Add deprecation warnings for policies CMP0036 and below

The OLD behaviors of all policies are deprecated, but only by
documentation.  Add an explicit deprecation diagnostic for some policies
to encourage projects to port away from setting policies to OLD.

3 files changed, 24 insertions, 0 deletions

diff --git a/Source/cmMakefile.cxx b/Source/cmMakefile.cxx
index 4ed76c7..b8a5293 100644
--- a/Source/cmMakefile.cxx
+++ b/Source/cmMakefile.cxx
@@ -4002,6 +4002,13 @@ bool cmMakefile::SetPolicy(cmPolicies::PolicyID id,
     return false;
   }
 
+  // Deprecate old policies, especially those that require a lot
+  // of code to maintain the old behavior.
+  if (status == cmPolicies::OLD && id <= cmPolicies::CMP0036) {
+    this->IssueMessage(cmake::DEPRECATION_WARNING,
+                       cmPolicies::GetPolicyDeprecatedWarning(id));
+  }
+
   this->StateSnapshot.SetPolicy(id, status);
   return true;
 }
diff --git a/Source/cmPolicies.cxx b/Source/cmPolicies.cxx
index 837557b..da3edd4 100644
--- a/Source/cmPolicies.cxx
+++ b/Source/cmPolicies.cxx
@@ -275,6 +275,22 @@ std::string cmPolicies::GetPolicyWarning(cmPolicies::PolicyID id)
   return msg.str();
 }
 
+std::string cmPolicies::GetPolicyDeprecatedWarning(cmPolicies::PolicyID id)
+{
+  std::ostringstream msg;
+  /* clang-format off */
+  msg <<
+    "The OLD behavior for policy " << idToString(id) << " "
+    "will be removed from a future version of CMake.\n"
+    "The cmake-policies(7) manual explains that the OLD behaviors of all "
+    "policies are deprecated and that a policy should be set to OLD only "
+    "under specific short-term circumstances.  Projects should be ported "
+    "to the NEW behavior and not rely on setting a policy to OLD."
+    ;
+  /* clang-format on */
+  return msg.str();
+}
+
 ///! return an error string for when a required policy is unspecified
 std::string cmPolicies::GetRequiredPolicyError(cmPolicies::PolicyID id)
 {
diff --git a/Source/cmPolicies.h b/Source/cmPolicies.h
index 120beb6..69cbc18 100644
--- a/Source/cmPolicies.h
+++ b/Source/cmPolicies.h
@@ -279,6 +279,7 @@ public:
 
   ///! return a warning string for a given policy
   static std::string GetPolicyWarning(cmPolicies::PolicyID id);
+  static std::string GetPolicyDeprecatedWarning(cmPolicies::PolicyID id);
 
   ///! return an error string for when a required policy is unspecified
   static std::string GetRequiredPolicyError(cmPolicies::PolicyID id);