diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
index 823bfb5cd9b4..f254291b6ecc 100644
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -442,9 +442,6 @@ if (LIBCXX_ENABLE_ASSERTIONS)
   define_if_not(MSVC -D_DEBUG)
 endif()
 
-# Feature flags ===============================================================
-define_if(MSVC -D_CRT_SECURE_NO_WARNINGS)
-
 # Modules flags ===============================================================
 # FIXME The libc++ sources are fundamentally non-modular. They need special
 # versions of the headers in order to provide C++03 and legacy ABI definitions.
diff --git a/libcxx/lib/CMakeLists.txt b/libcxx/lib/CMakeLists.txt
index 53f93aed9b91..f37ac68ebc26 100644
--- a/libcxx/lib/CMakeLists.txt
+++ b/libcxx/lib/CMakeLists.txt
@@ -161,6 +161,28 @@ split_list(LIBCXX_LINK_FLAGS)
 
 # Add a object library that contains the compiled source files.
 add_library(cxx_objects OBJECT ${exclude_from_all} ${LIBCXX_SOURCES} ${LIBCXX_HEADERS})
+if(WIN32 AND NOT MINGW)
+  target_compile_definitions(cxx_objects
+                             PRIVATE
+                               # Ignore the -MSC_VER mismatch, as we may build
+                               # with a different compatibility version.
+                               _ALLOW_MSC_VER_MISMATCH
+                               # Don't check the msvcprt iterator debug levels
+                               # as we will define the iterator types; libc++
+                               # uses a different macro to identify the debug
+                               # level.
+                               _ALLOW_ITERATOR_DEBUG_LEVEL_MISMATCH
+                               # We are building the c++ runtime, don't pull in
+                               # msvcprt.
+                               _CRTBLD
+                               # Don't warn on the use of "deprecated"
+                               # "insecure" functions which are standards
+                               # specified.
+                               _CRT_SECURE_NO_WARNINGS
+                               # Use the ISO conforming behaviour for conversion
+                               # in printf, scanf.
+                               _CRT_STDIO_ISO_WIDE_SPECIFIERS)
+endif()
 
 set_target_properties(cxx_objects
   PROPERTIES