[perl #127987] Block.assuming method apparently not reentrant

[via RT]

# New Ticket Created by  Sam S. 
# Please include the string:  [perl #127987]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=127987 >


The following works fine:

    for (^100).race(batch=>1) {  {1 + $^a}(10)  }

But the following pretty reliably crashes:

    for (^100).race(batch=>1) {  {1 + $^a}.assuming(10)()  }

If even crashes when EVAL is used to make sure each thread has a completely 
different closure:

    for (^100).race(batch=>1) {  EVAL(q[ ({1 + $^a}) ]).assuming(10)()  }


It doesn't always crash with the same error message. But it always seems to be 
a variation of one of the following four:

    P6opaque: no such attribute '$!named'
      in any named at gen/moar/stage2/QASTNode.nqp line 30
      ...

    Parameter '$a' requires a type object of type Int, but an object instance 
was passed.
    Did you forget a 'multi'?
      in block

    Cannot call postfix:<++>(Int); none of these signatures match:
    ...
      in block

    No such symbol 'primers'
      at ...EVAL_10:4 ------> $self(primers⏏.list[0], |{ %ahash, %chash }); # 
|{} wo

The code fragment that the last one quotes, is from src/core/Block.pm.


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This is Rakudo version 2016.04-33-gf05c77b built on MoarVM version 2016.04 
implementing Perl 6.c.