Stop $^H |= 0x1c020000 from enabling all features

That set of bits sets the feature bundle to ‘custom’, which means that
the features are set by %^H, and also indicates that %^H has been did-
dled with, so it’s worth looking at.

In the specific case where %^H is untouched and there is no corres-
ponding cop hint hash behind the scenes, Perl_feature_is_enabled (in
toke.c) ends up returning TRUE.

Commit v5.15.6-55-g94250ae sped up feature checking by allowing
refcounted_he_fetch to return a boolean when checking for existence,
instead of converting the value to a scalar, whose contents we are not
even going to use.

This was when the bug started happening.  I did not update the code
path in refcounted_he_fetch that handles the absence of a hint hash.
So it was returning &PL_sv_placeholder instead of NULL; TRUE instead
of FALSE.

This did not cause problems for most code, but with the introduction
of the new bitwise ops in v5.21.8-150-g8823cb8, it started causing
uni::perl to fail, because they were implicitly enabled, making ^ a
numeric op, when it was being used as a string op.

1 files changed, 2 insertions, 1 deletions

diff --git a/hv.c b/hv.c
index 1fa6d6ff4e..e5bf629b4a 100644
--- a/hv.c
+++ b/hv.c
@@ -3189,7 +3189,7 @@ Perl_refcounted_he_fetch_pvn(pTHX_ const struct refcounted_he *chain,
 	Perl_croak(aTHX_ "panic: refcounted_he_fetch_pvn bad flags %"UVxf,
 	    (UV)flags);
     if (!chain)
-	return &PL_sv_placeholder;
+	goto ret;
     if (flags & REFCOUNTED_HE_KEY_UTF8) {
 	/* For searching purposes, canonicalise to Latin-1 where possible. */
 	const char *keyend = keypv + keylen, *p;
@@ -3249,6 +3249,7 @@ Perl_refcounted_he_fetch_pvn(pTHX_ const struct refcounted_he *chain,
 	    return sv_2mortal(refcounted_he_value(chain));
 	}
     }
+  ret:
     return flags & REFCOUNTED_HE_EXISTS ? NULL : &PL_sv_placeholder;
 }