From @lizmat

[21​:55​:53] <lizmat> m​: sub a($x is rw) { $x; }; a 42 # dies as expected
[21​:55​:54] <+camelia> rakudo-moar 8b834c​: OUTPUT«Parameter '$x' expected a writable container, but got Int value␤ in sub a at /tmp/2NK1Hbv1Ub​:1␤ in block <unit> at /tmp/2NK1Hbv1Ub​:1␤␤»
[21​:56​:11] <lizmat> m​: sub a($x is rw) { $x; }; { a 42 }() # but not if within a block ?
[21​:56​:11] <+camelia> rakudo-moar 8b834c​: ( no output )
[21​:58​:08] <lizmat> m​: sub a($x is rw) { $x; }; { a 42 } # seems to be optimizer related
[21​:58​:08] <+camelia> rakudo-moar 8b834c​: ( no output )
[21​:58​:19] <lizmat> $ perl6 --optimize=off -e 'sub a($x is rw) { $x }; { a 42 }'
[21​:58​:19] <lizmat> Parameter '$x' expected a writable container, but got Int value

From @peschwa

On Sat Nov 07 12​:59​:32 2015, elizabeth wrote​:

[21​:55​:53] <lizmat> m​: sub a($x is rw) { $x; }; a 42 # dies as
expected
[21​:55​:54] <+camelia> rakudo-moar 8b834c​: OUTPUT«Parameter '$x'
expected a writable container, but got Int value␤ in sub a at
/tmp/2NK1Hbv1Ub​:1␤ in block <unit> at /tmp/2NK1Hbv1Ub​:1␤␤»
[21​:56​:11] <lizmat> m​: sub a($x is rw) { $x; }; { a 42 }() # but
not if within a block ?
[21​:56​:11] <+camelia> rakudo-moar 8b834c​: ( no output )
[21​:58​:08] <lizmat> m​: sub a($x is rw) { $x; }; { a 42 } # seems
to be optimizer related
[21​:58​:08] <+camelia> rakudo-moar 8b834c​: ( no output )
[21​:58​:19] <lizmat> $ perl6 --optimize=off -e 'sub a($x is rw) {
$x }; { a 42 }'
[21​:58​:19] <lizmat> Parameter '$x' expected a writable container,
but got Int value

My previous fix for a related issue (in commit 76391ca9) was apparently bogus. I've reverted that in 8b0d257 and implemented the necessary logic (hopefully) correctly in b1a465c. The test in S06-traits/misc.t passes again (and I'll unfudge it after this comment) and camelia shows the correct behavior as well​:

16​:13 <psch> m​: sub a($x is rw) { $x; }; a 42
16​:13 <camelia> rakudo-moar b1a465​: OUTPUT«Parameter '$x' expected a writable container, but got Int value␤ in sub a at /tmp/iAUHG8Hg4F​:1␤ in
  block <unit> at /tmp/iAUHG8Hg4F​:1␤␤»
16​:13 <psch> m​: sub a($x is rw) { $x; }; { a 42 }()
16​:13 <camelia> rakudo-moar b1a465​: OUTPUT«Parameter '$x' expected a writable container, but got Int value␤ in sub a at /tmp/zX2zZQDT2u​:1␤ in
  block <unit> at /tmp/zX2zZQDT2u​:1␤␤»

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

@moritz - Status changed from 'open' to 'resolved'