Range with an alias parameter

[Steven Schveighoffer]

On 1/30/19 3:56 PM, Alex wrote:

> Ok... strange... it doesn't in fact... as this works:
> 
> ´´´
> import std.experimental.all;
> 
> void main()
> {
>      S!42 s;
>      auto res = s[];
>      static assert(isInputRange!(typeof(res)));
>      res.front.writeln;
> }
> 
> //static assert(isInputRange!(ReturnType!(produceS!(42))[]));

Apples and oranges :)

ReturnType!(produceS!(42)) is a TYPE, not a variable. When you apply the 
brackets, it's not calling your opindex, but rather changing it to an 
array. So let's make it clearer by saying:

alias T = ReturnType!(produceS!(42));

So now, your assert becomes:

static assert(isInputRange!(T[]));

Which, is not coming up as a valid range, because you can't copy the 
front value (this(this) is disabled).

In the other expression, you are first calling the index operator on an 
instance of the type, which returns a DIFFERENT type, and then asserting 
the type of that is an input range.

The equivalent (still using T) is:

static assert(isInputRange!(typeof(T.init[])));

replacing T with the original is:

static assert(isInputRange!(typeof(ReturnType!(produceS!(42)).init[])));

-Steve