Apparent deeply-nested missing error bug with gcc 7.3

[Soul Studios]

>> 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.
> 
> Told you so :-P
> 
> 
>> However, it does surprise me that GCC doesn't check this code.
> 
> It's a dependent expression so can't be fully checked until
> instantiated -- and as you've discovered, it wasn't being
> instantiated. There's a trade-off between compilation speed and doing
> additional work to check uninstantiated templates with arbitrarily
> complex expressions in them.
> 

Yeah, I get it - saves a lot of time with heavily-templated setups and 
large projects.