undef $0 shouldn't warn about $0

RT #123910

    $ perl -we'undef $0'
    Use of uninitialized value $0 in undef operator at -e line 1.

Generally, undef should ignore its arg when determining which var was
undef: only magic will trigger an undef warning.

1 files changed, 5 insertions, 1 deletions

diff --git a/sv.c b/sv.c
index 088359f62d..dd0b3d456a 100644
--- a/sv.c
+++ b/sv.c
@@ -15823,6 +15823,11 @@ S_find_uninit_var(pTHX_ const OP *const obase, const SV *const uninit_sv,
 
     switch (obase->op_type) {
 
+    case OP_UNDEF:
+        /* undef should care if its args are undef - any warnings
+         * will be from tied/magic vars */
+        break;
+
     case OP_RV2AV:
     case OP_RV2HV:
     case OP_PADAV:
@@ -16369,7 +16374,6 @@ S_find_uninit_var(pTHX_ const OP *const obase, const SV *const uninit_sv,
     case OP_ALARM:
     case OP_SEMGET:
     case OP_GETLOGIN:
-    case OP_UNDEF:
     case OP_SUBSTR:
     case OP_AEACH:
     case OP_EACH: