Type Alias IsNever<T, $O>

IsNever: $Special<
    T,
    {
        $any: $ResolveBranch<IsNever._O<$O>, [$Any, $Else]>;
        $else: $ResolveBranch<IsNever._O<$O>, [$Else]>;
        $never: $ResolveBranch<$O, [$Then], T>;
        $unknown: $ResolveBranch<IsNever._O<$O>, [$Unknown, $Else]>;
        $void: $ResolveBranch<IsNever._O<$O>, [$Void, $Else]>;
    },
>

🎭 predicate

Validate if T is never.

type R = IsNever<never> // true

type R = IsNever<1> // false

🔢 customize

Filter to ensure T is never, otherwise returns $NotNever.

Filter normally returns never in the $else clause. But since we are checking for never here, we have to return $NotNever instead.

type R = IsNever<never, { selection: 'filter' }> // never

type R = IsNever<1, { selection: 'filter' }> // $NotNever

🔢 customize

Use unique branch identifiers to allow precise processing of the result.

type R = IsNever<never, $SelectionBranch> // $Then
type R = IsNever<1, $SelectionBranch> // $Else