std.traits : Select - could it be better?

[Steven Schveighoffer]

On 7/5/18 2:50 PM, SrMordred wrote:
> alias U = Select!( isPointer!T, PointerTarget!T, T );
> 
> This don´t compile if T are not a pointer;
> 
> so you have to do this:
> 
> static if( isPointer!T )
>      alias U = PointerTarget!T;
> else
>      alias U = T;
> 
> Shouldnt the 'correct' way of Select to work is ignoring the choice that 
> was not taken?
> 
> I love the idea of Select, but because of this I almost never use it.

Hm... only thing I can think of is to do the instantiation within the 
template itself:

template BetterSelect(bool cond, alias temp1, alias temp2, Args...)
{
    import std.meta : Instantiate;
    static if(cond)
       alias BetterSelect = Instantiate!(temp1, Args);
    else
       alias BetterSelect = Instantiate!(temp2, Args);
}

alias U = BetterSelect!(isPointer!T, PointerTarget, Alias, T)

Of course, this requires you to have templates that take EXACTLY the 
same parameters.

I don't know if this (or something like it) is worth adding to 
std.traits or std.meta.

-Steve