"fold": a replacement for "reduce"

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On Friday, 28 March 2014 at 22:39:24 UTC, Timon Gehr wrote:
> You mean because the literal is accepted as an alias argument? 
> Alias template arguments are not actually the same thing as 
> alias declarations. (Eg. the latter can accept built-in types 
> like 'int', while the former will not, but otherwise allow 
> arbitrary expressions.) I used to think this was a bug, but 
> Walter stated that this is in fact by design. (I think this is 
> a gratuitous design mistake.)
>
> The expressions that are used in alias declarations are 'a' and 
> 'Id!(a=>a)'. Both of those can occur in a context where they 
> denote types.

I'm confused now. What exactly was it that you were saying didn't 
make sense?