> Otherwise, the caller must allocate memory for the return value and pass a pointer to it as the first argument. The remaining arguments are then shifted one a

> Otherwise, the caller must allocate memory for the return value and pass a pointer to it as the first argument. The remaining arguments are then shifted one argument to the right. I think the issue is that it isnt _fully_ documented anywhere. The MS docs do partially describe it, but it omits details about how this should be handled in the case of C++ member functions (which are the COM methods in question). C++ member functions always receive the hidden 'this' pointer as the first argument in Microsoft's x64 calling convention, which directly conflicts with this document stating the returned struct would be a "pointer to it as the first argument". You can see this in some of the godbolt links in the linked issue. Ultimately, some COM functions that take no arguments actually end up taking two hidden parameters in rcx and rdx (this pointer, and the return struct pointer respectively)