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feat(eslint-plugin): add no-unnecessary-type-constraint rule (typescript-eslint#2516)
* feat(eslint-plugin): add no-unnecessary-type-constraint rule * Inlined report * Improved message and removed type checker * Stop testing with type info
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‎packages/eslint-plugin/README.md

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@@ -143,6 +143,7 @@ Pro Tip: For larger codebases you may want to consider splitting our linting int
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| [`@typescript-eslint/no-unnecessary-qualifier`](./docs/rules/no-unnecessary-qualifier.md) | Warns when a namespace qualifier is unnecessary | | :wrench: | :thought_balloon: |
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| [`@typescript-eslint/no-unnecessary-type-arguments`](./docs/rules/no-unnecessary-type-arguments.md) | Enforces that type arguments will not be used if not required | | :wrench: | :thought_balloon: |
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| [`@typescript-eslint/no-unnecessary-type-assertion`](./docs/rules/no-unnecessary-type-assertion.md) | Warns if a type assertion does not change the type of an expression | :heavy_check_mark: | :wrench: | :thought_balloon: |
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| [`@typescript-eslint/no-unnecessary-type-constraint`](./docs/rules/no-unnecessary-type-constraint.md) | Disallows unnecessary constraints on generic types | | :wrench: | |
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| [`@typescript-eslint/no-unsafe-assignment`](./docs/rules/no-unsafe-assignment.md) | Disallows assigning any to variables and properties | :heavy_check_mark: | | :thought_balloon: |
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| [`@typescript-eslint/no-unsafe-call`](./docs/rules/no-unsafe-call.md) | Disallows calling an any type value | :heavy_check_mark: | | :thought_balloon: |
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| [`@typescript-eslint/no-unsafe-member-access`](./docs/rules/no-unsafe-member-access.md) | Disallows member access on any typed variables | :heavy_check_mark: | | :thought_balloon: |

‎packages/eslint-plugin/docs/rules/no-unnecessary-type-constraint.md

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# Disallows unnecessary constraints on generic types (`no-unnecessary-type-constraint`)
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## Rule Details
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Type parameters (`<T>`) may be "constrained" with an `extends` keyword ([docs](https://www.typescriptlang.org/docs/handbook/generics.html#generic-constraints)).
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When not provided, type parameters happen to default to:
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- As of TypeScript 3.9: `unknown` ([docs](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-9.html#type-parameters-that-extend-any-no-longer-act-as-any))
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- Before that, as of 3.5: `any` ([docs](https://devblogs.microsoft.com/typescript/announcing-typescript-3-5/#breaking-changes))
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It is therefore redundant to `extend` from these types in later versions of TypeScript.
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Examples of **incorrect** code for this rule:
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```ts
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interface FooAny<T extends any> {}
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interface FooUnknown<T extends unknown> {}
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type BarAny<T extends any> = {};
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type BarUnknown<T extends unknown> = {};
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class BazAny<T extends any> {
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quxUnknown<U extends unknown>() {}
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}
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class BazUnknown<T extends unknown> {
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quxUnknown<U extends unknown>() {}
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}
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const QuuxAny = <T extends any>() => {};
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const QuuxUnknown = <T extends unknown>() => {};
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function QuuzAny<T extends any>() {}
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function QuuzUnknown<T extends unknown>() {}
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```
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Examples of **correct** code for this rule:
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```ts
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interface Foo<T> {}
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type Bar<T> = {};
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class Baz<T> {
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qux<U> { }
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}
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const Quux = <T>() => {};
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function Quuz<T>() {}
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```
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## When Not To Use It
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If you don't care about the specific styles of your type constraints, or never use them in the first place, then you will not need this rule.

