cgen: small interface conversion fix (#11229)

vlib/v/gen/c/cgen.v

@@ -6466,6 +6466,9 @@ fn (mut g Gen) as_cast(node ast.AsCast) {
 		}
 	} else if expr_type_sym.kind == .interface_ && sym.kind == .interface_ {
 		g.write('I_${expr_type_sym.cname}_as_I_${sym.cname}(')
+		if node.expr_type.is_ptr() {
+			g.write('*')
+		}
 		g.expr(node.expr)
 		g.write(')')
 

vlib/v/gen/c/infix_expr.v

@@ -448,6 +448,9 @@ fn (mut g Gen) gen_interface_is_op(node ast.InfixExpr) {
 		return
 	}
 	g.write('I_${left_sym.cname}_is_I_${right_sym.cname}(')
+	if node.left_type.is_ptr() {
+		g.write('*')
+	}
 	g.expr(node.left)
 	g.write(')')
 }

vlib/v/tests/interface_runtime_conversions_test.v

@@ -21,6 +21,15 @@ fn draw(w Widget) {
 	}
 }
 
+fn draw_ref(w &Widget) {
+	if w is ResizableWidget {
+		rw := w as ResizableWidget
+		assert rw.resize(10, 20) == 200
+	} else {
+		assert false
+	}
+}
+
 // implements Widget, but not ResizableWidget
 struct WidgetA {
 }