[perl #122995] Hang with while(1) in a sub-list

It was hanging at compile time in some cases, e.g.:

sub foo { () = ($a, my $b, ($c, do { while(1) {} })) }

The optimisation added in 5.20 to turn list+pushmark into null ops
when they are in list context (effectively making ($a,($b,$c)) equiva-
lent to ($a,$b,$c) with regard to which ops are executed) followed
op_next pointers to find the last op that was a kid of the sublist.

You can’t just follow op_next pointers like that, because it will loop
at compile time on infinite loops like while (1){}.

In this case, the last kid was being found in order to elide the erst-
while list op from the op_next chain, but that is not necessary, since
later OP_NULL handling takes care of it anyway.

1 files changed, 4 insertions, 0 deletions

diff --git a/t/op/list.t b/t/op/list.t
index d14873f3a2..3f42e6d8e2 100644
--- a/t/op/list.t
+++ b/t/op/list.t
@@ -190,3 +190,7 @@ sub {
  )
 }
  ->(("${\''}")[0,0]);
+
+# [perl #122995] Hang when compiling while(1) in a sub-list
+# No ok() or is() necessary.
+sub foo { () = ($a, my $b, ($c, do { while(1) {} })) }