7 files changed, 54 insertions, 9 deletions
diff --git a/Help/manual/cmake-variables.7.rst b/Help/manual/cmake-variables.7.rst
index d9d7a0c..f4b9666 100644
--- a/Help/manual/cmake-variables.7.rst
+++ b/Help/manual/cmake-variables.7.rst
@@ -131,6 +131,7 @@ Variables that Change Behavior
    /variable/CMAKE_MODULE_PATH
    /variable/CMAKE_NOT_USING_CONFIG_FLAGS
    /variable/CMAKE_POLICY_DEFAULT_CMPNNNN
+   /variable/CMAKE_POLICY_WARNING_CMPNNNN
    /variable/CMAKE_PREFIX_PATH
    /variable/CMAKE_PROGRAM_PATH
    /variable/CMAKE_PROJECT_PROJECT-NAME_INCLUDE
diff --git a/Help/policy/CMP0025.rst b/Help/policy/CMP0025.rst
index 5d63efa..8d19edf 100644
--- a/Help/policy/CMP0025.rst
+++ b/Help/policy/CMP0025.rst
@@ -18,6 +18,10 @@ to the invocation of either command.
 The OLD behavior for this policy is to use compiler id ``Clang``.  The
 NEW behavior for this policy is to use compiler id ``AppleClang``.
 
-This policy was introduced in CMake version 3.0.  CMake version
-|release| warns when the policy is not set and uses OLD behavior.  Use
-the :command:`cmake_policy` command to set it to OLD or NEW explicitly.
+This policy was introduced in CMake version 3.0.  Use the
+:command:`cmake_policy` command to set this policy to OLD or NEW explicitly.
+Unlike most policies, CMake version |release| does *not* warn
+by default when this policy is not set and simply uses OLD behavior.
+See documentation of the
+:variable:`CMAKE_POLICY_WARNING_CMP0025 <CMAKE_POLICY_WARNING_CMP<NNNN>>`
+variable to control the warning.
diff --git a/Help/policy/CMP0047.rst b/Help/policy/CMP0047.rst
index 896d702..26ae439 100644
--- a/Help/policy/CMP0047.rst
+++ b/Help/policy/CMP0047.rst
@@ -19,7 +19,10 @@ The OLD behavior for this policy is to use the ``GNU`` compiler id
 for the qcc and QCC compiler drivers. The NEW behavior for this policy
 is to use the ``QCC`` compiler id for those drivers.
 
-This policy was introduced in CMake version 3.0.
-CMake version |release| warns when the policy is not set and uses
-OLD behavior.  Use the :command:`cmake_policy` command to set it to
-OLD or NEW explicitly.
+This policy was introduced in CMake version 3.0.  Use the
+:command:`cmake_policy` command to set this policy to OLD or NEW explicitly.
+Unlike most policies, CMake version |release| does *not* warn
+by default when this policy is not set and simply uses OLD behavior.
+See documentation of the
+:variable:`CMAKE_POLICY_WARNING_CMP0047 <CMAKE_POLICY_WARNING_CMP<NNNN>>`
+variable to control the warning.
diff --git a/Help/variable/CMAKE_POLICY_WARNING_CMPNNNN.rst b/Help/variable/CMAKE_POLICY_WARNING_CMPNNNN.rst
new file mode 100644
index 0000000..b563aea
--- /dev/null
+++ b/Help/variable/CMAKE_POLICY_WARNING_CMPNNNN.rst
@@ -0,0 +1,17 @@
+CMAKE_POLICY_WARNING_CMP<NNNN>
+------------------------------
+
+Explicitly enable or disable the warning when CMake Policy ``CMP<NNNN>``
+is not set.  This is meaningful only for the few policies that do not
+warn by default:
+
+* ``CMAKE_POLICY_WARNING_CMP0025`` controls the warning for
+  policy :policy:`CMP0025`.
+* ``CMAKE_POLICY_WARNING_CMP0047`` controls the warning for
+  policy :policy:`CMP0047`.
+
+This variable should not be set by a project in CMake code.  Project
+developers running CMake may set this variable in their cache to
+enable the warning (e.g. ``-DCMAKE_POLICY_WARNING_CMP<NNNN>=ON``).
+Alternatively, running :manual:`cmake(1)` with the ``--debug-output``
+or ``--trace`` option will also enable the warning.
diff --git a/Source/cmGlobalGenerator.cxx b/Source/cmGlobalGenerator.cxx
index 4f3328d..beb10da 100644
--- a/Source/cmGlobalGenerator.cxx
+++ b/Source/cmGlobalGenerator.cxx
@@ -759,7 +759,8 @@ void cmGlobalGenerator::CheckCompilerIdCompatibility(cmMakefile* mf,
     switch(mf->GetPolicyStatus(cmPolicies::CMP0025))
       {
       case cmPolicies::WARN:
-        if(!this->CMakeInstance->GetIsInTryCompile())
+        if(!this->CMakeInstance->GetIsInTryCompile() &&
+           mf->PolicyOptionalWarningEnabled("CMAKE_POLICY_WARNING_CMP0025"))
           {
           cmOStringStream w;
           w << policies->GetPolicyWarning(cmPolicies::CMP0025) << "\n"
@@ -790,7 +791,8 @@ void cmGlobalGenerator::CheckCompilerIdCompatibility(cmMakefile* mf,
     switch(mf->GetPolicyStatus(cmPolicies::CMP0047))
       {
       case cmPolicies::WARN:
-        if(!this->CMakeInstance->GetIsInTryCompile())
+        if(!this->CMakeInstance->GetIsInTryCompile() &&
+           mf->PolicyOptionalWarningEnabled("CMAKE_POLICY_WARNING_CMP0047"))
           {
           cmOStringStream w;
           w << policies->GetPolicyWarning(cmPolicies::CMP0047) << "\n"
diff --git a/Source/cmMakefile.cxx b/Source/cmMakefile.cxx
index 10137ec..b195943 100644
--- a/Source/cmMakefile.cxx
+++ b/Source/cmMakefile.cxx
@@ -4350,6 +4350,22 @@ cmMakefile::GetPolicyStatusInternal(cmPolicies::PolicyID id) const
   return this->GetPolicies()->GetPolicyStatus(id);
 }
 
+//----------------------------------------------------------------------------
+bool cmMakefile::PolicyOptionalWarningEnabled(std::string const& var)
+{
+  // Check for an explicit CMAKE_POLICY_WARNING_CMP<NNNN> setting.
+  if(!var.empty())
+    {
+    if(const char* val = this->GetDefinition(var.c_str()))
+      {
+      return cmSystemTools::IsOn(val);
+      }
+    }
+  // Enable optional policy warnings with --debug-output or --trace.
+  cmake* cm = this->GetCMakeInstance();
+  return cm->GetDebugOutput() || cm->GetTrace();
+}
+
 bool cmMakefile::SetPolicy(const char *id,
                            cmPolicies::PolicyStatus status)
 {
diff --git a/Source/cmMakefile.h b/Source/cmMakefile.h
index 45f3b9f..f00fd20 100644
--- a/Source/cmMakefile.h
+++ b/Source/cmMakefile.h
@@ -875,6 +875,8 @@ public:
   std::set<cmStdString> const & GetSystemIncludeDirectories() const
     { return this->SystemIncludeDirectories; }
 
+  bool PolicyOptionalWarningEnabled(std::string const& var);
+
 protected:
   // add link libraries and directories to the target
   void AddGlobalLinkInformation(const char* name, cmTarget& target);