feat(eslint-plugin): [switch-exhaustiveness-check] add requireDefault…

…ForNonUnion option (#7880)

* rule(switch-exhaustiveness-check): add requireDefaultForNonUnion option

* Apply suggestions from code review

Co-authored-by: Josh Goldberg ✨ <git@joshuakgoldberg.com>

* chore: apply suggestions to more locations

* fix lint

* Fix linting and ordering of md

---------

Co-authored-by: Josh Goldberg ✨ <git@joshuakgoldberg.com>

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

@@ -1,5 +1,5 @@
 ---
-description: 'Require switch-case statements to be exhaustive with union types and enums.'
+description: 'Require switch-case statements to be exhaustive.'
 ---
 
 > 🛑 This file is source code, not the primary documentation location! 🛑
@@ -11,6 +11,8 @@ However, if the union type or the enum changes, it's easy to forget to modify th
 
 This rule reports when a `switch` statement over a value typed as a union of literals or as an enum is missing a case for any of those literal types and does not have a `default` clause.
 
+There is also an option to check the exhaustiveness of switches on non-union types by requiring a default clause.
+
 ## Examples
 
 When the switch doesn't have exhaustive cases, either filling them all out or adding a default will correct the rule's complaint.
@@ -179,6 +181,27 @@ switch (fruit) {
 
 <!--/tabs-->
 
+## Options
+
+### `requireDefaultForNonUnion`
+
+Examples of additional **incorrect** code for this rule with `{ requireDefaultForNonUnion: true }`:
+
+```ts option='{ "requireDefaultForNonUnion": true }' showPlaygroundButton
+const value: number = Math.floor(Math.random() * 3);
+
+switch (value) {
+  case 0:
+    return 0;
+  case 1:
+    return 1;
+}
+```
+
+Since `value` is a non-union type it requires the switch case to have a default clause only with `requireDefaultForNonUnion` enabled.
+
+<!--/tabs-->
+
 ## When Not To Use It
 
 If you don't frequently `switch` over union types or enums with many parts, or intentionally wish to leave out some parts.

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

@@ -12,25 +12,47 @@ import {
   requiresQuoting,
 } from '../util';
 
-export default createRule({
+type MessageIds = 'switchIsNotExhaustive' | 'addMissingCases';
+type Options = [
+  {
+    /**
+     * If `true`, require a `default` clause for switches on non-union types.
+     *
+     * @default false
+     */
+    requireDefaultForNonUnion?: boolean;
+  },
+];
+
+export default createRule<Options, MessageIds>({
   name: 'switch-exhaustiveness-check',
   meta: {
     type: 'suggestion',
     docs: {
-      description:
-        'Require switch-case statements to be exhaustive with union types and enums',
+      description: 'Require switch-case statements to be exhaustive',
       requiresTypeChecking: true,
     },
     hasSuggestions: true,
-    schema: [],
+    schema: [
+      {
+        type: 'object',
+        additionalProperties: false,
+        properties: {
+          requireDefaultForNonUnion: {
+            description: `If 'true', require a 'default' clause for switches on non-union types.`,
+            type: 'boolean',
+          },
+        },
+      },
+    ],
     messages: {
       switchIsNotExhaustive:
         'Switch is not exhaustive. Cases not matched: {{missingBranches}}',
       addMissingCases: 'Add branches for missing cases.',
     },
   },
-  defaultOptions: [],
-  create(context) {
+  defaultOptions: [{ requireDefaultForNonUnion: false }],
+  create(context, [{ requireDefaultForNonUnion }]) {
     const sourceCode = getSourceCode(context);
     const services = getParserServices(context);
     const checker = services.program.getTypeChecker();
@@ -39,9 +61,9 @@ export default createRule({
     function fixSwitch(
       fixer: TSESLint.RuleFixer,
       node: TSESTree.SwitchStatement,
-      missingBranchTypes: ts.Type[],
+      missingBranchTypes: (ts.Type | null)[], // null means default branch
       symbolName?: string,
-    ): TSESLint.RuleFix | null {
+    ): TSESLint.RuleFix {
       const lastCase =
         node.cases.length > 0 ? node.cases[node.cases.length - 1] : null;
       const caseIndent = lastCase
@@ -52,6 +74,10 @@ export default createRule({
 
       const missingCases = [];
       for (const missingBranchType of missingBranchTypes) {
+        if (missingBranchType == null) {
+          missingCases.push(`default: { throw new Error('default case') }`);
+          continue;
+        }
         // While running this rule on checker.ts of TypeScript project
         // the fix introduced a compiler error due to:
         //
@@ -159,7 +185,7 @@ export default createRule({
           suggest: [
             {
               messageId: 'addMissingCases',
-              fix(fixer): TSESLint.RuleFix | null {
+              fix(fixer): TSESLint.RuleFix {
                 return fixSwitch(
                   fixer,
                   node,
@@ -170,6 +196,28 @@ export default createRule({
             },
           ],
         });
+      } else if (requireDefaultForNonUnion) {
+        const hasDefault = node.cases.some(
+          switchCase => switchCase.test == null,
+        );
+
+        if (!hasDefault) {
+          context.report({
+            node: node.discriminant,
+            messageId: 'switchIsNotExhaustive',
+            data: {
+              missingBranches: 'default',
+            },
+            suggest: [
+              {
+                messageId: 'addMissingCases',
+                fix(fixer): TSESLint.RuleFix {
+                  return fixSwitch(fixer, node, [null]);
+                },
+              },
+            ],
+          });
+        }
       }
     }
 

packages/eslint-plugin/tests/rules/switch-exhaustiveness-check.test.ts

@@ -209,6 +209,21 @@ function test(value: ObjectUnion): number {
   }
 }
     `,
+    // switch with default clause on non-union type
+    {
+      code: `
+declare const value: number;
+switch (value) {
+  case 0:
+    return 0;
+  case 1:
+    return 1;
+  default:
+    return -1;
+}
+      `,
+      options: [{ requireDefaultForNonUnion: true }],
+    },
   ],
   invalid: [
     {
@@ -595,6 +610,38 @@ function test(arg: Enum): string {
   case Enum['9test']: { throw new Error('Not implemented yet: Enum[\\'9test\\'] case') }
   case Enum.test: { throw new Error('Not implemented yet: Enum.test case') }
   }
+}
+      `,
+            },
+          ],
+        },
+      ],
+    },
+    {
+      code: `
+const value: number = Math.floor(Math.random() * 3);
+switch (value) {
+  case 0:
+    return 0;
+  case 1:
+    return 1;
+}
+      `,
+      options: [{ requireDefaultForNonUnion: true }],
+      errors: [
+        {
+          messageId: 'switchIsNotExhaustive',
+          suggestions: [
+            {
+              messageId: 'addMissingCases',
+              output: `
+const value: number = Math.floor(Math.random() * 3);
+switch (value) {
+  case 0:
+    return 0;
+  case 1:
+    return 1;
+  default: { throw new Error('default case') }
 }
       `,
             },