[perl #126118] Nil.gist goes missing via type parameter

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# New Ticket Created by  Zefram 
# Please include the string:  [perl #126118]
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# <URL: https://rt.perl.org/Ticket/Display.html?id=126118 >


$ ./perl6 -e 'sub aa (Mu:U ::T) { say T; my T $a = T; say $a.WHAT; }; aa(Int); 
aa(Nil)'
(Int)
(Int)
Nil
Method 'gist' not found for invocant of class 'T'
  in sub aa at -e:1
  in block <unit> at -e:1

I'm mystified as to how the gist method goes missing there.  It seems
entirely available when I'm not using a type parameter:

$ ./perl6 -e 'say Int; my Int $a = Int; say $a.WHAT; say Nil; my Nil $b = Nil; 
say $b.WHAT;'
(Int)
(Int)
Nil
Nil

I also don't get an error if the "= T" is omitted, which is bizarre
because the variable contains the same value by default.  The error
doesn't occur at the time of the assignment, though: it only happens
with the say statement.

-zefram