[Issue 13204] recursive alias declaration

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=13204

Jonathan M Davis <jmdavisProg at gmx.com> changed:

           What    |Removed                     |Added
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             Status|NEW                         |RESOLVED
                 CC|                            |jmdavisProg at gmx.com
         Resolution|---                         |INVALID

--- Comment #1 from Jonathan M Davis <jmdavisProg at gmx.com> ---
Okay. Think this through. When you declare

alias A1 = ABase!1; // L6

it now has to instantiate ABase with 1. When it does that it has to define

alias whatever = Foo;

Foo is then an alias to A1, which is an alias to ABase!1. So, to determine what
Foo is, it has to instantiate ABase!1, which it's already in the middle of
doing. It's clearly recursive and isn't going to end. You always have to be
_very_ careful when attempting to instantiate a template within itself. You
need template constraints or static ifs to fix the problem. So, in this case,
you'd probably need to do something like this (untested).

struct ABase(uint v)
{
    static if(v == 1)
        alias whatever = ABase;
    else
        alias whatever = Foo;
}

And as to an improved error message, I don't know what else would help you. It
told you the line that where the problem was, and the definition of Foo clearly
is attempting to instantiate the template that the error was in. If you can
suggest an error message that would be more helpful, that would be great, but
as far as I can tell, the compiler is telling you exactly what's wrong.

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