sort: correctly sort "0+" overloaded values

embed.fnc

@@ -5362,8 +5362,8 @@ i	|I32	|sv_i_ncmp	|NN SV * const a			\
 				|NN SV * const b
 i	|I32	|sv_i_ncmp_desc |NN SV * const a			\
 				|NN SV * const b
-i	|I32	|sv_ncmp	|NN SV * const a			\
-				|NN SV * const b
+i	|I32	|sv_ncmp	|NN SV *a				\
+				|NN SV *b
 i	|I32	|sv_ncmp_desc	|NN SV * const a			\
 				|NN SV * const b
 # if defined(USE_LOCALE_COLLATE)

pod/perldelta.pod

@@ -386,7 +386,13 @@ manager will later use a regex to expand these into links.
 
 =item *
 
-XXX
+sort() optimizes well known comparisons from calling the OP tree for a
+comparison block into a call to a C function.  The C function used for
+overloaded numeric comparisons did not handle the case where there was
+no comparison overload but there was a numeric ("0+") overload
+correct, losing precision for large overloaded integer arguments that
+are not exactly representable as a Perl floating point value (NV).
+[GH #23956]
 
 =back
 

pp_sort.c

@@ -1310,12 +1310,23 @@ S_sortcv_xsub(pTHX_ SV *const a, SV *const b)
 
 
 PERL_STATIC_FORCE_INLINE I32
-S_sv_ncmp(pTHX_ SV *const a, SV *const b)
+S_sv_ncmp(pTHX_ SV *a, SV *b)
 {
-    I32 cmp = do_ncmp(a, b);
-
     PERL_ARGS_ASSERT_SV_NCMP;
 
+    /* Numify since do_ncmp will just SvNV() non-IVs.
+
+       Even for the non-overloading case, if RVs are allocated with
+       large 64-bit addresses (only theoretically possible I think)
+       the bottom bits of the RV might be lost.
+     */
+    if (SvROK(a))
+        a = sv_2num(a);
+    if (SvROK(b))
+        b = sv_2num(b);
+
+    I32 cmp = do_ncmp(a, b);
+
     if (cmp == 2) {
         if (ckWARN(WARN_UNINITIALIZED)) report_uninit(NULL);
         return 0;
@@ -1359,7 +1370,7 @@ S_sv_i_ncmp_desc(pTHX_ SV *const a, SV *const b)
 #define SORT_NORMAL_RETURN_VALUE(val)  (((val) > 0) ? 1 : ((val) ? -1 : 0))
 
 PERL_STATIC_FORCE_INLINE I32
-S_amagic_ncmp(pTHX_ SV *const a, SV *const b)
+S_amagic_ncmp(pTHX_ SV *a, SV *b)
 {
     SV * const tmpsv = tryCALL_AMAGICbin(a,b,ncmp_amg);
 
@@ -1375,6 +1386,7 @@ S_amagic_ncmp(pTHX_ SV *const a, SV *const b)
             return SORT_NORMAL_RETURN_VALUE(d);
         }
      }
+
      return S_sv_ncmp(aTHX_ a, b);
 }
 

t/op/sort.t

@@ -7,7 +7,7 @@ BEGIN {
     set_up_inc('../lib');
 }
 use warnings;
-plan(tests => 205);
+plan(tests => 206);
 use Tie::Array; # we need to test sorting tied arrays
 
 # these shouldn't hang
@@ -964,6 +964,21 @@ is("@b", "1 2 3 3 4 5 7", "comparison result as string");
     is($cs, 2, 'overload string called twice');
 }
 
+{
+    # GH 23956 - amagic_ncmp didn't handle numeric conversions
+    # properly
+    package GH23956 {
+        use overload
+          fallback => 1,
+          "0+" => sub { $_[0][0] };
+    }
+    my @data = map {
+        bless [ $_ ], "GH23956"
+    } ~0, ~0-1;
+    my @sorted = sort { $a <=> $b } @data;
+    is $sorted[0]+0, $data[1], "sort of 0+ overloaded values";
+}
+
 fresh_perl_is('sub w ($$) {my ($l, $r) = @_; my $v = \@_; undef @_; $l <=> $r}; print join q{ }, sort w 3, 1, 2, 0',
              '0 1 2 3',
              {stderr => 1, switches => ['-w']},