[@mantine/form] fix type inference of validation for union types

[jvdsande]

Union types were incorrectly distributed when computing validation functions. This was not visible for the most classic unions, those including undefined and null, because Mantine's codebase is not in strict mode.

In strict mode, if the type of a value includes null or undefined in a union, it will get incorrectly distributed, as undefined and null will be inferred independently.

To avoid that, I wrap the Value inferred type into NonNullable before checking it against Array and Record cases. This prevents early distribution of the Value inferred type since the check is not directly against it.

It is safe since NonNullable will get rid of undefined and null, which would be "false" in those checks anyway (not an Array, not a Record), so the comparison is still correct.

[auronsan1st]

just note
I don't know why my vscode not complained. typescript: 4.8.2

[jvdsande]

@auronsan1st if you are not using strict mode for Typescript, then union with undefined or null work correctly already.

Unions with other types such as string | number would fail.

And with TS in strict mode, all unions fail, including with null and undefined

[rtivital]

Thanks!