oleaut32: ITypeInfo_fnInvoke behavior on PARAMFLAG_FOPT without PARAMFLAG_FHASDEFAULT

In a tax app written in Visual Basic/ADO, I found the following problem:

In dlls/oleaut32/typelib.c, in function ITypeInfo_fnInvoke(), the
following code is found (around line 5569).

VARIANTARG *missing_arg = INVBUF_GET_MISSING_ARG_ARRAY(buffer,
func_desc->cParams);

V_VT(arg) = VT_VARIANT | VT_BYREF;
V_VARIANTREF(arg) = &missing_arg[i];
V_VT(V_VARIANTREF(arg)) = VT_ERROR;
V_ERROR(V_VARIANTREF(arg)) = DISP_E_PARAMNOTFOUND;

This code runs when a parameter is found that has the PARAMFLAG_FIN and
PARAMFLAG_FOPT flags, but not the PARAMFLAG_FHASDEFAULT flag. As far as I
understand, this code is supposed to supply a variant that might hold an
output value. However, in the Append method of the Fields object within
the Recordset object of Microsoft ADODB 2.7, the following declaration is
found:

trace:ole:ITypeInfo_fnInvoke invoking:
L"Append"(5)
    parm0: L"Name"
    parm1: L"Type"
    parm2: L"DefinedSize"
    parm3: L"Attrib"
    parm4: L"FieldValue"
memid is 00000003
Param 0:
    	tdesc.vartype 8 (VT_BSTR)
    	u.paramdesc.wParamFlags PARAMFLAG_FIN
    	u.paramdesc.lpex (nil)
Param 1:
    	tdesc.vartype 29 (VT_USERDEFINED ref = 2bc)
    	u.paramdesc.wParamFlags PARAMFLAG_FIN
    	u.paramdesc.lpex (nil)
Param 2:
    	tdesc.vartype 29 (VT_USERDEFINED ref = 1c84)
    	u.paramdesc.wParamFlags PARAMFLAG_FIN PARAMFLAG_FOPT PARAMFLAG_FHASDEFAULT
    	u.paramdesc.lpex 0x199868
Param 3:
    	tdesc.vartype 29 (VT_USERDEFINED ref = 320)
    	u.paramdesc.wParamFlags PARAMFLAG_FIN PARAMFLAG_FOPT PARAMFLAG_FHASDEFAULT
    	u.paramdesc.lpex 0x1998d0
Param 4:
    	tdesc.vartype 12 (VT_VARIANT)
    	u.paramdesc.wParamFlags PARAMFLAG_FIN PARAMFLAG_FOPT
    	u.paramdesc.lpex (nil)
    funckind: 1 (pure virtual)
    invkind: 1 (func)
    callconv: 4 (stdcall)
    oVft: 52
    cParamsOpt: 1
    wFlags: 0
    elemdescFunc (return value type):
    	tdesc.vartype 25 (VT_HRESULT)
    	u.paramdesc.wParamFlags PARAMFLAGS_NONE
    	u.paramdesc.lpex (nil)
    helpstring: (null)
    entry (ordinal): 0x0000

The implementation of the method in Microsoft ADODB 2.7 last parameter
(FieldValue) absolutely refuses to accept a VT_VARIANT | VT_BYREF variant.
This result is not dependent of the type of variant pointed to by the
first-level variant - changing VT_ERROR to VT_EMPTY has no effect. Only by
passing a VT_EMPTY instead of a VT_VARIANT | VT_BYREF can the application
proceed normally. All oleaut32 tests still pass. Please comment if this is
an appropriate fix for this issue.

Changelog:
* (PARAMFLAG_FIN | PARAMFLAG_FOPT) & ~PARAMFLAG_FHASDEFAULT should result
in a default parameter of VT_EMPTY, not a VT_VARIANT | VT_BYREF in
ITypeInfo_fnInvoke

Alex Villacís Lasso