"SFINAE is Evil"

[Russell Lewis]

Bill Baxter wrote:
> How do I know if I'm using SFINAE or not?  :-)
> 
> Seriously, I might be using it.  I'm not sure.  I'm not sure how to go 
> about checking.  And that in and of itself seems like an undesirable 
> quality.

Indeed!

> I will say that I have most definitely been bitten by errors that came 
> from failure to instantiate the template I thought I was instantiating 
> (most often due to IFTI failures).  These things can usually be debugged 
> by switching the code I suspect to specify all template arguments.  Then 
> I'll get the sensible error telling me why it doesn't match.

Tuples hurt you here, because if you specify some Tuple template as a 
"last resort" template, you can end up using it for things you thought 
you'd handled.  Or, at least, I think that maybe that's the case...with 
SFINAE, it's hard to know for sure.