[perl #131673] `is rw` on anon parameter causes "inconsistent bind result" error

[Zoffix Znet via RT]

On Thu, 29 Jun 2017 03:13:07 -0700, c...@zoffix.com wrote:
> On Thu, 29 Jun 2017 00:45:02 -0700, elizabeth wrote:
> > Shouldn’t:
> >
> > sub ($ is rw) { }
> >
> > be a compile-time error?  I mean, there is no way to actually assign
> > to the anonymous variable, is there?  So there is no point in the “is
> > rw”.  So most likely indicates a typo on the user side.
> >
> 

Here's a less convoluted example for the last usecase. Taking an iterable with 
2 rw containers, while not necessarily wanting to assign each of the items into 
their own variables. I'd expect the first example to error out just like the 
last example instead of accepting the values (or dying with inconsistent bind 
error):

    <Zoffix__> m: sub foo (@a ($ is rw, $ is rw)) { $_++ for @a }; my @a := 42, 
42; foo @a; dd @a
    <camelia> rakudo-moar 2a8d1e: OUTPUT: «Cannot resolve caller 
postfix:<++>(Int); the following candidates␤match the type but require mutable 
arguments:␤    (Mu:D $a is rw)␤    (Int:D $a is rw)␤␤The following do not match 
for other reasons:␤    (Bool:D $a is rw)␤    (Bool:U $a is …»
    <Zoffix__> m: sub foo (@a ($ is rw, $ is rw)) { $_++ for @a }; my @a = 42, 
42; foo @a; dd @a
    <camelia> rakudo-moar 2a8d1e: OUTPUT: «Array @a = [43, 43]␤»
    <Zoffix__> m: sub foo (@a ($z is rw, $zz is rw)) { $_++ for @a }; my @a := 
42, 42; foo @a; dd @a
    <camelia> rakudo-moar 2a8d1e: OUTPUT: «Parameter '$z' expected a writable 
container, but got Int value␤  in sub foo at <tmp> line 1␤  in block <unit> at 
<tmp> line 1␤␤»