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feat(eslint-plugin): add switch-exhaustiveness-check rule (#972)
Co-authored-by: Serg Nesterov <[email protected]> Co-authored-by: Brad Zacher <[email protected]>
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Diff for: ‎packages/eslint-plugin/README.md

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@@ -148,6 +148,7 @@ Pro Tip: For larger codebases you may want to consider splitting our linting int
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| [`@typescript-eslint/restrict-plus-operands`](./docs/rules/restrict-plus-operands.md) | When adding two variables, operands must both be of type number or of type string | | | :thought_balloon: |
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| [`@typescript-eslint/restrict-template-expressions`](./docs/rules/restrict-template-expressions.md) | Enforce template literal expressions to be of string type | | | :thought_balloon: |
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| [`@typescript-eslint/strict-boolean-expressions`](./docs/rules/strict-boolean-expressions.md) | Restricts the types allowed in boolean expressions | | | :thought_balloon: |
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| [`@typescript-eslint/switch-exhaustiveness-check`](./docs/rules/switch-exhaustiveness-check.md) | Exhaustiveness checking in switch with union type | | | :thought_balloon: |
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| [`@typescript-eslint/triple-slash-reference`](./docs/rules/triple-slash-reference.md) | Sets preference level for triple slash directives versus ES6-style import declarations | :heavy_check_mark: | | |
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| [`@typescript-eslint/type-annotation-spacing`](./docs/rules/type-annotation-spacing.md) | Require consistent spacing around type annotations | :heavy_check_mark: | :wrench: | |
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| [`@typescript-eslint/typedef`](./docs/rules/typedef.md) | Requires type annotations to exist | | | |

Diff for: ‎packages/eslint-plugin/docs/rules/switch-exhaustiveness-check.md

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# Exhaustiveness checking in switch with union type (`switch-exhaustiveness-check`)
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Union type may have a lot of parts. It's easy to forget to consider all cases in switch. This rule reminds which parts are missing. If domain of the problem requires to have only a partial switch, developer may _explicitly_ add a default clause.
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Examples of **incorrect** code for this rule:
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```ts
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type Day =
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| 'Monday'
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| 'Tuesday'
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| 'Wednesday'
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| 'Thursday'
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| 'Friday'
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| 'Saturday'
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| 'Sunday';
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const day = 'Monday' as Day;
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let result = 0;
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switch (day) {
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case 'Monday': {
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result = 1;
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break;
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}
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}
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```
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Examples of **correct** code for this rule:
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```ts
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type Day =
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| 'Monday'
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| 'Tuesday'
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| 'Wednesday'
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| 'Thursday'
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| 'Friday'
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| 'Saturday'
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| 'Sunday';
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const day = 'Monday' as Day;
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let result = 0;
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switch (day) {
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case 'Monday': {
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result = 1;
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break;
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}
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case 'Tuesday': {
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result = 2;
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break;
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}
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case 'Wednesday': {
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result = 3;
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break;
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}
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case 'Thursday': {
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result = 4;
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break;
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}
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case 'Friday': {
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result = 5;
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break;
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}
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case 'Saturday': {
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result = 6;
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break;
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}
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case 'Sunday': {
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result = 7;
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break;
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}
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}
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```
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or
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```ts
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type Day =
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| 'Monday'
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| 'Tuesday'
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| 'Wednesday'
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| 'Thursday'
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| 'Friday'
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| 'Saturday'
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| 'Sunday';
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const day = 'Monday' as Day;
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let result = 0;
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switch (day) {
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case 'Monday': {
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result = 1;
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break;
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}
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default: {
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result = 42;
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}
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}
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```
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## When Not To Use It
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If program doesn't have union types with many parts. Downside of this rule is the need for type information, so it's slower than regular rules.

Diff for: ‎packages/eslint-plugin/src/configs/all.json

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"space-before-function-paren": "off",
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"@typescript-eslint/space-before-function-paren": "error",
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"@typescript-eslint/strict-boolean-expressions": "error",
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"@typescript-eslint/switch-exhaustiveness-check": "error",
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"@typescript-eslint/triple-slash-reference": "error",
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"@typescript-eslint/type-annotation-spacing": "error",
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"@typescript-eslint/typedef": "error",

Diff for: ‎packages/eslint-plugin/src/rules/index.ts

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@@ -79,6 +79,7 @@ import returnAwait from './return-await';
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import semi from './semi';
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import spaceBeforeFunctionParen from './space-before-function-paren';
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import strictBooleanExpressions from './strict-boolean-expressions';
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import switchExhaustivenessCheck from './switch-exhaustiveness-check';
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import tripleSlashReference from './triple-slash-reference';
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import typeAnnotationSpacing from './type-annotation-spacing';
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import typedef from './typedef';
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semi: semi,
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'space-before-function-paren': spaceBeforeFunctionParen,
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'strict-boolean-expressions': strictBooleanExpressions,
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'switch-exhaustiveness-check': switchExhaustivenessCheck,
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'triple-slash-reference': tripleSlashReference,
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'type-annotation-spacing': typeAnnotationSpacing,
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typedef: typedef,

