Deprecate compatibility with CMake versions older than 2.8.12

Issue a deprecation warning on calls to `cmake_minimum_required` or
`cmake_policy` that set policies based on versions older than 2.8.12.
Note that the effective policy version includes `...<max>` treatment.
This is important in combination with commit ca24b70d31 (Export: Specify
a policy range in exported files, 2020-05-16, v3.18.0-rc1~133^2).

3 files changed, 41 insertions, 7 deletions

diff --git a/Source/cmMakefile.cxx b/Source/cmMakefile.cxx
index 3c1cba2..891e290 100644
--- a/Source/cmMakefile.cxx
+++ b/Source/cmMakefile.cxx
@@ -1659,7 +1659,8 @@ void cmMakefile::Configure()
         this->SetCheckCMP0000(true);
 
         // Implicitly set the version for the user.
-        this->SetPolicyVersion("2.4", std::string());
+        cmPolicies::ApplyPolicyVersion(this, 2, 4, 0,
+                                       cmPolicies::WarnCompat::Off);
       }
     }
     bool hasProject = false;
@@ -4621,7 +4622,8 @@ void cmMakefile::PopSnapshot(bool reportError)
 bool cmMakefile::SetPolicyVersion(std::string const& version_min,
                                   std::string const& version_max)
 {
-  return cmPolicies::ApplyPolicyVersion(this, version_min, version_max);
+  return cmPolicies::ApplyPolicyVersion(this, version_min, version_max,
+                                        cmPolicies::WarnCompat::On);
 }
 
 bool cmMakefile::HasCMP0054AlreadyBeenReported(
diff --git a/Source/cmPolicies.cxx b/Source/cmPolicies.cxx
index dea3f8a..01e8c04 100644
--- a/Source/cmPolicies.cxx
+++ b/Source/cmPolicies.cxx
@@ -7,9 +7,11 @@
 #include <sstream>
 #include <vector>
 
+#include "cmListFileCache.h"
 #include "cmMakefile.h"
 #include "cmMessageType.h"
 #include "cmState.h"
+#include "cmStateSnapshot.h"
 #include "cmStateTypes.h"
 #include "cmStringAlgorithms.h"
 #include "cmSystemTools.h"
@@ -157,7 +159,8 @@ static bool GetPolicyDefault(cmMakefile* mf, std::string const& policy,
 
 bool cmPolicies::ApplyPolicyVersion(cmMakefile* mf,
                                     std::string const& version_min,
-                                    std::string const& version_max)
+                                    std::string const& version_max,
+                                    WarnCompat warnCompat)
 {
   // Parse components of the minimum version.
   unsigned int minMajor = 2;
@@ -244,13 +247,34 @@ bool cmPolicies::ApplyPolicyVersion(cmMakefile* mf,
     polPatch = maxPatch;
   }
 
-  return cmPolicies::ApplyPolicyVersion(mf, polMajor, polMinor, polPatch);
+  return cmPolicies::ApplyPolicyVersion(mf, polMajor, polMinor, polPatch,
+                                        warnCompat);
 }
 
 bool cmPolicies::ApplyPolicyVersion(cmMakefile* mf, unsigned int majorVer,
                                     unsigned int minorVer,
-                                    unsigned int patchVer)
+                                    unsigned int patchVer,
+                                    WarnCompat warnCompat)
 {
+  // Warn about policy versions for which support will be removed.
+  if (warnCompat == WarnCompat::On &&
+      (majorVer < 2 || (majorVer == 2 && minorVer < 8) ||
+       (majorVer == 2 && minorVer == 8 && patchVer < 12)) &&
+      // Avoid warning on calls generated by install(EXPORT)
+      // in CMake versions prior to 3.18.
+      !(majorVer == 2 && minorVer == 6 && patchVer == 0 &&
+        mf->GetStateSnapshot().CanPopPolicyScope() &&
+        cmSystemTools::Strucmp(mf->GetBacktrace().Top().Name.c_str(),
+                               "cmake_policy") == 0)) {
+    mf->IssueMessage(
+      MessageType::DEPRECATION_WARNING,
+      "Compatibility with CMake < 2.8.12 will be removed from "
+      "a future version of CMake.\n"
+      "Update the VERSION argument <min> value or use a ...<max> suffix "
+      "to tell CMake that the project does not need compatibility with "
+      "older versions.");
+  }
+
   // now loop over all the policies and set them as appropriate
   std::vector<cmPolicies::PolicyID> ancientPolicies;
   for (PolicyID pid = cmPolicies::CMP0000; pid != cmPolicies::CMPCOUNT;
diff --git a/Source/cmPolicies.h b/Source/cmPolicies.h
index 49dadc7..bba8b03 100644
--- a/Source/cmPolicies.h
+++ b/Source/cmPolicies.h
@@ -399,12 +399,20 @@ public:
   //! Get the default status for a policy
   static cmPolicies::PolicyStatus GetPolicyStatus(cmPolicies::PolicyID id);
 
+  enum class WarnCompat
+  {
+    Off,
+    On
+  };
+
   //! Set a policy level for this listfile
   static bool ApplyPolicyVersion(cmMakefile* mf,
                                  std::string const& version_min,
-                                 std::string const& version_max);
+                                 std::string const& version_max,
+                                 WarnCompat warnCompat);
   static bool ApplyPolicyVersion(cmMakefile* mf, unsigned int majorVer,
-                                 unsigned int minorVer, unsigned int patchVer);
+                                 unsigned int minorVer, unsigned int patchVer,
+                                 WarnCompat warnCompat);
 
   //! return a warning string for a given policy
   static std::string GetPolicyWarning(cmPolicies::PolicyID id);