MSVC: Do not add /GR to CMAKE_CXX_FLAGS by default

The `/GR` flag has been on by default since MSVC cl 14.0 from VS 2005.
Remove it from the default flags to make it easier for projects to pass
`/GR-` themselves to turn it off.

Projects may be using string processing to replace `/GR` with another
flag, so we cannot simply drop it.  Add a policy to drop it in a
compatible way.

Fixes: #21428

Help/manual/cmake-policies.7.rst

@@ -57,6 +57,7 @@ Policies Introduced by CMake 3.20
 .. toctree::
    :maxdepth: 1
 
+   CMP0117: MSVC RTTI flag /GR is not added to CMAKE_CXX_FLAGS by default. </policy/CMP0117>
    CMP0116: Ninja generators transform DEPFILEs from add_custom_command(). </policy/CMP0116>
    CMP0115: Source file extensions must be explicit. </policy/CMP0115>
 

Help/policy/CMP0117.rst

@@ -0,0 +1,43 @@
+CMP0117
+-------
+
+.. versionadded:: 3.20
+
+MSVC RTTI flag ``/GR`` is not added to
+:variable:`CMAKE_CXX_FLAGS <CMAKE_<LANG>_FLAGS>` by default.
+
+When using MSVC-like compilers in CMake 3.19 and below, the RTTI flag
+``/GR`` is added to :variable:`CMAKE_CXX_FLAGS <CMAKE_<LANG>_FLAGS>` by
+default.  This behavior is left from support for MSVC versions from Visual
+Studio 2003 and below that did not enable RTTI by default.  It is no longer
+necessary.  Furthermore, it is problematic for projects that want to change
+to ``/GR-`` programmatically.  In particular, it requires string editing of
+the :variable:`CMAKE_CXX_FLAGS <CMAKE_<LANG>_FLAGS>` variable with knowledge
+of the CMake builtin default so it can be replaced.
+
+CMake 3.20 and above prefer to leave out ``/GR`` from the value of
+:variable:`CMAKE_CXX_FLAGS <CMAKE_<LANG>_FLAGS>` by default.
+
+This policy provides compatibility with projects that have not been updated
+to expect the lack of the ``/GR`` flag.  The policy setting takes effect as
+of the first :command:`project` or :command:`enable_language` command that
+initializes :variable:`CMAKE_CXX_FLAGS <CMAKE_<LANG>_FLAGS>`.
+
+.. note::
+
+  Once the policy has taken effect at the top of a project for a given
+  language, that choice must be used throughout the tree for that language.
+  In projects that have nested projects in subdirectories, be sure to
+  convert everything together.
+
+The ``OLD`` behavior for this policy is to place the MSVC ``/GR`` flag in the
+default :variable:`CMAKE_CXX_FLAGS <CMAKE_<LANG>_FLAGS>` cache entry.  The
+``NEW`` behavior for this policy is to *not* place the MSVC ``/GR`` flag in
+the default cache entry.
+
+This policy was introduced in CMake version 3.20.  Use the
+:command:`cmake_policy` command to set it to ``OLD`` or ``NEW`` explicitly.
+Unlike many policies, CMake version |release| does *not* warn
+when this policy is not set and simply uses ``OLD`` behavior.
+
+.. include:: DEPRECATED.txt

Help/release/dev/msvc-no-GR.rst

@@ -0,0 +1,7 @@
+msvc-no-GR
+----------
+
+* With MSVC-like compilers the value of
+  :variable:`CMAKE_CXX_FLAGS <CMAKE_<LANG>_FLAGS>` no longer contains
+  the ``/GR`` flag for runtime type information by default.
+  See policy :policy:`CMP0117`.

Modules/Platform/Windows-MSVC.cmake

@@ -163,7 +163,13 @@ foreach(lang C CXX)
   endif()
 endforeach()
 
-set(_GR " /GR")
+cmake_policy(GET CMP0117 __WINDOWS_MSVC_CMP0117)
+if(__WINDOWS_MSVC_CMP0117 STREQUAL "NEW")
+  set(_GR "")
+else()
+  set(_GR " /GR")
+endif()
+unset(__WINDOWS_MSVC_CMP0117)
 
 if(WINCE)
   foreach(lang C CXX)

Source/cmPolicies.h

@@ -345,6 +345,9 @@ class cmMakefile;
          0, cmPolicies::WARN)                                                 \
   SELECT(POLICY, CMP0116,                                                     \
          "Ninja generators transform DEPFILEs from add_custom_command().", 3, \
+         20, 0, cmPolicies::WARN)                                             \
+  SELECT(POLICY, CMP0117,                                                     \
+         "MSVC RTTI flag /GR is not added to CMAKE_CXX_FLAGS by default.", 3, \
          20, 0, cmPolicies::WARN)
 
 #define CM_SELECT_ID(F, A1, A2, A3, A4, A5, A6) F(A1)

Tests/RunCMake/CMakeLists.txt

@@ -262,6 +262,7 @@ if(CMAKE_CXX_COMPILER_ID MATCHES "^(Cray|PGI|XL|XLClang)$")
 endif()
 if(MSVC)
   add_RunCMake_test(MSVCRuntimeLibrary)
+  add_RunCMake_test(MSVCRuntimeTypeInfo)
   add_RunCMake_test(MSVCWarningFlags)
 endif()
 add_RunCMake_test(ObjectLibrary)

Tests/RunCMake/MSVCRuntimeTypeInfo/CMP0117-NEW.cmake

@@ -0,0 +1,2 @@
+cmake_policy(SET CMP0117 NEW)
+include(CMP0117-common.cmake)

Tests/RunCMake/MSVCRuntimeTypeInfo/CMP0117-OLD.cmake

@@ -0,0 +1,2 @@
+cmake_policy(SET CMP0117 OLD)
+include(CMP0117-common.cmake)

Tests/RunCMake/MSVCRuntimeTypeInfo/CMP0117-WARN.cmake

@@ -0,0 +1,2 @@
+
+include(CMP0117-common.cmake)

Tests/RunCMake/MSVCRuntimeTypeInfo/CMP0117-common.cmake

@@ -0,0 +1,12 @@
+enable_language(CXX)
+
+cmake_policy(GET CMP0117 cmp0117)
+if(cmp0117 STREQUAL "NEW")
+  if(" ${CMAKE_CXX_FLAGS} " MATCHES " ([/-]GR) ")
+    message(SEND_ERROR "CMAKE_CXX_FLAGS has '${CMAKE_MATCH_1}' under NEW behavior")
+  endif()
+else()
+  if(NOT " ${CMAKE_CXX_FLAGS} " MATCHES " /GR ")
+    message(SEND_ERROR "CMAKE_CXX_FLAGS does not have '/GR' under OLD behavior")
+  endif()
+endif()

Tests/RunCMake/MSVCRuntimeTypeInfo/CMakeLists.txt

@@ -0,0 +1,3 @@
+cmake_minimum_required(VERSION 3.14)
+project(${RunCMake_TEST} NONE)
+include(${RunCMake_TEST}.cmake)

Tests/RunCMake/MSVCRuntimeTypeInfo/RunCMakeTest.cmake

@@ -0,0 +1,5 @@
+include(RunCMake)
+
+run_cmake(CMP0117-WARN)
+run_cmake(CMP0117-OLD)
+run_cmake(CMP0117-NEW)