[perl #128187] Forbid sub :lvalue{keys} in aassign

This commit makes perl die when keys(%hash) is returned from an lvalue
sub and the lvalue sub call is assigned to in list assignment:

    sub foo : lvalue { keys(%INC) }
    (foo) = 3; # death

This prevents an assignment that is completely useless and probably a
mistake, and it makes the lvalue-sub use of keys behave the same way
as (keys(%INC)) = 3.

Author: Father Chrysostomos

doop.c

@@ -1273,6 +1273,13 @@ Perl_do_kv(pTHX)
 	RETURN;
     }
 
+    if (UNLIKELY(PL_op->op_private & OPpMAYBE_LVSUB)) {
+	const I32 flags = is_lvalue_sub();
+	if (flags && !(flags & OPpENTERSUB_INARGS))
+	    /* diag_listed_as: Can't modify %s in %s */
+	    Perl_croak(aTHX_ "Can't modify keys in list assignment");
+    }
+
     /* 2*HvUSEDKEYS() should never be big enough to truncate or wrap */
     assert(HvUSEDKEYS(keys) <= (SSize_t_MAX >> 1));
     extend_size = (SSize_t)HvUSEDKEYS(keys) * (dokeys + dovalues);

t/op/sub_lval.t

@@ -5,7 +5,7 @@ BEGIN {
     @INC = '../lib';
     require './test.pl';
 }
-plan tests=>209;
+plan tests=>210;
 
 sub a : lvalue { my $a = 34; ${\(bless \$a)} }  # Return a temporary
 sub b : lvalue { ${\shift} }
@@ -553,6 +553,9 @@ sub keeze : lvalue { keys %__ }
 %__ = ("a","b");
 keeze = 64;
 is scalar %__, '1/64', 'keys assignment through lvalue sub';
+eval { (keeze) = 64 };
+like $@, qr/^Can't modify keys in list assignment at /,
+  'list assignment to keys through lv sub is forbidden';
 
 # Bug 20001223.002: split thought that the list had only one element
 @ary = qw(4 5 6);