Apparent deeply-nested missing error bug with gcc 7.3

[Soul Studios]

> 
> It's never called.
> 
> I added a call to abort() to that function, and the tests all pass. So
> the function is never used, so GCC never compiles it and doesn't
> notice that the return type is invalid. That's allowed by the
> standard. The compiler is not required to diagnose ill-formed code in
> uninstantiated templates.
> 

As I mentioned in the original message, it is called several times.
Both MSVC and Clang pick up the error, but GCC does not. It's a bug.