--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/clang/test/Driver/cl-idl.cpp	Thu Feb 13 15:10:13 2020 +0900
@@ -0,0 +1,18 @@
+// Note: %s must be preceded by --, otherwise it may be interpreted as a
+// command-line option, e.g. on Mac where %s is commonly under /Users.
+
+// Test that 'clang-cl /E' treats inputs as C++ if the extension is
+// unrecognized. midl relies on this. See PR40140.
+
+// Use a plain .cpp extension first.
+// RUN: %clang_cl /E -- %s | FileCheck %s
+
+// Copy to use .idl as the extension.
+// RUN: cp %s %t.idl
+// RUN: %clang_cl /E -- %t.idl | FileCheck %s
+
+#ifdef __cplusplus
+struct IsCPlusPlus {};
+#endif
+
+// CHECK: struct IsCPlusPlus {};