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.
> 


UPDATE: My bad.
The original compiler feature detection on the test suite was broken/not 
matching the correct libstdc++ versions.
Hence the emplace_back/emplace_front tests were not running.
However, it does surprise me that GCC doesn't check this code.
Cheers-