fix check order for filetest overload

The tryAMAGICftest_MG() macro was doing two checks:

1) seeing whether the filetest operator's arg (*PL_stack_sp) looks
   to have magic or be a reference,
2) and if the op has children (which will have pushed an arg, unlike
   (-X _),
If both are true, then do full-on magic and/or overload processing.

The problem with this is that it checks the arg *before* checking whether
there's even an arg. Thus in the case of (-X _), it is actually examining
a random SV on the stack (or in the case of nothing on the stack,
examining the PL_sv_undef pointer always stored at PL_stack_base[0] as a
guard.)

It turns out this was harmless - the test for (1) will examine a random
(but real) SV and get garbage results, but then the 2nd test will fail
anyway, so overloading won't be called.

So the fix is to swap the (1) and (2) test order.

In addition, I changed the 'has an argument' test from OPf_KIDS to
!OPf_REF. These should be mutually exclusive, but the OPf_REF flag
formally indicates (-X _), i.e. that no arg has been pushed on the
stack. Whether the op has children or not could potentially change in
the future, independent of whether it's the (-X _) form.

So overall this commit makes no visible functional difference, but may
make the code more robust against future changes.

1 files changed, 4 insertions, 2 deletions

diff --git a/pp_sys.c b/pp_sys.c
index 26a9dda006..348c43a261 100644
--- a/pp_sys.c
+++ b/pp_sys.c
@@ -3108,9 +3108,11 @@ S_ft_return_true(pTHX_ SV *ret) {
 #define FT_RETURNUNDEF	return S_ft_return_false(aTHX_ &PL_sv_undef)
 #define FT_RETURNYES	return S_ft_return_true(aTHX_ &PL_sv_yes)
 
+/* NB: OPf_REF implies '-X _' and thus no arg on the stack */
 #define tryAMAGICftest_MG(chr) STMT_START { \
-        if ( (SvFLAGS(*PL_stack_sp) & (SVf_ROK|SVs_GMG)) \
-                && PL_op->op_flags & OPf_KIDS) {     \
+        if (   !(PL_op->op_flags & OPf_REF)                   \
+            && (SvFLAGS(*PL_stack_sp) & (SVf_ROK|SVs_GMG)))   \
+        {                                                     \
             OP *next = S_try_amagic_ftest(aTHX_ chr);	\
             if (next) return next;			  \
         }						   \