http://bugs.winehq.org/show_bug.cgi?id=13332
--- Comment #3 from Sachin sachin_bhandare@persistent.co.in 2008-05-22 00:04:46 --- Yes, I believe following is a known issue with widl.
$ widl test.idl -I /path/to/wine/include error: encode_type: VT_USERDEFINED - unhandled type 17
However, I am trying to generate only headers and server stubs and it fails with the reported problem.
If you don't generate server stubs (-s option) then it works and generates header file.