From @zoffixznet

<Zoffix___> c​: HEAD with "/tmp/foo2121".IO { .spurt​: "fo♥o"; with .open(​:enc<ascii>) { say .slurp } }
<committable6> Zoffix___, ¦HEAD(0c5fe56)​: «MoarVM panic​: MVM_nfg_get_synthetic_info called with out-of-range synthetic «exit code = 1»»

This is not the result of encoding refactor; still exists in 2017.05​:
<Zoffix___> c​: 2017.05 with "/tmp/foo2121".IO { .spurt​: "fo♥o"; with .open(​:enc<ascii>) { say .slurp } }
<committable6> Zoffix___, ¦2017.05​: «MoarVM panic​: MVM_nfg_get_synthetic_info called with out-of-range synthetic «exit code = 1»»

From @zoffixznet

Bisecting this shows the issue existed before The Christmas​:

Zoffix___> bisect​: old=2015.12,new=b4dfed2a86b8af1 with "/tmp/foo2121".IO { .spurt​: "fo♥o"; with .open(​:enc<ascii>) { say .get } }
<bisectable6> Zoffix___, On both starting points (old=2015.12 new=b4dfed2a86b8af1) the exit code is 1 and the output is identical as well
<bisectable6> Zoffix___, Output on both points​: «MVM_nfg_get_synthetic_info called with out-of-range synthetic»

From @jnthn

On Sat, 27 May 2017 08​:01​:00 -0700, cpan@​zoffix.com wrote​:

<Zoffix___> c​: HEAD with "/tmp/foo2121".IO { .spurt​: "fo♥o"; with
.open(​:enc<ascii>) { say .slurp } }
<committable6> Zoffix___, ¦HEAD(0c5fe56)​: «MoarVM panic​:
MVM_nfg_get_synthetic_info called with out-of-range synthetic «exit
code = 1»»

This is not the result of encoding refactor; still exists in 2017.05​:
<Zoffix___> c​: 2017.05 with "/tmp/foo2121".IO { .spurt​: "fo♥o"; with
.open(​:enc<ascii>) { say .slurp } }
<committable6> Zoffix___, ¦2017.05​: «MoarVM panic​:
MVM_nfg_get_synthetic_info called with out-of-range synthetic «exit
code = 1»»

Fixed (now it gives a normal decoding exception) and tested in S32-io/io-handle.t.

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

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