feat(eslint-plugin): add no-redundant-type-constituents rule (types…

…cript-eslint#4378)

.cspell.json

@@ -71,8 +71,8 @@
     "IIFE",
     "IIFEs",
     "linebreaks",
-    "markdownlint",
     "lzstring",
+    "markdownlint",
     "necroing",
     "nocheck",
     "nullish",
@@ -101,14 +101,15 @@
     "transpiled",
     "transpiles",
     "transpiling",
-    "tsvfs",
     "tsconfigs",
     "tsutils",
+    "tsvfs",
     "typedef",
     "typedefs",
     "unfixable",
     "unoptimized",
     "unprefixed",
+    "upsert",
     "Zacher"
   ],
   "overrides": [

packages/eslint-plugin/README.md

@@ -136,6 +136,7 @@ Pro Tip: For larger codebases you may want to consider splitting our linting int
 | [`@typescript-eslint/no-non-null-asserted-optional-chain`](./docs/rules/no-non-null-asserted-optional-chain.md)         | Disallows using a non-null assertion after an optional chain expression                                                 | :white_check_mark: |          |                   |
 | [`@typescript-eslint/no-non-null-assertion`](./docs/rules/no-non-null-assertion.md)                                     | Disallows non-null assertions using the `!` postfix operator                                                            | :white_check_mark: |          |                   |
 | [`@typescript-eslint/no-parameter-properties`](./docs/rules/no-parameter-properties.md)                                 | Disallow the use of parameter properties in class constructors                                                          |                    |          |                   |
+| [`@typescript-eslint/no-redundant-type-constituents`](./docs/rules/no-redundant-type-constituents.md)                   | Disallow members of unions and intersections that do nothing or override type information                               |                    |          | :thought_balloon: |
 | [`@typescript-eslint/no-require-imports`](./docs/rules/no-require-imports.md)                                           | Disallows invocation of `require()`                                                                                     |                    |          |                   |
 | [`@typescript-eslint/no-this-alias`](./docs/rules/no-this-alias.md)                                                     | Disallow aliasing `this`                                                                                                | :white_check_mark: |          |                   |
 | [`@typescript-eslint/no-type-alias`](./docs/rules/no-type-alias.md)                                                     | Disallow the use of type aliases                                                                                        |                    |          |                   |

packages/eslint-plugin/docs/rules/no-redundant-type-constituents.md

@@ -0,0 +1,85 @@
+# `no-redundant-type-constituents`
+
+Disallow members of unions and intersections that do nothing or override type information.
+
+## Rule Details
+
+Some types can override some other types ("constituents") in a union or intersection and/or be overridden by some other types.
+TypeScript's set theory of types includes cases where a constituent type might be useless in the parent union or intersection.
+
+Within `|` unions:
+
+- `any` and `unknown` "override" all other union members
+- `never` is dropped from unions in any position except when in a return type position
+- primitive types such as `string` "override" any of their literal types such as `""`
+
+Within `&` intersections:
+
+- `any` and `never` "override" all other intersection members
+- `unknown` is dropped from intersections
+- literal types "override" any primitive types in an intersection
+- literal types such as `""` "override" any of their primitive types such as `string`
+
+Examples of code for this rule:
+
+<!--tabs-->
+
+### ❌ Incorrect
+
+```ts
+type UnionAny = any | 'foo';
+type UnionUnknown = unknown | 'foo';
+type UnionNever = never | 'foo';
+
+type UnionBooleanLiteral = boolean | false;
+type UnionNumberLiteral = number | 1;
+type UnionStringLiteral = string | 'foo';
+
+type IntersectionAny = any & 'foo';
+type IntersectionUnknown = string & unknown;
+type IntersectionNever = string | never;
+
+type IntersectionBooleanLiteral = boolean & false;
+type IntersectionNumberLiteral = number & 1;
+type IntersectionStringLiteral = string & 'foo';
+```
+
+### ✅ Correct
+
+```ts
+type UnionAny = any;
+type UnionUnknown = unknown;
+type UnionNever = never;
+
+type UnionBooleanLiteral = boolean;
+type UnionNumberLiteral = number;
+type UnionStringLiteral = string;
+
+type IntersectionAny = any;
+type IntersectionUnknown = string;
+type IntersectionNever = string;
+
+type IntersectionBooleanLiteral = false;
+type IntersectionNumberLiteral = 1;
+type IntersectionStringLiteral = 'foo';
+
+type ReturnUnionNever = () => string | never;
+```
+
+## Limitations
+
+This rule plays it safe and only works with bottom types, top types, and comparing literal types to primitive types.
+It also does not provide an auto-fixer just yet.
+
+## Further Reading
+
+- [Union Types](https://www.typescriptlang.org/docs/handbook/2/everyday-types.html#union-types)
+- [Intersection Types](https://www.typescriptlang.org/docs/handbook/2/objects.html#intersection-types)
+- [Bottom Types](https://en.wikipedia.org/wiki/Bottom_type)
+- [Top Types](https://en.wikipedia.org/wiki/Top_type)
+
+## Attributes
+
+- [ ] ✅ Recommended
+- [ ] 🔧 Fixable
+- [x] 💭 Requires type information

packages/eslint-plugin/src/configs/all.ts

@@ -88,6 +88,7 @@ export = {
     '@typescript-eslint/no-parameter-properties': 'error',
     'no-redeclare': 'off',
     '@typescript-eslint/no-redeclare': 'error',
+    '@typescript-eslint/no-redundant-type-constituents': 'error',
     '@typescript-eslint/no-require-imports': 'error',
     'no-restricted-imports': 'off',
     '@typescript-eslint/no-restricted-imports': 'error',

packages/eslint-plugin/src/rules/index.ts

@@ -60,6 +60,7 @@ import noNonNullAssertedOptionalChain from './no-non-null-asserted-optional-chai
 import noNonNullAssertion from './no-non-null-assertion';
 import noParameterProperties from './no-parameter-properties';
 import noRedeclare from './no-redeclare';
+import noRedundantTypeConstituents from './no-redundant-type-constituents';
 import noRequireImports from './no-require-imports';
 import noRestrictedImports from './no-restricted-imports';
 import noShadow from './no-shadow';
@@ -183,6 +184,7 @@ export default {
   'no-non-null-assertion': noNonNullAssertion,
   'no-parameter-properties': noParameterProperties,
   'no-redeclare': noRedeclare,
+  'no-redundant-type-constituents': noRedundantTypeConstituents,
   'no-require-imports': noRequireImports,
   'no-restricted-imports': noRestrictedImports,
   'no-shadow': noShadow,