[perl #132794] Junction as default value of MAIN param hangs forever

On Wed, 31 Jan 2018 07:56:31 -0800, [email protected] wrote:
> On Wed, 31 Jan 2018 07:26:24 -0800, [email protected]
> wrote:
> > $ perl6 -e 'sub MAIN ( $x = 1|2 ) { }'
> >
> > Following oneliner will hang forever on Rakudo 2018.01.
> > Error exists for all type of Junctions.
> > Doesn't matter if they are built by infix "1|2" or by method
> > "any(1,2)".
> 
> We can probably improve that behaviour with an error message (it
> applies to any routine, not just MAIN).
> 
> The problem is you're declaring an Any parameter, but your default
> value is a Junction, so when the default is used, it's trying to
> autothread the Junction and I'm guessing it keeps looping there in the
> guts somewhere.
> 
> Looking at your use, I'm suspecting you meant to use a `where` clause
> instead of the default value:
> 
> `perl6 -e 'sub MAIN ( $x where 1|2 ) { }'

I started with this, but likely won't have time to finish it this week. In case someone else wants
to give it a go, some notes:

1) jnthn++ agreed[^1] that this should throw
2) This code doesn't go through `check_param_default_type`[^2] in Actions. I tried passing the   
    thunked[^3] defaults through the routine, but compilation fails
    with `Cannot unbox a type object (Str) to a str.`
3) The infiniloop is in the BOOTSTRAP[^4]. It basically keeps autothreading itself with no
    params over and over.

[1] https://irclog.perlgeek.de/perl6/2018-01-31#i_15761475
[2] https://github.com/rakudo/rakudo/blob/72948e8477fdb92801a59a6e101b29c2da2f844e/src/Perl6/Actions.nqp#L5050-L5058
[3] https://github.com/rakudo/rakudo/blob/72948e8477fdb92801a59a6e101b29c2da2f844e/src/Perl6/Actions.nqp#L5061
[4] https://github.com/rakudo/rakudo/blob/72948e8477fdb92801a59a6e101b29c2da2f844e/src/Perl6/Metamodel/BOOTSTRAP.nqp#L3336-L3362