[clang-cl] Treat inputs as C++ with /E, like MSVC

midl invokes the compiler on .idl files with /E. Before this change, we
would treat unrecognized inputs as object files. Now we pre-process to
stdout as expected. I checked that MSVC defines __cplusplus when invoked
this way, so treating the input as C++ seems like the right thing to do.

After this change, I was able to run midl like this with clang-cl:
$ midl -cpp_cmd clang-cl.exe foo.idl

Things worked for the example IDL file in the Microsoft documentation,
but beyond that, I don't know if this will work well.

Fixes PR40140

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@350072 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 18 insertions, 0 deletions

diff --git a/test/Driver/cl-idl.cpp b/test/Driver/cl-idl.cpp
new file mode 100644
index 0000000000..7843ab3133
--- /dev/null
+++ b/test/Driver/cl-idl.cpp
@@ -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 {};