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fix special case for null pointer constants in cond expressions

	Section 6.5.15 of the C99 spec requires that if
	one argument of a ?: expression is a null pointer
	constant, and the other has a pointer type T*, then
	the type of the expression is T*.

	We were attempting to follow this rule, however,
	we only handled literal expressions when checking
	for null pointers.

	This change looks through casts, so 'nil' and 'NULL',
	and their expansion '(void*)0' are all detected as
	null pointer constants.

--- a/sys/src/cmd/cc/cc.h

+++ b/sys/src/cmd/cc/cc.h

@@ -691,6 +691,7 @@

 void	typeext(Type*, Node*);

 void	typeext1(Type*, Node*);

 int	side(Node*);

+int	zpconst(Node*);

 int	vconst(Node*);

 int	log2(uvlong);

 int	vlog(Node*);

--- a/sys/src/cmd/cc/com.c

+++ b/sys/src/cmd/cc/com.c

@@ -341,11 +341,13 @@

 		o |= tcom(r->left);

 		if(o | tcom(r->right))

 			goto bad;

-		if(r->right->type->etype == TIND && vconst(r->left) == 0) {

+		if(r->right->type->etype == TIND && zpconst(r->left)) {

+			r->type = r->right->type;

 			r->left->type = r->right->type;

 			r->left->vconst = 0;

 		}

-		if(r->left->type->etype == TIND && vconst(r->right) == 0) {

+		if(r->left->type->etype == TIND && zpconst(r->right)) {

+			r->type = r->left->type;

 			r->right->type = r->left->type;

 			r->right->vconst = 0;

 		}

--- a/sys/src/cmd/cc/sub.c

+++ b/sys/src/cmd/cc/sub.c

@@ -1026,6 +1026,21 @@

 }

 int

+zpconst(Node *n)

+{

+	while(n->op == OCAST){

+		if(n->type == T)

+			break;

+		if(n->type->etype != TIND)

+			break;

+		if(n->type->link->etype != TVOID)

+			break;

+		n = n->left;

+	}

+	return vconst(n) == 0;

+}

+

+int

 vconst(Node *n)

 {

 	int i;