cmPolicies: Simplify representation of removed policies

A policy that can be set defaults to `WARN` internally.
A policy that has been removed always uses `NEW`.
The `REQUIRED_IF_USED` and `REQUIRED_ALWAYS` states
are not needed.

Source/cmMakefile.cxx

@@ -4629,10 +4629,9 @@ bool cmMakefile::SetPolicy(const char* id, cmPolicies::PolicyStatus status)
 bool cmMakefile::SetPolicy(cmPolicies::PolicyID id,
                            cmPolicies::PolicyStatus status)
 {
-  // A REQUIRED_ALWAYS policy may be set only to NEW.
-  if (status != cmPolicies::NEW &&
-      cmPolicies::GetPolicyStatus(id) == cmPolicies::REQUIRED_ALWAYS) {
-    std::string msg = cmPolicies::GetRequiredAlwaysPolicyError(id);
+  // A removed policy may be set only to NEW.
+  if (cmPolicies::IsRemoved(id) && status != cmPolicies::NEW) {
+    std::string msg = cmPolicies::GetRemovedPolicyError(id);
     this->IssueMessage(MessageType::FATAL_ERROR, msg);
     return false;
   }

Source/cmPolicies.cxx

@@ -57,6 +57,10 @@ static bool stringToId(const char* input, cmPolicies::PolicyID& pid)
 #define CM_FOR_EACH_POLICY_ID_DOC(POLICY)                                     \
   CM_FOR_EACH_POLICY_TABLE(POLICY, CM_SELECT_ID_DOC)
 
+#define CM_SELECT_ID_STATUS(F, A1, A2, A3, A4, A5, A6) F(A1, A6)
+#define CM_FOR_EACH_POLICY_ID_STATUS(POLICY)                                  \
+  CM_FOR_EACH_POLICY_TABLE(POLICY, CM_SELECT_ID_STATUS)
+
 static const char* idToString(cmPolicies::PolicyID id)
 {
   switch (id) {
@@ -119,6 +123,23 @@ static const char* idToShortDescription(cmPolicies::PolicyID id)
   return nullptr;
 }
 
+namespace {
+cmPolicies::PolicyStatus idToStatus(cmPolicies::PolicyID id)
+{
+  switch (id) {
+#define POLICY_CASE(ID, STATUS)                                               \
+  case cmPolicies::ID:                                                        \
+    return cmPolicies::STATUS;
+    // NOLINTNEXTLINE(bugprone-branch-clone)
+    CM_FOR_EACH_POLICY_ID_STATUS(POLICY_CASE)
+#undef POLICY_CASE
+    case cmPolicies::CMPCOUNT:
+      break;
+  }
+  return cmPolicies::WARN;
+}
+}
+
 static void DiagnoseAncientPolicies(
   std::vector<cmPolicies::PolicyID> const& ancient, unsigned int majorVer,
   unsigned int minorVer, unsigned int patchVer, cmMakefile* mf)
@@ -284,7 +305,7 @@ bool cmPolicies::ApplyPolicyVersion(cmMakefile* mf, unsigned int majorVer,
   for (PolicyID pid = cmPolicies::CMP0000; pid != cmPolicies::CMPCOUNT;
        pid = static_cast<PolicyID>(pid + 1)) {
     if (isPolicyNewerThan(pid, majorVer, minorVer, patchVer)) {
-      if (cmPolicies::GetPolicyStatus(pid) == cmPolicies::REQUIRED_ALWAYS) {
+      if (cmPolicies::IsRemoved(pid)) {
         ancientPolicies.push_back(pid);
       } else {
         cmPolicies::PolicyStatus status = cmPolicies::WARN;
@@ -377,14 +398,12 @@ std::string cmPolicies::GetRequiredPolicyError(cmPolicies::PolicyID id)
     "Run \"cmake --help-command cmake_policy\" for more information.");
 }
 
-//! Get the default status for a policy
-cmPolicies::PolicyStatus cmPolicies::GetPolicyStatus(
-  cmPolicies::PolicyID /*unused*/)
+bool cmPolicies::IsRemoved(cmPolicies::PolicyID id)
 {
-  return cmPolicies::WARN;
+  return idToStatus(id) == cmPolicies::NEW;
 }
 
-std::string cmPolicies::GetRequiredAlwaysPolicyError(cmPolicies::PolicyID id)
+std::string cmPolicies::GetRemovedPolicyError(cmPolicies::PolicyID id)
 {
   std::string pid = idToString(id);
   return cmStrCat(

Source/cmPolicies.h

@@ -9,6 +9,10 @@
 
 class cmMakefile;
 
+// Each policy has a short description, the version of CMake in which
+// it was introduced, and its default status.  The status is WARN for
+// existing policies and NEW for removed policies.
+
 #define CM_FOR_EACH_POLICY_TABLE(POLICY, SELECT)                              \
   SELECT(POLICY, CMP0000,                                                     \
          "A minimum required CMake version must be specified.", 2, 6, 0,      \
@@ -634,8 +638,8 @@ public:
   //! convert a string policy ID into a number
   static bool GetPolicyID(const char* id, /* out */ cmPolicies::PolicyID& pid);
 
-  //! Get the default status for a policy
-  static cmPolicies::PolicyStatus GetPolicyStatus(cmPolicies::PolicyID id);
+  //! Return whether a policy has been removed.
+  static bool IsRemoved(cmPolicies::PolicyID id);
 
   enum class WarnCompat
   {
@@ -660,7 +664,7 @@ public:
   static std::string GetRequiredPolicyError(cmPolicies::PolicyID id);
 
   //! return an error string for when a required policy is unspecified
-  static std::string GetRequiredAlwaysPolicyError(cmPolicies::PolicyID id);
+  static std::string GetRemovedPolicyError(cmPolicies::PolicyID id);
 
   /** Represent a set of policy values.  */
   struct PolicyMap

Source/cmStateSnapshot.cxx

@@ -165,13 +165,12 @@ void cmStateSnapshot::SetPolicy(cmPolicies::PolicyID id,
 cmPolicies::PolicyStatus cmStateSnapshot::GetPolicy(cmPolicies::PolicyID id,
                                                     bool parent_scope) const
 {
-  cmPolicies::PolicyStatus status = cmPolicies::GetPolicyStatus(id);
-
-  if (status == cmPolicies::REQUIRED_ALWAYS ||
-      status == cmPolicies::REQUIRED_IF_USED) {
-    return status;
+  if (cmPolicies::IsRemoved(id)) {
+    return cmPolicies::NEW;
   }
 
+  cmPolicies::PolicyStatus status = cmPolicies::WARN;
+
   cmLinkedTree<cmStateDetail::BuildsystemDirectoryStateType>::iterator dir =
     this->Position->BuildSystemDirectory;