Index: libcxx/CMakeLists.txt
===================================================================
--- libcxx/CMakeLists.txt
+++ libcxx/CMakeLists.txt
@@ -93,7 +93,11 @@
 option(LIBCXX_ENABLE_STATIC "Build libc++ as a static library." ON)
 option(LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY "Build libc++experimental.a" ON)
 set(ENABLE_FILESYSTEM_DEFAULT ON)
-if (WIN32)
+if (WIN32 AND NOT MINGW)
+  # Filesystem is buildable for windows, but it requires __int128 helper
+  # functions, that currently are provided by libgcc or compiler_rt builtins.
+  # These are available in MinGW environments, but not currently in MSVC
+  # environments.
   set(ENABLE_FILESYSTEM_DEFAULT OFF)
 endif()
 option(LIBCXX_ENABLE_FILESYSTEM "Build filesystem as part of the main libc++ library"