Preserve behaviour of allowing an undefined key for tied hash lookups

The previous change prevented the warning, but forced the hash key for
defelem to the empty string if the original key was undefined.

This wasn't a problem if the hash was already tied when the function
was called (and the PVLV created), since the resulting PVLV used
packelem magic instead of defelem.

But if the hash was tied after the PVLV was created references through
the PVLV would be working on $hash{""} instead of $hash{+undef},
breaking this possibly used feature.

One issue is whether we want to allow undef keys for tied hash, if we
do we should probably make them warn less.

Fixes Perl#22423

Author: tonycoz

mg.c

@@ -2649,7 +2649,15 @@ Perl_defelem_target(pTHX_ SV *sv, MAGIC *mg)
     if (LvTARGLEN(sv)) {
         if (mg->mg_obj) {
             SV * const ahv = LvTARG(sv);
-            SV * const index_sv = SvOK(mg->mg_obj) ? mg->mg_obj : &PL_sv_no;
+            /* A call like $h{$s} with $s not defined would warn
+               here, which could be confusing.  A tied hash could treat
+               an undef index specially, so we need to preserve undef
+               for a tied hash.
+            */
+            SV * const index_sv =
+                SvOK(mg->mg_obj) ||
+                (SvGMAGICAL(ahv) && mg_find((const SV *)ahv, PERL_MAGIC_tied))
+                ? mg->mg_obj : &PL_sv_no;
             HE * const he = hv_fetch_ent(MUTABLE_HV(ahv), index_sv, FALSE, 0);
             if (he)
                 targ = HeVAL(he);

t/lib/warnings/mg

@@ -93,7 +93,10 @@ Use of uninitialized value $s in hash element at - line 6.
 # NAME Use of uninitialized value $_[0] in defined operator (tied)
 # github 22423
 # should we allow tied hashes to distinguish between undef and ""
-# without warning?  For now test the current behaviour
+# without warning?  For now test the current behaviour, this
+# didn't produce the warning described in github #22423 since
+# if the hash is tied for the call the PVLV uses packelem (tie)
+# magic rather than defelem magic
 use v5.36;
 ++$|;
 sub f { defined $_[0] }
@@ -124,9 +127,49 @@ sub EXISTS {
 }
 
 EXPECT
-Use of uninitialized value $s in hash element at - line 9.
-Use of uninitialized value in hash element at - line 10.
-Use of uninitialized value in hash element at - line 12.
+Use of uninitialized value $s in hash element at - line 12.
+Use of uninitialized value in hash element at - line 13.
+Use of uninitialized value in hash element at - line 15.
 1
-Use of uninitialized value $s in hash element at - line 13.
+Use of uninitialized value $s in hash element at - line 16.
+########
+# NAME Use of uninitialized value $_[0] in defined operator (tied2)
+# github 22423
+# In this case we have a tied hash, but it's only tied after the
+# PVLV is created for the element. This *does* produce the warning
+# complained about in #22423
+use v5.36;
+++$|;
+my %h;
+sub f {
+  tie %h, "Foo";
+  my $x = defined $_[0];
+  $_[0] = "tied-undef";
+}
+my $s;
+f($h{$s});
+$h{""} = "tied";
+say $h{+undef};
+
+package Foo;
 
+sub TIEHASH {
+  bless {}, shift;
+}
+sub STORE($self, $index, $val) {
+  $self->{defined $index ? $index : "+undef"} = $val;
+}
+sub FETCH($self, $index) {
+  $self->{defined $index ? $index : "+undef"};
+}
+sub EXISTS($self, $index) {
+  exists $self->{defined $index ? $index : "+undef"};
+}
+sub DELETE($self, $index) {
+  delete $self->{defined $index ? $index : "+undef"};
+}
+EXPECT
+Use of uninitialized value $s in hash element at - line 14.
+Use of uninitialized value $_[0] in scalar assignment at - line 11.
+Use of uninitialized value in hash element at - line 16.
+tied-undef