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Allow cast between different type of pointers

The cast between pointers of different type must generate
the code in order to generate a valid pointer of the destine
type (usually some alignment code).

--- a/expr.c

+++ b/expr.c

@@ -96,7 +96,7 @@

  * Convert a Node to a type

  */

 static Node *

-convert(Node *np, Type *tp1)

+convert(Node *np, Type *tp1, char iscast)

 {

 	Type *tp2;

 	register uint8_t t1, t2;

@@ -118,15 +118,17 @@

 		case ARY: case FTN:

 			np = addr2ptr(np);

 		case PTR:

-			if (tp1 != pvoidtype && tp2 != pvoidtype)

-				return NULL;

-			/* TODO:

-			 * we assume conversion between pointers

-			 * do not need any operation, but due to

-			 * alignment problems that may be false

-			 */

-			np->type = tp1;

-			return np;

+			if (iscast || tp1 == pvoidtype ||  tp2 == pvoidtype) {

+				/* TODO:

+				 * we assume conversion between pointers

+				 * do not need any operation, but due to

+				 * alignment problems that may be false

+				 */

+				np->type = tp1;

+				return np;

+			}

+			return NULL;

+

 		}

 	}

 	return NULL;

@@ -495,7 +497,7 @@

 			tp = typename();

 			expect(')');

 			np1 = cast();

-			if ((np2 = convert(np1,  tp)) == NULL)

+			if ((np2 = convert(np1,  tp, 1)) == NULL)

 				error("bad type convertion requested");

 			np2->b.lvalue = np1->b.lvalue;

 			return np1;

@@ -703,7 +705,7 @@

 		if (isconst(np1->type->op))

 			goto const_mod;

 		np2 = eval(np2);

-		if ((np2 = convert(np2, np1->type)) == NULL)

+		if ((np2 = convert(np2, np1->type, 0)) == NULL)

 			goto incompatibles;

 		np1 = bincode(op, np1->type, np1, np2);

 	}