compiler: implement direct cast for sum type

vlib/compiler/expression.v

@@ -70,8 +70,17 @@ fn (p mut Parser) bool_expression() string {
 		if typ == cast_typ {
 			p.warn('casting `$typ` to `$cast_typ` is not needed')
 		}
-		p.cgen.set_placeholder(start_ph, '($cast_typ)(')
-		p.gen(')')
+		if typ in p.table.sum_types {
+			T := p.table.find_type(cast_typ)
+			if T.parent != typ {
+				p.error('cannot cast `$typ` to `$cast_typ`. `$cast_typ` is not a variant of `$typ`')
+			}
+			p.cgen.set_placeholder(start_ph, '*($cast_typ*)')
+			p.gen('.obj')
+		} else {
+			p.cgen.set_placeholder(start_ph, '($cast_typ)(')
+			p.gen(')')
+		}
 		return cast_typ
 	}
 	return typ

vlib/compiler/tests/type_test.v

@@ -28,12 +28,12 @@ struct BoolExpr {
 }
 
 struct BinExpr {
-
+	name string
 }
 
 fn expr1() Expr {
 	mut e := Expr{}
-	e = BinExpr{}
+	e = BinExpr{'binexpr'}
 	return e
 	//return BinExpr{}
 }
@@ -55,6 +55,12 @@ fn parse_bool() BoolExpr {
 	return BoolExpr{}
 }
 
+fn test_sum_type_cast() {
+	a := expr1()
+	b := a as BinExpr
+	assert b.name == 'binexpr'
+}
+
 fn test_sum_types() {
 	b := parse_bool()
 	handle_expr(b)