diff --git a/libcxx/src/support/win32/locale_win32.cpp b/libcxx/src/support/win32/locale_win32.cpp
--- a/libcxx/src/support/win32/locale_win32.cpp
+++ b/libcxx/src/support/win32/locale_win32.cpp
@@ -97,7 +97,10 @@
         ret, n, format, loc, ap);
 #else
     __libcpp_locale_guard __current(loc);
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wformat-nonliteral"
     int result = vsnprintf( ret, n, format, ap );
+#pragma clang diagnostic pop
 #endif
     va_end(ap);
     return result;
diff --git a/libcxx/src/support/win32/support.cpp b/libcxx/src/support/win32/support.cpp
--- a/libcxx/src/support/win32/support.cpp
+++ b/libcxx/src/support/win32/support.cpp
@@ -23,7 +23,10 @@
     // Query the count required.
     va_list ap_copy;
     va_copy(ap_copy, ap);
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wformat-nonliteral"
     int count = vsnprintf( NULL, 0, format, ap_copy );
+#pragma clang diagnostic pop
     va_end(ap_copy);
     if (count < 0)
         return count;
@@ -33,7 +36,10 @@
         return -1;
     // If we haven't used exactly what was required, something is wrong.
     // Maybe bug in vsnprintf. Report the error and return.
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wformat-nonliteral"
     if (vsnprintf(p, buffer_size, format, ap) != count) {
+#pragma clang diagnostic pop
         free(p);
         return -1;
     }
diff --git a/libcxx/utils/ci/run-buildbot b/libcxx/utils/ci/run-buildbot
--- a/libcxx/utils/ci/run-buildbot
+++ b/libcxx/utils/ci/run-buildbot
@@ -590,6 +590,7 @@
     # further back in PATH. By calling the compiler with an explicit
     # triple prefix, we use the one that is bundled with a mingw sysroot.
     generate-cmake \
+          -DLIBCXX_ENABLE_WERROR=ON \
           -DCMAKE_C_COMPILER=x86_64-w64-mingw32-clang \
           -DCMAKE_CXX_COMPILER=x86_64-w64-mingw32-clang++ \
           -DLIBCXX_TEST_CONFIG="llvm-libc++-mingw.cfg.in" \
@@ -600,6 +601,7 @@
 mingw-static)
     clean
     generate-cmake \
+          -DLIBCXX_ENABLE_WERROR=ON \
           -DCMAKE_C_COMPILER=x86_64-w64-mingw32-clang \
           -DCMAKE_CXX_COMPILER=x86_64-w64-mingw32-clang++ \
           -DLIBCXX_TEST_CONFIG="llvm-libc++-mingw.cfg.in" \