[perl #114498] lslice returning empty vs undef

Formerly, list slice would return an empty list if all the indices
fell outside the actual list, but would return ‘undef’ for every index
specified if but one of them fell within the list.

This was not implemented according to the original design, according
to which list slice would give an empty list (for a non-zero number of
indices) only if the list on the left-hand side were empty.

This commit rectifies that.  See ticket #114498 for the discussion.

2 files changed, 19 insertions, 17 deletions

diff --git a/pp.c b/pp.c
index 3ec401d8b2..49798e2d93 100644
--- a/pp.c
+++ b/pp.c
@@ -4974,7 +4974,6 @@ PP(pp_lslice)
     SV ** const lastlelem = PL_stack_base + POPMARK;
     SV ** const firstlelem = PL_stack_base + POPMARK + 1;
     SV ** const firstrelem = lastlelem + 1;
-    I32 is_something_there = FALSE;
     const U8 mod = PL_op->op_flags & OPf_MOD;
 
     const I32 max = lastrelem - lastlelem;
@@ -5004,7 +5003,6 @@ PP(pp_lslice)
 	if (ix < 0 || ix >= max)
 	    *lelem = &PL_sv_undef;
 	else {
-	    is_something_there = TRUE;
 	    if (!(*lelem = firstrelem[ix]))
 		*lelem = &PL_sv_undef;
 	    else if (mod && SvPADTMP(*lelem)) {
@@ -5012,10 +5010,7 @@ PP(pp_lslice)
             }
 	}
     }
-    if (is_something_there)
-	SP = lastlelem;
-    else
-	SP = firstlelem - 1;
+    SP = lastlelem;
     RETURN;
 }
 
diff --git a/t/op/list.t b/t/op/list.t
index d3f6b50661..c7056d0245 100644
--- a/t/op/list.t
+++ b/t/op/list.t
@@ -3,10 +3,10 @@
 BEGIN {
     chdir 't' if -d 't';
     @INC = qw(. ../lib);
+    require "./test.pl";
 }
 
-require "./test.pl";
-plan( tests => 67 );
+plan( tests => 69 );
 
 @foo = (1, 2, 3, 4);
 cmp_ok($foo[0], '==', 1, 'first elem');
@@ -146,21 +146,28 @@ cmp_ok(join('',(1,2),3,(4,5)),'eq','12345','list (..).(..)');
 
     my $size = scalar(()[1..1]);
     cmp_ok($size,'==','0','size nil');
+
+    $size = scalar(()=((1,2,3,4,5)[()])[2,3,4]);
+    is $size, 0, 'slice of empty list from complex expr is empty list';
+
+    @a = (1)[2,3,4];
+    is "@{[ map $_//'undef', @a ]}", "undef undef undef",
+       'slice beyond the end of non-empty list returns undefs';
 }
 
 {
     # perl #39882
-    sub test_zero_args {
+    sub test_two_args {
         my $test_name = shift;
-        is(scalar(@_), 0, $test_name);
+        is(scalar(@_), 2, $test_name);
     }
-    test_zero_args("simple list slice",      (10,11)[2,3]);
-    test_zero_args("grepped list slice",     grep(1, (10,11)[2,3]));
-    test_zero_args("sorted list slice",      sort((10,11)[2,3]));
-    test_zero_args("assigned list slice",    my @tmp = (10,11)[2,3]);
-    test_zero_args("do-returned list slice", do { (10,11)[2,3]; });
-    test_zero_args("list literal slice",     qw(a b)[2,3]);
-    test_zero_args("empty literal slice",    qw()[2,3]);
+    test_two_args("simple list slice",      (10,11)[2,3]);
+    test_two_args("grepped list slice",     grep(1, (10,11)[2,3]));
+    test_two_args("sorted list slice",      sort((10,11)[2,3]));
+    test_two_args("assigned list slice",    my @tmp = (10,11)[2,3]);
+    test_two_args("do-returned list slice", do { (10,11)[2,3]; });
+    test_two_args("list literal slice",     qw(a b)[2,3]);
+    is (()=qw()[2,3], 0, "empty literal slice");
 }
 
 {