[TextField] Required `variant` prop

[dmtrKovalenko]

• The issue is present in the latest release.
• I have searched the issues of this repository and believe that this is not a duplicate.

Current Behavior 😯

The following code failing with error Type 'undefined' is not assignable to type '"outlined"'

const textFieldProps: Partial<TextFieldProps> = { }

<TextField value="" onChange={console.log} {...textFieldProps} />

Expected Behavior 🤔

No errors

Steps to Reproduce 🕹

https://codesandbox.io/s/gallant-hermann-k06ld?file=/src/index.tsx

Context 🔦

This loc should have ?

Your Environment 🌎

Tech	Version
Material-UI	v4.9.11
TypeScript	v3.8.3
etc.

[oliviertassinari]

Duplicate of #15697

[dmtrKovalenko]

No it is not a duplicate. It is about handing it is optional ?

[eps1lon]

It's a duplicate of #15697. The error message is just misleading. See #15697 (comment) for a more detailed explanation.

[dmtrKovalenko]

It's not about determination union, but about missing ? for according props.

type Props = {
  variant?: "outlined"
} | {
  variant?: "standard"
} 

Instead of what you have for now

type Props = {
  variant?: "outlined"
} | {
  variant: "standard"
}