From @masak

<sorear> p6​: @​*ARGS ».=« flip
<p6eval> ..rakudo 90333b​: OUTPUT«===SORRY!===␤ResizablePMCArray​: Can't
shift from an empty array!␤»
<sorear> hey, I broke Rakudo
* masak submits sorear's ResizablePMCArray rakudobug
<sorear> o/ masakbot
* masak bobs in mid-air
<masak> r​: (my @​) ».=« flip
<p6eval> rakudo 90333b​: OUTPUT«===SORRY!===␤ResizablePMCArray​: Can't
shift from an empty array!␤»
<masak> I'm tryin' to figger out what ».=« means, semantically.
<masak> so, infix​:<.=> is an operator that makes the rhs a bit special.
<masak> 'flip' is still a method.
<sorear> masak​: I vote for 'nothing'
<sorear> actually, I somewhat think .= should be treated syntactically
as an adverb
<sorear> accepted in infix position but otherwise parses as a postfix

From @coke

On Wed Jun 13 23​:13​:01 2012, masak wrote​:

<sorear> p6​: @​*ARGS ».=« flip
<p6eval> ..rakudo 90333b​: OUTPUT«===SORRY!===␤ResizablePMCArray​: Can't
shift from an empty array!␤»
<sorear> hey, I broke Rakudo
* masak submits sorear's ResizablePMCArray rakudobug
<sorear> o/ masakbot
* masak bobs in mid-air
<masak> r​: (my @​) ».=« flip
<p6eval> rakudo 90333b​: OUTPUT«===SORRY!===␤ResizablePMCArray​: Can't
shift from an empty array!␤»
<masak> I'm tryin' to figger out what ».=« means, semantically.
<masak> so, infix​:<.=> is an operator that makes the rhs a bit special.
<masak> 'flip' is still a method.
<sorear> masak​: I vote for 'nothing'
<sorear> actually, I somewhat think .= should be treated syntactically
as an adverb
<sorear> accepted in infix position but otherwise parses as a postfix

Error message has changed, and no longer exposing an internal parrot error.

23​:26 <[Coke]> r​: (my @​) ».=« flip
23​:26 <p6eval> rakudo 8230ae​: OUTPUT«===SORRY!===␤Error while compiling block :
  Error while compiling op call​: Error while compiling block :
  Error while compiling op call​: Error while compiling op
  callmethod​: Method call node requires at least one child␤»

--
Will "Coke" Coleda

The RT System itself - Status changed from 'new' to 'open'

From @smls

On Wed, 13 Jun 2012 23​:13​:01 -0700, masak wrote​:

<masak> r​: (my @​) ».=« flip
<p6eval> rakudo 90333b​: OUTPUT«===SORRY!===␤ResizablePMCArray​: Can't
shift from an empty array!␤»

Still present in​:

  This is Rakudo version 2017.08-104-g76f1d8970
  built on MoarVM version 2017.08.1-148-g1059eed1
  implementing Perl 6.c.

It addition to an error, it now also prints a bogus "useless use" warning.

  ➜ my @​a = <ab xy>; @​a »~=» '1'; dd @​a;
  Array @​a = ["ab1", "xy1"]

  ➜ my @​a = <ab xy>; @​a ».=» flip; dd @​a;
  Potential difficulties​:
  Useless use of ».=« in sink context
  at -e​:1
  ------> my @​a = <ab xy>; @​a ⏏».=» flip; dd @​a
  ===SORRY!===
  Method call node requires at least one child

Here shown with the the right-facing hyper meta-op `» »`, which makes more sense here (though all forms currently fail in the same way).

Expected result for each form, IMO​:

  ».=»

  Array @​a = ["ba", "yx"]

  «.=»

  Array @​a = ["ba", "yx"]

  ».=«

  ERROR​:
  Lists on either side of non-dwimmy hyperop of .= are not of
  the same length
  left​: 2 elements, right​: 1 elements

  «.=«

  ERROR​:
  Lists on either side of non-dwimmy hyperop of .= are not of
  the expected length
  left​: 2 elements, right​: 1 elements