https://issues.dlang.org/show_bug.cgi?id=8577

hsteoh@quickfur.ath.cx changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |hsteoh@quickfur.ath.cx

--- Comment #1 from hsteoh@quickfur.ath.cx ---
Workarounds:

1)
-----
template T(Ts...)
    if (Ts.length > 1)
{
    alias Ts[1] Z;
}
-----

2)
-----
template T(Ts...)
{
    static if (Ts.length > 1)
        alias Ts[1] Z;
    else
        static assert(0);
}
-----

--

https://issues.dlang.org/show_bug.cgi?id=8577

RazvanN <razvan.nitu1305@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |razvan.nitu1305@gmail.com
         Resolution|---                         |INVALID

--- Comment #2 from RazvanN <razvan.nitu1305@gmail.com> ---
Today, the code yields:

onlineapp.d(2): Error: tuple index 1 exceeds 1
onlineapp.d(7): Error: template instance `onlineapp.T!int` error instantiating

And I am arguing that this is the correct behavior. The static assert is
generally used to test that things inside the template instantiation are
correct.
Template constraints are used to check that the template arguments are correct.

The static assert cannot be evaluated before the template is instantiated because that will make it useless in 99% of the cases. The general case for static asserts in template declarations is to check that specific template properties are correct, therefore, you first need to have the instantiation before you actually evaluate the static assert.

In conclusion: use template constraints for this scenario:

template T(Ts...)
if (Ts.length > 1)
{
    alias Ts[1] Z;
}

Closing as invalid.

--