cgen: fix `in` op usage on array of sumtypes without cast (#12141)

vlib/v/gen/c/infix_expr.v

@@ -394,6 +394,22 @@ fn (mut g Gen) infix_expr_in_op(node ast.InfixExpr) {
 				return
 			}
 		}
+		if right.sym.info is ast.Array {
+			elem_type := right.sym.info.elem_type
+			elem_type_ := g.unwrap(elem_type)
+			if elem_type_.sym.kind == .sum_type {
+				if node.left_type in elem_type_.sym.sumtype_info().variants {
+					new_node_left := ast.CastExpr{
+						arg: ast.EmptyExpr{}
+						typ: elem_type
+						expr: node.left
+						expr_type: node.left_type
+					}
+					g.gen_array_contains(node.right_type, node.right, new_node_left)
+					return
+				}
+			}
+		}
 		g.gen_array_contains(node.right_type, node.right, node.left)
 	} else if right.unaliased_sym.kind == .map {
 		g.write('_IN_MAP(')

vlib/v/tests/in_expression_test.v

@@ -280,4 +280,10 @@ fn test_in_sumtype_array() {
 		println(foo)
 		assert true
 	}
+
+	// without sumtype cast
+	mut foos := []Foo{}
+	foos << Foo1{}
+	assert Foo1{} in foos
+	assert Foo2{} !in foos
 }