diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -572,7 +572,15 @@
 # Warning flags ===============================================================
 function(cxx_add_warning_flags target)
   target_compile_definitions(${target} PUBLIC -D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-  target_add_compile_flags_if_supported(${target} PRIVATE -Wall -Wextra -W -Wwrite-strings
+  if (NOT MSVC)
+    # Don't add -Wall with clang-cl; in cl.exe mode, -Wall (or /Wall) means
+    # the same as -Weverything in a GCC like driver.
+    target_add_compile_flags_if_supported(${target} PRIVATE -Wall)
+  else()
+    # -W4 is the cl.exe equivalent of -Wall
+    target_add_compile_flags_if_supported(${target} PRIVATE -W4)
+  endif()
+  target_add_compile_flags_if_supported(${target} PRIVATE -Wextra -W -Wwrite-strings
                                                           -Wno-unused-parameter -Wno-long-long
                                                           -Werror=return-type -Wextra-semi)
   if ("${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")