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

[redi at gcc dot gnu.org]

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

--- Comment #21 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #20)
> (In reply to Giuseppe Ottaviano from comment #19)
> > 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.