[perl #128726] Arrays and hashes don't parameterize inside role methods

[via RT]

# New Ticket Created by  Faye 
# Please include the string:  [perl #128726]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=128726 >


$ perl6 -e 'role A[::T = Numeric] { method B() { my T $a; say $a.WHAT } }; 
A.new.B'
(Numeric)

$ perl6 -e 'role A[::T = Numeric] { method B() { my T @a; say @a.WHAT } }; 
A.new.B'
Method 'instantiate_generic' not found for invocant of class 'Array[T]'
  in method B at -e line 1
  in block <unit> at -e line 1

$ perl6 -e 'role A[::T = Numeric] { method B() { my T %a; say %a.WHAT } }; 
A.new.B'
Method 'instantiate_generic' not found for invocant of class 'Hash[T]'
  in method B at -e line 1
  in block <unit> at -e line 1

The variable declarations here show a problem in likely the MOP. You can see 
this problem in a different light beyond type constraining variables:

$ perl6 -e 'role A[::T = Numeric] { method B() { say T; say Array[T]; say 
Hash[T]; } }; A.new.B'
(Numeric)
(Array[T])
(Hash[T])

Namely, the fact that the parameter to arrays and hashes doesn't get "resolved" 
like it does as a scalar.