Relax no-null-undefined-union rule

[adidahiya]

especially as it pertains to React render functions, null | undefined is sometimes unavoidable when dealing with external type definitions.

from @styu:

the rule should be enforced on type definitions rather than on consumed types, so you wouldn’t be able to define types with the union, but could still consume types that have defined the union already. That way you can trend towards the right direction without encouraging the disabling of this rule due to external types

[VincentLanglet]

Something like this

type Text = string | null

interface Props {
  title?: Text
}

Should be allowed, or at least having an options to be allowed.

It's more convenient to use ? to inform it's an optional props, even if the type already have null or undefined somewhere lost in his definition.

[adidahiya]

@VincentLanglet if you want to allow that then it seems like you should just disable the rule altogether. It is a relatively strict style choice, definitely not for everyone

[VincentLanglet]

See #4625 (comment)

? is more than undefined

interface A {
 a: number | undefined
}
interface B {
 b?: number
}

const a: A = {} // $error
const b: B = {} // no error

Plus the actual tslint:all has conflict between this rule and no-null-keyword.

[glen-84]

@adidahiya This was fixed in #4625.