How could HRESULT appear in an MIDL file?

I am developing some COM interfaces with IDL files. Some interface methods return HRESULT, but I have checked the MIDL language reference on MSDN, there's not a clue of HRESULT. So where could I find the official definition of this data type?

Update

Thanks to Shog9, I found it in wtypes.idl. I paste it here for other's view:

    ...
    cpp_quote("#ifndef _HRESULT_DEFINED")
    cpp_quote("#define _HRESULT_DEFINED")
    #if defined(_STRICT_HRESULT)
    typedef struct _HRESULT_STRUCT {
            DWORD Data1;
    } HRESULT_STRUCT, *PHRESULT_STRUCT;
    typedef PHRESULT_STRUCT HRESULT;
    #else // defined(_STRICT_HRESULT)
    cpp_quote("#ifdef __midl")
    typedef LONG HRESULT;
    ...

However, when I use the DWORD or LONG explicitly in my IDL files, the MIDL compiler will report an error saying:

"error MIDL2269: procedures in an object interface must return an HRESUL开发者_Go百科T" 

Kind of ridiculous...

Any practical .idl file should start with

import "oaidl.idl";
import "ocidl.idl";

Which declares essential types. Like HRESULT and VARIANT. Etcetera.

It's actually just an alias for DWORD (which is an alias for unsigned long). It's #defined for MIDL in wtypes.idl, which as Hans notes is brought in by the standard import mechanism.

The purpose of HRESULT is to represent return codes in a consistently recognizable manner - success and failure can be identified regardless of the source or other information encoded in the value, and certain system-defined values are given special meaning when coupled with standard interfaces. See MSDN for details.