[perl #126477] [glr] .assuming on sub with is copy parameter fails

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# New Ticket Created by  Wenzel Peppmeyer 
# Please include the string:  [perl #126477]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=126477 >


sub foo(@a is copy, :$flag){ say @a };
my @a = 1,2; 
(&foo.assuming(:flag))(@a);

# OUTPUT«replace this Array is copy logic␤  in sub __PRIMED_ANON at EVAL_2:2␤  
in sub __PRIMED_ANON at 
EVAL_2:1␤  in block <unit> at /tmp/TlcJQPUFbc:1␤