[Bug c++/60976] Compilation with G++ 4.9.0 is 2-3 times slower than with 4.8.2

["ott at fb dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60976

--- Comment #22 from Giuseppe Ottaviano <ott at fb dot com> ---
>> The regression might have been already solved in r225244, which uses
>> yet another SFINAE pattern without extra template arguments, which I
>> believe are the cause of the regression. However I haven't tested it
>> yet.


> That would be nice to know, because I now use that kind of
> void_t-style constraint in a few places, and plan to use it more
> widely. My measurements do show that using void_t-style constraints
> result in small but measurable reductions in compile time and memory
> use.
> Oh, I looked at the wrong bit of r225244, it's using SFINAE in a
> trailing-return-type that matters here, not the __detected_or_t_
> changes.

Yes I referred to the trailing return type. Unfortunately it's not trivial to
test it with our code because alloc_traits.h is not anymore a drop-in
replacement. Maybe the test code included in this bug is enough? Is r225244
already included in a GCC release?