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, 8 insertions, 13 deletions

diff --git a/op.c b/op.c
index 313087d34a..812ece2bb1 100644
--- a/op.c
+++ b/op.c
@@ -4672,17 +4672,13 @@ Perl_utilize(pTHX_ int aver, I32 floor, OP *version, OP *idop, OP *arg)
     if (use_version) {
 	HV * const hinthv = GvHV(PL_hintgv);
 	const bool hhoff = !hinthv || !(PL_hints & HINT_LOCALIZE_HH);
+	SV *importsv;
 
 	/* Turn features off */
-	if (hhoff)
-	    /* avoid loading feature.pm */
-	    PL_hints &= ~HINT_UNI_8_BIT;
-	else {
-	    ENTER_with_name("load_feature");
-	    Perl_load_module(aTHX_
+	ENTER_with_name("load_feature");
+	Perl_load_module(aTHX_
 		PERL_LOADMOD_DENY, newSVpvs("feature"), NULL, NULL
-	    );
-	}
+	);
 
 	/* If we request a version >= 5.9.5, load feature.pm with the
 	 * feature bundle that corresponds to the required version. */
@@ -4690,13 +4686,12 @@ Perl_utilize(pTHX_ int aver, I32 floor, OP *version, OP *idop, OP *arg)
 
 	if (vcmp(use_version,
 		 sv_2mortal(upg_version(newSVnv(5.009005), FALSE))) >= 0) {
-	    SV *const importsv = vnormal(use_version);
-	    if (hhoff) ENTER_with_name("load_feature");
+	    importsv = vnormal(use_version);
 	    *SvPVX_mutable(importsv) = ':';
-	    Perl_load_module(aTHX_ 0, newSVpvs("feature"), NULL, importsv, NULL);
-	    LEAVE_with_name("load_feature");
 	}
-	else if (!hhoff) LEAVE_with_name("load_feature");
+	else importsv = newSVpvs(":default");
+	Perl_load_module(aTHX_ 0, newSVpvs("feature"), NULL, importsv, NULL);
+	LEAVE_with_name("load_feature");
 	/* If a version >= 5.11.0 is requested, strictures are on by default! */
 	if (vcmp(use_version,
 		 sv_2mortal(upg_version(newSVnv(5.011000), FALSE))) >= 0) {