Disable $[ under 5.16

This adds the array_base feature to feature.pm

Perl_feature_is_enabled has been modified to use PL_curcop, rather
than PL_hintgv, so it can work with run-time hints as well.
(PL_curcop holds the current state op at run time, and &PL_compiling
at compile time, so it works for both.)  The hints in $^H are not
stored in the same place at compile time and run time, so the FEATURE_IS_ENABLED macro has been modified to check first whether
PL_curop == &PL_compiling.

Since array_base is on by default with no hint for it in %^H, it is
a ‘negative’ feature, whose entry in %^H turns it off.  feature.pm
has been modified to support such negative features.  The new FEATURE_IS_ENABLED_d can check whether such default features
are enabled.

This does make things less efficient, as every version declaration
now loads feature.pm to disable all features (including turning off
array_base, which entails adding an entry to %^H) before loading the
new bundle.  I have plans to make this more efficient.

1 files changed, 3 insertions, 1 deletions

diff --git a/gv.c b/gv.c
index 2af41a87c4..37a1bd9510 100644
--- a/gv.c
+++ b/gv.c
@@ -1939,11 +1939,13 @@ Perl_gv_fetchpvn_flags(pTHX_ const char *nambeg, STRLEN full_len, I32 flags,
 	    }
 	    goto magicalize;
 	case '[':		/* $[ */
-	    if (sv_type == SVt_PV || sv_type == SVt_PVGV) {
+	    if ((sv_type == SVt_PV || sv_type == SVt_PVGV)
+	     && FEATURE_IS_ENABLED_d("$[")) {
 		if (addmg) (void)hv_store(stash,name,len,(SV *)gv,0);
 		require_tie_mod(gv,name,newSVpvs("arybase"),"FETCH",0);
 		addmg = 0;
 	    }
+	    else goto magicalize;
             break;
 	case '\023':	/* $^S */
 	ro_magicalize: