In unions and intersections, redundant types should not be used.

== Why is this an issue?

When defining a union or intersection in TypeScript, it is possible to mistakenly include type constituents that encompass other constituents, that don't have any effect, or that are more restrictive. For instance,

- The type `something` in `any | something` is redundant because `any` covers all possible types, whatever `something` is.
- The types `never` in unions like `never | something` or `unknown` in intersections like `unknown & something` are effectless.
- More restrictive types in intersections like the literal type `1` in `1 & number` reduce the set of possible values to specific ones.

Eliminating redundant types from a union or intersection type simplifies the code and enhances its readability. Moreover, it provides a clearer representation of the actual values that a variable can hold.

== How to fix it

The redundant and overridden types should be removed.

=== Code examples

==== Noncompliant code example

[source,typescript,diff-id=1,diff-type=noncompliant]
----
type UnionWithAny = any | 'redundant'; // Noncompliant
type UnionWithNever = never | 'override'; // Noncompliant
type UnionWithLiteral = number | 1; // Noncompliant

type IntersectionWithAny = any & 'redundant'; // Noncompliant
type IntersectionWithUnknown = string & unknown; // Noncompliant
type IntersectionWithLiteral = string & 'override'; // Noncompliant
----

==== Compliant solution

[source,typescript,diff-id=1,diff-type=compliant]
----
type UnionWithAny = any;
type UnionWithNever = never;
type UnionWithLiteral = number;

type IntersectionWithAny = any;
type IntersectionWithUnknown = string;
type IntersectionWithLiteral = 'override';
----

== Resources
=== Documentation

* TypeScript Documentation - https://www.typescriptlang.org/docs/handbook/2/everyday-types.html#union-types[Union Types]
* TypeScript Documentation - https://www.typescriptlang.org/docs/handbook/2/objects.html#intersection-types[Intersection Types]