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.