Allow restricted hashes containing COWs to be cleared

Author: Father Chrysostomos

dist/base/t/fields.t

@@ -6,7 +6,7 @@ BEGIN {
 }
 
 use strict;
-use Test::More tests => 17;
+use Test::More tests => 18;
 
 BEGIN { use_ok('fields'); }
 
@@ -108,8 +108,10 @@ package main;
     ok(exists $x->{b}, 'x has b');
 
     SKIP: {
-        skip "This test triggers a perl bug", 1 if $] < 5.014001;
+        skip "These tests trigger a perl bug", 1 if $] < 5.014001;
         $x->{a} = __PACKAGE__;
         ok eval { delete $x->{a}; 1 }, 'deleting COW values';
+        $x->{a} = __PACKAGE__;
+        ok eval { %$x = (); 1 }, 'clearing a restr hash containing COWs';
     }
 }

hv.c

@@ -1554,7 +1554,8 @@ Perl_hv_clear(pTHX_ HV *hv)
 	    for (; entry; entry = HeNEXT(entry)) {
 		/* not already placeholder */
 		if (HeVAL(entry) != &PL_sv_placeholder) {
-		    if (HeVAL(entry) && SvREADONLY(HeVAL(entry))) {
+		    if (HeVAL(entry) && SvREADONLY(HeVAL(entry))
+		     && !SvIsCOW(HeVAL(entry))) {
 			SV* const keysv = hv_iterkeysv(entry);
 			Perl_croak(aTHX_
 				   "Attempt to delete readonly key '%"SVf"' from a restricted hash",