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).

1 files changed, 27 insertions, 3 deletions

diff --git a/Source/cmPolicies.cxx b/Source/cmPolicies.cxx
index dea3f8a356..01e8c047a0 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;