warn on redefining an array constant sub

The code in report_redefined_cv() would check whether the old
constant and the new constant were the same SV to avoid warning on
duplicate imports of the same constant.  Since both gv_setref()
and report_redefined_cv() used cv_const_sv() to fetch the constant
SV, which returns NULL for an AV based constant sub, the check
for the equivalent SV would compare NULL to NULL and
report_redefined_cv() would return early.

Additional checks were required further down to prevent sv_cmp()
being called on AVs.

The check for simple SV's allow redefinition of constant subs if the
new value string compares the same as the old value.  The AV check
does not try to allow that.

Fixed #20742

Author: tonycoz

op.c

@@ -15321,7 +15321,7 @@ Perl_report_redefined_cv(pTHX_ const SV *name, const CV *old_cv,
 {
     const char *hvname;
     bool is_const = cBOOL(CvCONST(old_cv));
-    SV *old_const_sv = is_const ? cv_const_sv(old_cv) : NULL;
+    SV *old_const_sv = is_const ? cv_const_sv_or_av(old_cv) : NULL;
 
     PERL_ARGS_ASSERT_REPORT_REDEFINED_CV;
 
@@ -15344,14 +15344,20 @@ Perl_report_redefined_cv(pTHX_ const SV *name, const CV *old_cv,
         )
      || (is_const
          && ckWARN_d(WARN_REDEFINE)
-         && (!new_const_svp || sv_cmp(old_const_sv, *new_const_svp))
+         && (!new_const_svp ||
+             !*new_const_svp ||
+             !old_const_sv ||
+             SvTYPE(old_const_sv) == SVt_PVAV ||
+             SvTYPE(*new_const_svp) == SVt_PVAV ||
+             sv_cmp(old_const_sv, *new_const_svp))
         )
-    )
+        ) {
         Perl_warner(aTHX_ packWARN(WARN_REDEFINE),
                           is_const
                             ? "Constant subroutine %" SVf " redefined"
                             : "Subroutine %" SVf " redefined",
                           SVfARG(name));
+    }
 }
 
 /*

sv.c

@@ -3913,7 +3913,7 @@ Perl_gv_setref(pTHX_ SV *const dsv, SV *const ssv)
                     {
                         SV * const new_const_sv =
                             CvCONST((const CV *)sref)
-                                 ? cv_const_sv((const CV *)sref)
+                                 ? cv_const_sv_or_av((const CV *)sref)
                                  : NULL;
                         HV * const stash = GvSTASH((const GV *)dsv);
                         report_redefined_cv(

t/lib/warnings/op

@@ -284,7 +284,6 @@ my @s = @f["]", "a"];
 @h[glob ""];
 @h[readline ""];
 @h[m ""];
-use constant phoo => 1..3;
 @h[+phoo]; # rv2av
 @h[sort foo];
 @h[reverse foo];
@@ -1019,6 +1018,13 @@ sub fred () { 1 }
 EXPECT
 Constant subroutine main::fred redefined at - line 3.
 ########
+# op.c github #20742
+use constant fred => 1, 2;
+use constant fred => 2, 3;
+EXPECT
+OPTIONS regex
+Constant subroutine main::fred redefined at .*lib/constant\.pm line \d+
+########
 # op.c
 use feature "lexical_subs", "state";
 my sub fred () { 1 }