I used dumpcpp to import a typelib of ours that contains a method named clear().
DumpCPP dutifully wrote a definition of this interface, as
So, problem one is that this is inadvertently an override of
But the second problem is that, as an inline definition of a virtual method, it would seldom actually be inlined, and would usually be called through the vtbl, and instantiated only by the pointer within. The vtbl will in turn usually only be instantiated by one translation unit, whichever one the compiler contains the definition of the key function; i.e. the first non-pure virtual function that is not inline at the point of class definition. At least, I think MSVC uses that same itanium ABI rule that clang/g++ does for this (but I can't find a reference). In any case it seems to be picking the ecdiffenginelib.cpp that dumpcpp created, which makes sense by that rule, as it contains IFileSet::IFileSet().
But there is a usually-benign (but not this time) ODR-ish issue here. I'm not sure it's strictly an ODR violation, but it's that sort of issue. That is, ecdiffenginelib.cpp has set QAX_DUMPCPP_ECDIFFENGINELIB_NOINLINES. So the only place the compiler would have instantiated the inline function, is a place where its definition was deleted by the preprocessor. Everywhere else is going to see that they don't have a definition of the key method, and while they do have an inline definition for the method body, unless they actually want to inline it they're going to just make an linker reference to the one out-of-line definition they assume some other translation unit will have written. So the end result is that nobody emits it, and you get an undefined symbol at link time, due to the missing definition in in the .cpp that was supposed to have instantiated it. Other places do have a definition, but I'm not sure how to make one of them emit it, because any attempt to call it, take a member function pointer, etc, is going to end up going referencing the vtbl instead of actually inlining it.
Even if the linker issue could was fixed, though, we'd still have the inadvertent and unwanted override of QAxBase::clear. So I'm not sure the definition issue really needs a solution, it's just the long explanation of a very weird symptom; I think that dumpcpp needs a way to rename imported names that clash with its inherited method names (or even better, a built-in list of reserved member names that it just automatically appends prefixes or suffixes somehow to prevent clashes. In that sense, this is kind of adjacent to QTBUG-48895, in that I think he way forward is a way to rename the imported method, though for somewhat different reasons...