Wrong value of isValid() when using DE

[TomekStaszkiewicz]

Environment

Tech	Version
@material-ui/pickers	4.0.0-alpha.8
material-ui	4.10.2
TypeScript
React	16.12.0
Browser
Peer library

Steps to reproduce

Open code sandbox link: https://codesandbox.io/s/billowing-snowflake-gfgts?file=/package.json

1. Use English, then edit it using input(NOT THE POPOVER)
   

2. When it's an invalid date, the textfield is shown as red
   
   if you change it back to correct date it turns green.
   

3. Change to de and do the same things

4. When it's an invalid date, it's red (as expected).
   
   But when it's valid again, it remains red.
   If you check the console, you will see that the isValid() returns false for the correct date.

Expected behavior

isValid() returning true for correct date

Actual behavior

What is important is that this bug occurs only when changing the date by typing in the date. It does not occur when you change the date by clicking on the popover.

[oliviertassinari]

@TomekStaszkiewicz Thanks for the detailed reproduction. Did you have a chance to look at why it behaves wrongly like this?

[oliviertassinari]

I have simplified the reproduction down to:

import "dayjs/locale/de";
import dayjs from "dayjs";
import * as React from "react";
import DayJsAdapter from "@material-ui/pickers/adapter/dayjs";

const util = new DayJsAdapter({ locale: "de" });
console.log("1", util.parse("Januar 2020", "MMMM YYYY")); // KO
console.log("2", dayjs("Januar 2020", "MMMM YYYY", "de")); // OK
console.log("3", dayjs("Januar 2020", "MMMM YYYY")); // KO

export default function App() {
  return <div />;
}

https://codesandbox.io/s/dreamy-goldstine-xty48?file=/src/App.jsx

The problem is in https://github.com/dmtrKovalenko/date-io/blob/400e9f60f0990995c374978b3be4867aecea539c/packages/dayjs/src/dayjs-utils.ts#L105

We should be doing:

  public parse(value: any, format: any) {
    if (value === "") {
      return null;
    }

-   return this.dayjs(value, format);
+   return this.dayjs(value, format, this.locale);
  }

@TomekStaszkiewicz Do you want to work on a pull request :)?

[oliviertassinari]

Could it be worth adding a test case for? It seems that we should.