Hi all,
I am having issues finding a solution for this, i want to check if an alias is an istance of a variadic template nested in another variadic template.
Here is an example:

template A(As...) {
    template B(Bs...) {
    }
}

alias BI = A!(1,2).B!(3,4,5);

Now i want to test if BI is an istance of A.B, but i can't do something like this:

static if ( is( BI == A!As.B!Bs, As..., Bs... ) ) {
    // Do Something
}

there is some sort of workaround?

Thanks,
Gianni

On Friday, 4 November 2016 at 15:50:36 UTC, Gianni Pisetta wrote:
> Hi all,
> I am having issues finding a solution for this, i want to check if an alias is an istance of a variadic template nested in another variadic template.
> [...]
> there is some sort of workaround?
>
> Thanks,
> Gianni

Hello, I'm not sure that's exactly what you want but check this:

============================
template A(As...) {
    template B(Bs...) {
    }
}

alias BI = A!(1,2).B!(3,4,5);

import std.traits;

template NestedTemplateArgsOf(alias T)
{
    alias NestedTemplateArgsOf = TemplateArgsOf!(__traits(parent, T));
}

alias Bs = TemplateArgsOf!BI;
alias As = NestedTemplateArgsOf!BI;


static if (is(typeof(BI) == typeof(A!As.B!Bs)))
{
    pragma(msg, "for the win");
}
============================

The missing key was NestedTemplateArgsOf. With it you can solve the problem.

On Friday, 4 November 2016 at 17:37:10 UTC, Basile B. wrote:
> On Friday, 4 November 2016 at 15:50:36 UTC, Gianni Pisetta wrote:
>> Hi all,
>> I am having issues finding a solution for this, i want to check if an alias is an istance of a variadic template nested in another variadic template.
>> [...]
>> there is some sort of workaround?
>>
>> Thanks,
>> Gianni
>
> Hello, I'm not sure that's exactly what you want but check this:
>
> ============================
> [...]
> ============================

A better generic solution:

https://github.com/BBasile/iz/blob/1a6452b376a3a1977c8287f9ef294d2dde76952d/import/iz/types.d#L398

On Friday, 4 November 2016 at 17:37:10 UTC, Basile B. wrote:
>
> Hello, I'm not sure that's exactly what you want but check this:
>
> ============================
> template A(As...) {
>     template B(Bs...) {
>     }
> }
>
> alias BI = A!(1,2).B!(3,4,5);
>
> import std.traits;
>
> template NestedTemplateArgsOf(alias T)
> {
>     alias NestedTemplateArgsOf = TemplateArgsOf!(__traits(parent, T));
> }
>
> alias Bs = TemplateArgsOf!BI;
> alias As = NestedTemplateArgsOf!BI;
>
>
> static if (is(typeof(BI) == typeof(A!As.B!Bs)))
> {
>     pragma(msg, "for the win");
> }
> ============================
>
> The missing key was NestedTemplateArgsOf. With it you can solve the problem.

Well, kind of. But i think i can make it with what i got from your example, so thanks.
Another thing that I encountered and while messing with your code is that __traits( parent, T ) does not work as expected when you have structs instead of template. I think because something like

struct A(As...) {}

is downgraded to

template A(As...) {
  struct A {}
}

when i use __traits( parent, A!(1,2) ) i get in return A!(1,2), looping around the same symbol.
When you compile this

struct A(As...) {}

import std.conv;

pragma( msg, "The parent symbol is the same? " ~ to!string( __traits( isSame, A!(1,2), __traits( parent, A!(1,2) ) ) ) );

void main() {}

you get a really interesting result:

The parent symbol is the same? true

Gianni Pisetta

When i have time i will test it with ldc and see if the result is the same, then it will probably be a front-end bug and i will report it as an issue.

On Saturday, 5 November 2016 at 13:34:51 UTC, Gianni Pisetta wrote:
> When i have time i will test it with ldc and see if the result is the same, then it will probably be a front-end bug and i will report it as an issue.

I think it has already been reported.

https://issues.dlang.org/show_bug.cgi?id=13372

On Saturday, 5 November 2016 at 13:34:51 UTC, Gianni Pisetta wrote:
> When i have time i will test it with ldc and see if the result is the same, then it will probably be a front-end bug and i will report it as an issue.

Indeed, I've been fighting against that since a few minutes. We cant select the Base of an eponymous template.


template isEponymousTemplate(T)
{
    static if (is(T == class) || is(T == interface) || is(T == struct) || is(T == union))
    {
        enum p = __traits(parent, T).stringof == T.stringof;
        enum isEponymousTemplate = p && isTemplateInstance!T;
    }
    else
        enum isEponymousTemplate = false;
}

template TemplateBase(T : Base!Args, alias Base, Args...)
if (is(T == class) || is(T == interface) || is(T == struct) || is(T == union))
{
    alias TemplateBase = Base;
}

On Saturday, 5 November 2016 at 14:37:53 UTC, Basile B. wrote:
> On Saturday, 5 November 2016 at 13:34:51 UTC, Gianni Pisetta wrote:
>> [...]
>
> Indeed, I've been fighting against that since a few minutes. We cant select the Base of an eponymous template.
>
>
> template isEponymousTemplate(T)
> {
>     static if (is(T == class) || is(T == interface) || is(T == struct) || is(T == union))
>     {
>         enum p = __traits(parent, T).stringof == T.stringof;
>         enum isEponymousTemplate = p && isTemplateInstance!T;
>     }
>     else
>         enum isEponymousTemplate = false;
> }
>
> template TemplateBase(T : Base!Args, alias Base, Args...)
> if (is(T == class) || is(T == interface) || is(T == struct) || is(T == union))
> {
>     alias TemplateBase = Base;
> }

To be more accurate the template base can be selected for

    template A(T) {class A{}}

but not for

    class A(T){}

On Saturday, 5 November 2016 at 13:43:34 UTC, Lodovico Giaretta wrote:
> On Saturday, 5 November 2016 at 13:34:51 UTC, Gianni Pisetta wrote:
>> When i have time i will test it with ldc and see if the result is the same, then it will probably be a front-end bug and i will report it as an issue.
>
> I think it has already been reported.
>
> https://issues.dlang.org/show_bug.cgi?id=13372

Yes. Though the report is more generalized now. It applies to any eponymous type template.