interfaces: fix `is`

vlib/v/gen/cgen.v

@@ -3614,6 +3614,11 @@ fn (mut g Gen) is_expr(node ast.InfixExpr) {
 	sym := g.table.get_type_symbol(node.left_type)
 	if sym.kind == .interface_ {
 		g.write('_interface_idx == ')
+		// `_Animal_Dog_index`
+		sub_type := node.right as ast.Type
+		sub_sym := g.table.get_type_symbol(sub_type.typ)
+		g.write('_${sym.name}_${sub_sym.name}_index')
+		return
 	} else if sym.kind == .sum_type {
 		g.write('typ == ')
 	}

vlib/v/tests/interface_test.v

@@ -225,3 +225,16 @@ fn test_interface_ptr_array() {
 	assert true
 	assert animals.len == 3
 }
+
+fn test_is() {
+	dog := Dog{}
+	assert foo2(dog) == 1
+}
+
+fn foo2(a Animal) int {
+	if a is Dog {
+		return 1
+	} else {
+		return 0
+	}
+}