[core] Warn about Children.map & Fragment

[oliviertassinari]

Following #10932 (comment) of @ViolentCrumble

[nbkhope]

I have a MenuList defined similar to the following:

<MenuList>
  <MenuItem>...</MenuItem>
  <MenuItem>...</MenuItem>
  {someConditionIsTrue && (
    <Fragment>
      <Divider />
      <MenuItem>...</MenuItem>
    </Fragment>
  }
</MenuList>

I use a React.Fragment at the end to conditionally add something extra to the list. There is no problem there. After upgrading MUI to a newer version (1.2.0 to 1.5.1.), I started getting the following warning:

warning.js:34 Warning: Material-UI: the MenuList component doesn't accept a Fragment as a child.
Consider providing an array instead.

Because of the changes in this PR. Everything is working fine, but I would like to get rid of this new warning. Is that possible? I feel like adding this warning is regressive in this case since Fragment is there so you don't add an extra <div>

[eps1lon]

@nbkhope Well have you considered using an array instead as the warning suggested? You could just define an array before and conditionally push new items.

[nbkhope]

@eps1lon defining as an array require me add a "key" to every single entry. I feel like it gets more convoluted. I will just replace the Fragment with a div, but I still think doing this is regressive.

[eps1lon]

@nbkhope
The warning is not just to force some code style onto you. This is to pass some handlers to the children. You basically block this by wrapping them in a Fragment. This is explained in the linked issue in the description of this PR. Wrapping more than one relevant child might cause bugs depending on the behavior you expect.

[oliviertassinari]

@nbkhope I want to remove the MenuList children loop in #10847. The current implementation has too many drawbacks.