For now, forbid all list assignment initialisation of state variables,

as the precise semantics in Perl 6 are not clear. Better to make it a
syntax error, than to have one behaviour now, but change it later.
[I believe that this is the consensus. If not, it will be backed out]

p4raw-id: //depot/perl@31824

Author: nwc10

op.c

@@ -3968,6 +3968,8 @@ Perl_newASSIGNOP(pTHX_ I32 flags, OP *left, I32 optype, OP *right)
     }
 
     if (is_list_assignment(left)) {
+	static const char no_list_state[] = "Initialization of state variables"
+	    " in list context currently forbidden";
 	OP *curop;
 
 	PL_modcount = 0;
@@ -4061,6 +4063,54 @@ Perl_newASSIGNOP(pTHX_ I32 flags, OP *left, I32 optype, OP *right)
 		o->op_private |= OPpASSIGN_COMMON;
 	}
 
+	if ((left->op_type == OP_LIST
+	     || (left->op_type == OP_NULL && left->op_targ == OP_LIST))) {
+	    OP* lop = ((LISTOP*)left)->op_first;
+	    while (lop) {
+		if (lop->op_type == OP_PADSV ||
+		    lop->op_type == OP_PADAV ||
+		    lop->op_type == OP_PADHV ||
+		    lop->op_type == OP_PADANY) {
+		    if (lop->op_private & OPpPAD_STATE) {
+			if (left->op_private & OPpLVAL_INTRO) {
+			    /* Each variable in state($a, $b, $c) = ... */
+			}
+			else {
+			    /* Each state variable in
+			       (state $a, my $b, our $c, $d, undef) = ... */
+			}
+			yyerror(no_list_state);
+		    } else {
+			/* Each my variable in
+			   (state $a, my $b, our $c, $d, undef) = ... */
+		    }
+		} else {
+		    /* Other ops in the list. undef may be interesting in
+		       (state $a, undef, state $c) */
+		}
+		lop = lop->op_sibling;
+	    }
+	}
+	else if (((left->op_private & (OPpLVAL_INTRO | OPpPAD_STATE))
+		    == (OPpLVAL_INTRO | OPpPAD_STATE))
+		&& (   left->op_type == OP_PADSV
+		    || left->op_type == OP_PADAV
+		    || left->op_type == OP_PADHV
+		    || left->op_type == OP_PADANY))
+	{
+	    /* All single variable list context state assignments, hence
+	       state ($a) = ...
+	       (state $a) = ...
+	       state @a = ...
+	       state (@a) = ...
+	       (state @a) = ...
+	       state %a = ...
+	       state (%a) = ...
+	       (state %a) = ...
+	    */
+	    yyerror(no_list_state);
+	}
+
 	if (right && right->op_type == OP_SPLIT && !PL_madskills) {
 	    OP* tmpop = ((LISTOP*)right)->op_first;
 	    if (tmpop && (tmpop->op_type == OP_PUSHRE)) {

pod/perldiag.pod

@@ -1999,6 +1999,13 @@ either consume text or fail.
 The <-- HERE shows in the regular expression about where the problem was
 discovered.
 
+=item Initialization of state variables in list context currently forbidden
+
+(F) Currently the implementation of "state" only permits the initialization
+of scalar variables in scalar context. Re-write C<state ($a) = 42> as
+C<state $a = 42> to change from list to scalar context. Constructions such
+as C<state (@a) = foo()> will be supported in a future perl release.
+
 =item Insecure dependency in %s
 
 (F) You tried to do something that the tainting mechanism didn't like.

t/op/state.t

@@ -10,7 +10,7 @@ BEGIN {
 use strict;
 use feature ":5.10";
 
-plan tests => 108;
+plan tests => 117;
 
 ok( ! defined state $uninit, q(state vars are undef by default) );
 
@@ -300,17 +300,6 @@ foreach my $x (0 .. 4) {
 }
 
 
-#
-# List context reassigns, but scalar doesn't.
-#
-my @swords = qw [Stormbringer Szczerbiec Grimtooth Corrougue];
-foreach my $sword (@swords) {
-    state ($s1) = state $s2 = $sword;
-    is $s1, $swords [0], 'mixed context';
-    is $s2, $swords [0], 'mixed context';
-}
-
-
 #
 # Use with given.
 #
@@ -331,3 +320,28 @@ foreach my $spam (@spam) {
     state $x = "two";
     is $x, "two", "masked"
 }
+
+foreach my $forbidden (<DATA>) {
+    chomp $forbidden;
+    no strict 'vars';
+    eval $forbidden;
+    like $@, qr/Initialization of state variables in list context currently forbidden/, "Currently forbidden: $forbidden";
+}
+__DATA__
+state ($a) = 1;
+(state $a) = 1;
+state @a = 1;
+state (@a) = 1;
+(state @a) = 1;
+state %a = ();
+state (%a) = ();
+(state %a) = ();
+state ($a, $b) = ();
+state ($a, @b) = ();
+(state $a, state $b) = ();
+(state $a, $b) = ();
+(state $a, my $b) = ();
+(state $a, state @b) = ();
+(state $a, local @b) = ();
+(state $a, undef, state $b) = ();
+state ($a, undef, $b) = ();