MSVC: Do not add /W3 to CMAKE_<LANG>_FLAGS by default

We do not add default warning flags on other compilers, and having
a warning flag in the default flags makes it hard for projects to
customize the warning level.  They need to use string processing
to remove `/W3` from `CMAKE_{C,CXX}_FLAGS`.  Therefore we should
drop it.

However, projects may be using string processing to replace `/W3`
with another flag, so we cannot simply drop it.  Add a policy to
drop it in a compatible way.

Fixes: #18317

7 files changed, 27 insertions, 0 deletions

diff --git a/Tests/RunCMake/CMakeLists.txt b/Tests/RunCMake/CMakeLists.txt
index a8dcb4b..4973e55 100644
--- a/Tests/RunCMake/CMakeLists.txt
+++ b/Tests/RunCMake/CMakeLists.txt
@@ -197,6 +197,7 @@ if(CMAKE_CXX_COMPILER_ID MATCHES "^(Cray|PGI|XL|XLClang)$")
 endif()
 if(MSVC)
   add_RunCMake_test(MSVCRuntimeLibrary)
+  add_RunCMake_test(MSVCWarningFlags)
 endif()
 add_RunCMake_test(ObjectLibrary)
 add_RunCMake_test(ParseImplicitIncludeInfo)
diff --git a/Tests/RunCMake/MSVCWarningFlags/CMP0092-NEW.cmake b/Tests/RunCMake/MSVCWarningFlags/CMP0092-NEW.cmake
new file mode 100644
index 0000000..15c52d2
--- /dev/null
+++ b/Tests/RunCMake/MSVCWarningFlags/CMP0092-NEW.cmake
@@ -0,0 +1,2 @@
+cmake_policy(SET CMP0092 NEW)
+include(CMP0092-common.cmake)
diff --git a/Tests/RunCMake/MSVCWarningFlags/CMP0092-OLD.cmake b/Tests/RunCMake/MSVCWarningFlags/CMP0092-OLD.cmake
new file mode 100644
index 0000000..ea75445
--- /dev/null
+++ b/Tests/RunCMake/MSVCWarningFlags/CMP0092-OLD.cmake
@@ -0,0 +1,2 @@
+cmake_policy(SET CMP0092 OLD)
+include(CMP0092-common.cmake)
diff --git a/Tests/RunCMake/MSVCWarningFlags/CMP0092-WARN.cmake b/Tests/RunCMake/MSVCWarningFlags/CMP0092-WARN.cmake
new file mode 100644
index 0000000..45e183f
--- /dev/null
+++ b/Tests/RunCMake/MSVCWarningFlags/CMP0092-WARN.cmake
@@ -0,0 +1,2 @@
+
+include(CMP0092-common.cmake)
diff --git a/Tests/RunCMake/MSVCWarningFlags/CMP0092-common.cmake b/Tests/RunCMake/MSVCWarningFlags/CMP0092-common.cmake
new file mode 100644
index 0000000..87d7f67
--- /dev/null
+++ b/Tests/RunCMake/MSVCWarningFlags/CMP0092-common.cmake
@@ -0,0 +1,12 @@
+enable_language(C)
+
+cmake_policy(GET CMP0092 cmp0092)
+if(cmp0092 STREQUAL "NEW")
+  if("${CMAKE_C_FLAGS}" MATCHES "([/-]W[0-9])")
+    message(SEND_ERROR "CMAKE_C_FLAGS has '${CMAKE_MATCH_1}' under NEW behavior")
+  endif()
+else()
+  if(NOT " ${CMAKE_C_FLAGS} " MATCHES " /W3 ")
+    message(SEND_ERROR "CMAKE_C_FLAGS does not have '/W3' under OLD behavior")
+  endif()
+endif()
diff --git a/Tests/RunCMake/MSVCWarningFlags/CMakeLists.txt b/Tests/RunCMake/MSVCWarningFlags/CMakeLists.txt
new file mode 100644
index 0000000..3e470a2
--- /dev/null
+++ b/Tests/RunCMake/MSVCWarningFlags/CMakeLists.txt
@@ -0,0 +1,3 @@
+cmake_minimum_required(VERSION 3.14)
+project(${RunCMake_TEST} NONE)
+include(${RunCMake_TEST}.cmake)
diff --git a/Tests/RunCMake/MSVCWarningFlags/RunCMakeTest.cmake b/Tests/RunCMake/MSVCWarningFlags/RunCMakeTest.cmake
new file mode 100644
index 0000000..7ce448d
--- /dev/null
+++ b/Tests/RunCMake/MSVCWarningFlags/RunCMakeTest.cmake
@@ -0,0 +1,5 @@
+include(RunCMake)
+
+run_cmake(CMP0092-WARN)
+run_cmake(CMP0092-OLD)
+run_cmake(CMP0092-NEW)