vec(): defer lvalue out-of-range croaking

RT #131083

Recent commits v5.25.10-81-gd69c430 and v5.25.10-82-g67dd6f3 added
out-of-range/overflow checks for the offset arg of vec().  However in
lvalue context, these croaks now happen before the SVt_PVLV was created,
rather than when its set magic was called. This means that something like

    sub f { $x = $_[0] }
    f(vec($s, -1, 8))

now croaks even though the out-of-range value never ended up getting used
in lvalue context.

This commit fixes things by, in pp_vec(), rather than croaking, just set
flag bits in LvFLAGS() to indicate that the offset is -Ve / out-of-range.

Then in Perl_magic_getvec(), return 0 if these flags are set, and in
Perl_magic_setvec() croak with a suitable error.

1 files changed, 19 insertions, 1 deletions

diff --git a/t/op/vec.t b/t/op/vec.t
index e50ffb7af8..5fa1879686 100644
--- a/t/op/vec.t
+++ b/t/op/vec.t
@@ -8,7 +8,7 @@ BEGIN {
 
 use Config;
 
-plan(tests => 74);
+plan(tests => 78);
 
 
 is(vec($foo,0,1), 0);
@@ -223,3 +223,21 @@ like($@, qr/^Modification of a read-only value attempted at /,
         }
     }
 }
+
+# RT #131083 maybe-lvalue out of range should only croak if assigned to
+
+{
+    sub  RT131083 { if ($_[0]) { $_[1] = 1; } $_[1]; }
+    my $s = "abc";
+    my $off = -1;
+    my $v = RT131083(0, vec($s, $off, 8));
+    is($v, 0, "RT131083 rval -1");
+    $v = eval { RT131083(1, vec($s, $off, 8)); };
+    like($@, qr/Negative offset to vec in lvalue context/, "RT131083 lval -1");
+
+    $off = ~0;
+    my $v = RT131083(0, vec($s, $off, 8));
+    is($v, 0, "RT131083 rval ~0");
+    $v = eval { RT131083(1, vec($s, $off, 8)); };
+    like($@, qr/Out of memory!/, "RT131083 lval ~0");
+}