struct bar( T ) {
    auto baz( U )( U arg ) {
        bar!( typeof( this ) ) tmp;
        return tmp;
    }
}

void main( ) {
    bar!int n;
    n.baz( 3 );
}

This code fails with
Error: recursive template expansion for template argument bar!(int)
Now, I agree it is recursive, but it is not infinitely recursive, so
there shouldn't really be a problem.
Is this a bug? Is there a workaround?

-- 
Simen

On Wed, 04 Aug 2010 15:37:32 -0400, Simen kjaeraas <simen.kjaras@gmail.com> wrote:

> struct bar( T ) {
>      auto baz( U )( U arg ) {
>          bar!( typeof( this ) ) tmp;
>          return tmp;
>      }
> }
>
> void main( ) {
>      bar!int n;
>      n.baz( 3 );
> }
>
> This code fails with
> Error: recursive template expansion for template argument bar!(int)
> Now, I agree it is recursive, but it is not infinitely recursive, so
> there shouldn't really be a problem.
> Is this a bug? Is there a workaround?

It's lazily recursive.  Meaning, it *is* infinitely recursive, but the compiler does not have to evaluate all the recursions until they are used.

I'm not sure it should be disallowed, but it's definitely on the edge.  Do you have some real-world case for this?  If you want to get something like this fixed, you'll need a good example, not an academic one.

-Steve

Steven Schveighoffer <schveiguy@yahoo.com> wrote:

> On Wed, 04 Aug 2010 15:37:32 -0400, Simen kjaeraas <simen.kjaras@gmail.com> wrote:
>
>> struct bar( T ) {
>>      auto baz( U )( U arg ) {
>>          bar!( typeof( this ) ) tmp;
>>          return tmp;
>>      }
>> }
>>
>> void main( ) {
>>      bar!int n;
>>      n.baz( 3 );
>> }
>>
>> This code fails with
>> Error: recursive template expansion for template argument bar!(int)
>> Now, I agree it is recursive, but it is not infinitely recursive, so
>> there shouldn't really be a problem.
>> Is this a bug? Is there a workaround?
>
> It's lazily recursive.  Meaning, it *is* infinitely recursive, but the compiler does not have to evaluate all the recursions until they are used.
>
> I'm not sure it should be disallowed, but it's definitely on the edge.  Do you have some real-world case for this?  If you want to get something like this fixed, you'll need a good example, not an academic one.

I sorta do. I have a struct that keeps track of the operations performed
on it, to create a template stack for calculating stuff at the end.
Basically, I build a function by performing the operations on the
struct. Not sure if this is an expression template, but possibly related.

-- 
Simen

On Wed, Aug 4, 2010 at 22:06, Steven Schveighoffer <schveiguy@yahoo.com>wrote:

> On Wed, 04 Aug 2010 15:37:32 -0400, Simen kjaeraas <simen.kjaras@gmail.com> wrote:
>
>  struct bar( T ) {
>>     auto baz( U )( U arg ) {
>>         bar!( typeof( this ) ) tmp;
>>         return tmp;
>>     }
>> }
>>
>> void main( ) {
>>     bar!int n;
>>     n.baz( 3 );
>> }
>>
>> This code fails with
>> Error: recursive template expansion for template argument bar!(int)
>> Now, I agree it is recursive, but it is not infinitely recursive, so
>> there shouldn't really be a problem.
>> Is this a bug? Is there a workaround?
>>
>
> It's lazily recursive.  Meaning, it *is* infinitely recursive, but the compiler does not have to evaluate all the recursions until they are used.
>
> I'm not sure it should be disallowed, but it's definitely on the edge.  Do you have some real-world case for this?  If you want to get something like this fixed, you'll need a good example, not an academic one.
>
> -Steve
>

I could get this to work, using a factory function:

import std.stdio;

struct Bar( T ) {
   auto baz( U )( U arg ) {
       return bar(this);
   }
}

Bar!T bar(T)(T t)
{
    return Bar!T();
}

void main( ) {
   Bar!int n;
   auto nn = n.baz( 3 );
   writeln(typeof(n).stringof);
   writeln(typeof(nn).stringof);
}

It's... fragile. Changes a few things here and there, and it's back to infinite recursion.



Philippe

Simen kjaeraas wrote:
> struct bar( T ) {
>     auto baz( U )( U arg ) {
>         bar!( typeof( this ) ) tmp;
>         return tmp;
>     }
> }
> 
> void main( ) {
>     bar!int n;
>     n.baz( 3 );
> }
> 
> This code fails with
> Error: recursive template expansion for template argument bar!(int)
> Now, I agree it is recursive, but it is not infinitely recursive, so
> there shouldn't really be a problem.
> Is this a bug? Is there a workaround?
> 
Duplicate of bug 3869.