Diff for: ‎packages/eslint-plugin/src/rules/switch-exhaustiveness-check.ts

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import { TSESLint, TSESTree } from '@typescript-eslint/experimental-utils';
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import * as ts from 'typescript';
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import {
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createRule,
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getParserServices,
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getConstrainedTypeAtLocation,
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} from '../util';
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import { isTypeFlagSet, unionTypeParts } from 'tsutils';
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import { isClosingBraceToken, isOpeningBraceToken } from 'eslint-utils';
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export default createRule({
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name: 'switch-exhaustiveness-check',
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meta: {
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type: 'suggestion',
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docs: {
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description: 'Exhaustiveness checking in switch with union type',
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category: 'Best Practices',
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recommended: false,
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requiresTypeChecking: true,
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},
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schema: [],
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messages: {
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switchIsNotExhaustive:
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'Switch is not exhaustive. Cases not matched: {{missingBranches}}',
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addMissingCases: 'Add branches for missing cases',
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},
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},
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defaultOptions: [],
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create(context) {
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const sourceCode = context.getSourceCode();
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const service = getParserServices(context);
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const checker = service.program.getTypeChecker();
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function getNodeType(node: TSESTree.Node): ts.Type {
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const tsNode = service.esTreeNodeToTSNodeMap.get(node);
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return getConstrainedTypeAtLocation(checker, tsNode);
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}
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function fixSwitch(
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fixer: TSESLint.RuleFixer,
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node: TSESTree.SwitchStatement,
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missingBranchTypes: Array<ts.Type>,
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): TSESLint.RuleFix | null {
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const lastCase =
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node.cases.length > 0 ? node.cases[node.cases.length - 1] : null;
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const caseIndent = lastCase
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? ' '.repeat(lastCase.loc.start.column)
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: // if there are no cases, use indentation of the switch statement
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// and leave it to user to format it correctly
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' '.repeat(node.loc.start.column);
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const missingCases = [];
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for (const missingBranchType of missingBranchTypes) {
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// While running this rule on checker.ts of TypeScript project
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// the fix introduced a compiler error due to:
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//
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// type __String = (string & {
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// __escapedIdentifier: void;
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// }) | (void & {
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// __escapedIdentifier: void;
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// }) | InternalSymbolName;
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//
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// The following check fixes it.
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if (missingBranchType.isIntersection()) {
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continue;
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}
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const caseTest = checker.typeToString(missingBranchType);
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const errorMessage = `Not implemented yet: ${caseTest} case`;
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missingCases.push(
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`case ${caseTest}: { throw new Error('${errorMessage}') }`,
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);
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}
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const fixString = missingCases
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.map(code => `${caseIndent}${code}`)
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.join('\n');
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if (lastCase) {
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return fixer.insertTextAfter(lastCase, `\n${fixString}`);
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}
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// there were no existing cases
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const openingBrace = sourceCode.getTokenAfter(
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node.discriminant,
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isOpeningBraceToken,
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)!;
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const closingBrace = sourceCode.getTokenAfter(
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node.discriminant,
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isClosingBraceToken,
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)!;
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return fixer.replaceTextRange(
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[openingBrace.range[0], closingBrace.range[1]],
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['{', fixString, `${caseIndent}}`].join('\n'),
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);
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}
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function checkSwitchExhaustive(node: TSESTree.SwitchStatement): void {
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const discriminantType = getNodeType(node.discriminant);
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if (discriminantType.isUnion()) {
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const unionTypes = unionTypeParts(discriminantType);
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const caseTypes: Set<ts.Type> = new Set();
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for (const switchCase of node.cases) {
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if (switchCase.test === null) {
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// Switch has 'default' branch - do nothing.
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return;
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}
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caseTypes.add(getNodeType(switchCase.test));
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}
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const missingBranchTypes = unionTypes.filter(
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unionType => !caseTypes.has(unionType),
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);
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if (missingBranchTypes.length === 0) {
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// All cases matched - do nothing.
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return;
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}
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context.report({
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node: node.discriminant,
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messageId: 'switchIsNotExhaustive',
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data: {
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missingBranches: missingBranchTypes
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.map(missingType =>
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isTypeFlagSet(missingType, ts.TypeFlags.ESSymbolLike)
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? `typeof ${missingType.symbol.escapedName}`
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: checker.typeToString(missingType),
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)
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.join(' | '),
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},
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suggest: [
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{
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messageId: 'addMissingCases',
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fix(fixer): TSESLint.RuleFix | null {
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return fixSwitch(fixer, node, missingBranchTypes);
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},
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},
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],
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});
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}
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}
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return {
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SwitchStatement: checkSwitchExhaustive,
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};
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},
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});

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