&CORE::substr()

This commit makes &CORE::substr callable through references and via
&ampersand syntax.

It’s a bit awkward, as we need a substr op that is flagged as hav-
ing 4 arguments *and* possibly returning an lvalue.   The code in
op_lvalue_flags wasn’t really set up for that, so I needed to flag
the op with OPpMAYBE_LVSUB in coresub_op before it gets passed to
op_lvalue_flags.  It turned out that only that was necessary, as
op_lvalue_flags does an op_private == 4 check (rather than (op_private
& 7) == 4 or some such) when checking for the 4-arg case and croak-
ing.  When the op arrives in op_lvalue_flags, it’s already flagged
OPpMAYBE_LVSUB|4 which != 4.

pp_substr is also modified to check for nulls and, if necessary,
adjust its count of how many arguments were actually passed.)

1 files changed, 10 insertions, 0 deletions

diff --git a/t/op/coreamp.t b/t/op/coreamp.t
index b9b946fbe0..3347756675 100644
--- a/t/op/coreamp.t
+++ b/t/op/coreamp.t
@@ -670,6 +670,16 @@ $tests ++;
 &CORE::srand;
 pass '&srand with no args does not crash';
 
+test_proto 'substr';
+$tests += 5;
+$_ = "abc";
+is &mysubstr($_, 1, 1, "d"), 'b', '4-arg &substr';
+is $_, 'adc', 'what 4-arg &substr does';
+is &mysubstr("abc", 1, 1), 'b', '3-arg &substr';
+is &mysubstr("abc", 1), 'bc', '2-arg &substr';
+&mysubstr($_, 1) = 'long';
+is $_, 'along', 'lvalue &substr';
+
 test_proto 'symlink';
 test_proto 'syscall';
 test_proto 'sysread';