[perl #127393] [BUG] Something funny with modifying a subset where part of the check is calling a method on the item

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# New Ticket Created by  Jonathan Stowe 
# Please include the string:  [perl #127393]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=127393 >


   perl6 -e 'subset JJ where *.can("foo"); -> JJ:D $a { } '
   ===SORRY!===
   Method 'iterator' must be implemented by Iterable because it is required by 
a role


This does not happen without the Type modifier in the signature, i.e.

   subset JJ where *.can("foo"); -> JJ $a { }

But a modified subset that does not call a method is fine:

   subset JJ where { True }; -> JJ:D $a { }

But it appears that in fact that it isn't 'can' that is the culprit but using 
*any* method on the item:

   perl6 -e 'subset JJ where *.foo; -> JJ:D $a { } '
   ===SORRY!===
   Method 'iterator' must be implemented by Iterable because it is required by 
a role