cmMakefile::PushPolicy()
+void cmMakefile::PushPolicy(bool weak, cmPolicies::PolicyMap const& pm)
 {
   // Allocate a new stack entry.
-  this->PolicyStack.push_back(PolicyStackEntry());
+  this->PolicyStack.push_back(PolicyStackEntry(pm, weak));
 }
 
 //----------------------------------------------------------------------------

Source/cmMakefile.h

@@ -349,7 +349,9 @@ public:
   class PolicyPushPop
   {
   public:
-    PolicyPushPop(cmMakefile* m);
+    PolicyPushPop(cmMakefile* m,
+                  bool weak = false,
+                  cmPolicies::PolicyMap const& pm = cmPolicies::PolicyMap());
     ~PolicyPushPop();
     void Quiet() { this->ReportError = false; }
   private:
@@ -943,7 +945,8 @@ private:
   std::map<cmStdString, cmTarget*> ImportedTargets;
 
   // Internal policy stack management.
-  void PushPolicy();
+  void PushPolicy(bool weak = false,
+                  cmPolicies::PolicyMap const& pm = cmPolicies::PolicyMap());
   void PopPolicy();
   void PushPolicyBarrier();
   void PopPolicyBarrier(bool reportError = true);
@@ -955,9 +958,10 @@ private:
   struct PolicyStackEntry: public cmPolicies::PolicyMap
   {
     typedef cmPolicies::PolicyMap derived;
-    PolicyStackEntry(): derived() {}
-    PolicyStackEntry(derived const& d): derived(d) {}
-    PolicyStackEntry(PolicyStackEntry const& r): derived(r) {}
+    PolicyStackEntry(bool w = false): derived(), Weak(w) {}
+    PolicyStackEntry(derived const& d, bool w = false): derived(d), Weak(w) {}
+    PolicyStackEntry(PolicyStackEntry const& r): derived(r), Weak(r.Weak) {}
+    bool Weak;
   };
   typedef std::vector<PolicyStackEntry> PolicyStackType;
   PolicyStackType PolicyStack;