[Issue 1241] ICE on template instance parameter

[Manuel König]

d-bugmail at puremagic.com wrote:
> http://d.puremagic.com/issues/show_bug.cgi?id=1241
> 
> 
> shro8822 at vandals.uidaho.edu changed:
> 
>            What    |Removed                     |Added
> ----------------------------------------------------------------------------
>                  CC|                            |shro8822 at vandals.uidaho.edu
> 
> 
> 
> 
> ------- Comment #2 from shro8822 at vandals.uidaho.edu  2007-07-22 12:02 -------
> I have also run into this one. I was trying to use the tuples+templates as
> compile time trees
> 
> template Foo(A...)
> {
>   template Bar(B...)
>   {
>     alias A a;
>     alias B b;
>   }
> }
> 
> alias Foo!(Foo!(1,2,3).Bar!(4,5,6)).Bar!(Foo!(7,8,9).Bar!(10,11,12)) tree;
> 
> foreach( a; tree.a)
> {
>   foreach(aa; a.a);
>   foreach(ab; a.b);
> }
> foreach( b; tree.b)
> {
>   foreach(ba; b.a);
>   foreach(bb; b.b);
> }
> 
> Can we please get a fix on this one? It will drop a LOT of barriers to
> elaborate template systems. I have a few really cool things that use it and
> would like to put them in my talk without the disclaimer of "this would work
> but for bug #1241"
> 
> 

I just discovered a decent workaround. Instead of using templates with 
tuple members, just use structs:

Use

struct Tuple(T...)
{
	alias T tp;
}

instead of

template Tuple(T...)
{
	alias T tp;
}

I tested this with dmd1.020 and rewrote your tree structure using this 
trick:

import std.stdio;

struct Foo(A...)
{
   struct Bar(B...)
   {
     alias A a;
     alias B b;
   }
}

alias Foo!(Foo!(1,2,3).Bar!(4,5,6)).Bar!(Foo!(7,8,9).Bar!(10,11,12)) tree;

void main()
{
	foreach( a; tree.a)
	{
	  foreach(aa; a.a) writefln(aa);
	  foreach(ab; a.b) writefln(ab);
	}
	foreach( b; tree.b)
	{
	  foreach(ba; b.a) writefln(ba);
	  foreach(bb; b.b) writefln(bb);
	}
}

This prints
1
2
3
4
5
6
7
8
9
10
11
12

for me. Happy coding!