[perl #60204] Unhelpful error message from unpack

Nigel Sandever said:
> The error message produced by the following snippets is very unhelpful:
>
> c:\>perl -wle"print unpack 'v/a*', qq[a]"
> '/' must follow a numeric type in unpack at -e line 1.
>
> c:\>perl -wle"print unpack 'v/a*', ''"
> '/' must follow a numeric type in unpack at -e line 1.
>
> c:\>perl -wle"print unpack 'v/a*', ' '"
> '/' must follow a numeric type in unpack at -e line 1.

The "problem" is that the data string is too short. But
unpack doesn't generate a warning (or croak) in this case
for simple patterns:

mhx@r2d2 $ perl -MData::Dumper -we'print Dumper([unpack "n", "a"])'
$VAR1 = [];

So, I'd say your code should just behave in exactly the
same way. No warning, no return values.

2 files changed, 11 insertions, 3 deletions

diff --git a/pp_pack.c b/pp_pack.c
index 71dc22d67d..9f1c83a34b 100644
--- a/pp_pack.c
+++ b/pp_pack.c
@@ -2234,7 +2234,7 @@ S_unpack_rec(pTHX_ tempsym_t* symptr, const char *s, const char *strbeg, const c
 
         if (symptr->flags & FLAG_SLASH){
             if (SP - PL_stack_base - start_sp_offset <= 0)
-                Perl_croak(aTHX_ "'/' must follow a numeric type in unpack");
+		break;
             if( next_symbol(symptr) ){
               if( symptr->howlen == e_number )
 		Perl_croak(aTHX_ "Count after length/code in unpack" );
diff --git a/t/op/pack.t b/t/op/pack.t
index 43189ab538..34097adaa6 100644
--- a/t/op/pack.t
+++ b/t/op/pack.t
@@ -12,7 +12,7 @@ my $no_endianness = $] > 5.009 ? '' :
 my $no_signedness = $] > 5.009 ? '' :
   "Signed/unsigned pack modifiers not available on this perl";
 
-plan tests => 14700;
+plan tests => 14704;
 
 use strict;
 use warnings qw(FATAL all);
@@ -816,13 +816,21 @@ SKIP: {
 {
   # /
 
-  my ($x, $y, $z);
+  my ($x, $y, $z, @a);
   eval { ($x) = unpack '/a*','hello' };
   like($@, qr!'/' must follow a numeric type!);
   undef $x;
   eval { $x = unpack '/a*','hello' };
   like($@, qr!'/' must follow a numeric type!);
 
+  # [perl #60204] Unhelpful error message from unpack
+  eval { @a = unpack 'v/a*','h' };
+  is($@, '');
+  is(scalar @a, 0);
+  eval { $x = unpack 'v/a*','h' };
+  is($@, '');
+  is($x, undef);
+
   undef $x;
   eval { ($z,$x,$y) = unpack 'a3/A C/a* C/Z', "003ok \003yes\004z\000abc" };
   is($@, '');