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

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'@typescript-eslint/no-type-alias': 'error',
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'@typescript-eslint/no-unnecessary-boolean-literal-compare': 'error',
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'@typescript-eslint/no-unnecessary-condition': 'error',
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'@typescript-eslint/no-unnecessary-type-constraint': 'error',
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'@typescript-eslint/no-unnecessary-qualifier': 'error',
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'@typescript-eslint/no-unnecessary-type-arguments': 'error',
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'@typescript-eslint/no-unnecessary-type-assertion': 'error',

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

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import noUnnecessaryQualifier from './no-unnecessary-qualifier';
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import noUnnecessaryTypeArguments from './no-unnecessary-type-arguments';
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import noUnnecessaryTypeAssertion from './no-unnecessary-type-assertion';
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import noUnnecessaryTypeConstraint from './no-unnecessary-type-constraint';
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import noUnsafeAssignment from './no-unsafe-assignment';
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import noUnsafeCall from './no-unsafe-call';
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import noUnsafeMemberAccess from './no-unsafe-member-access';
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'no-unnecessary-qualifier': noUnnecessaryQualifier,
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'no-unnecessary-type-arguments': noUnnecessaryTypeArguments,
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'no-unnecessary-type-assertion': noUnnecessaryTypeAssertion,
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'no-unnecessary-type-constraint': noUnnecessaryTypeConstraint,
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'no-unsafe-assignment': noUnsafeAssignment,
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'no-unsafe-call': noUnsafeCall,
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'no-unsafe-member-access': noUnsafeMemberAccess,

‎packages/eslint-plugin/src/rules/no-unnecessary-type-constraint.ts

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import {
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AST_NODE_TYPES,
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TSESTree,
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} from '@typescript-eslint/experimental-utils';
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import * as semver from 'semver';
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import * as ts from 'typescript';
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import * as util from '../util';
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type MakeRequired<Base, Key extends keyof Base> = Omit<Base, Key> &
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Required<Pick<Base, Key>>;
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type TypeParameterWithConstraint = MakeRequired<
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TSESTree.TSTypeParameter,
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'constraint'
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>;
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const is3dot5 = semver.satisfies(
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ts.version,
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`>= 3.5.0 || >= 3.5.1-rc || >= 3.5.0-beta`,
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{
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includePrerelease: true,
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},
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);
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const is3dot9 =
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is3dot5 &&
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semver.satisfies(ts.version, `>= 3.9.0 || >= 3.9.1-rc || >= 3.9.0-beta`, {
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includePrerelease: true,
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});
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export default util.createRule({
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name: 'no-unnecessary-type-constraint',
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meta: {
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docs: {
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category: 'Best Practices',
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description: 'Disallows unnecessary constraints on generic types',
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recommended: false,
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suggestion: true,
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},
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fixable: 'code',
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messages: {
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unnecessaryConstraint:
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'Constraining the generic type `{{name}}` to `{{constraint}}` does nothing and is unnecessary.',
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},
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schema: [],
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type: 'suggestion',
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},
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defaultOptions: [],
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create(context) {
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if (!is3dot5) {
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return {};
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}
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// In theory, we could use the type checker for more advanced constraint types...
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// ...but in practice, these types are rare, and likely not worth requiring type info.
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// https://github.com/typescript-eslint/typescript-eslint/pull/2516#discussion_r495731858
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const unnecessaryConstraints = is3dot9
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? new Map([
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[AST_NODE_TYPES.TSAnyKeyword, 'any'],
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[AST_NODE_TYPES.TSUnknownKeyword, 'unknown'],
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])
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: new Map([[AST_NODE_TYPES.TSUnknownKeyword, 'unknown']]);
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const inJsx = context.getFilename().toLowerCase().endsWith('tsx');
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const checkNode = (
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node: TypeParameterWithConstraint,
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inArrowFunction: boolean,
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): void => {
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const constraint = unnecessaryConstraints.get(node.constraint.type);
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if (constraint) {
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context.report({
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data: {
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constraint,
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name: node.name.name,
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},
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fix(fixer) {
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return fixer.replaceTextRange(
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[node.name.range[1], node.constraint.range[1]],
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inArrowFunction && inJsx ? ',' : '',
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);
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},
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messageId: 'unnecessaryConstraint',
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node,
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});
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}
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};
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return {
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':not(ArrowFunctionExpression) > TSTypeParameterDeclaration > TSTypeParameter[constraint]'(
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node: TypeParameterWithConstraint,
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): void {
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checkNode(node, false);
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},
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'ArrowFunctionExpression > TSTypeParameterDeclaration > TSTypeParameter[constraint]'(
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node: TypeParameterWithConstraint,
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): void {
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checkNode(node, true);
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},
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};
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},
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});

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