fix(eslint-plugin): [no-unnecessary-condition] don't report on unnecessary optional array index access when noUncheckedIndexedAccess is enabled (typescript-eslint#10961)

* initial implementation

* add tests

* cover missing edge cases

Author: ronami

packages/eslint-plugin/src/rules/no-unnecessary-condition.ts

@@ -357,7 +357,7 @@ export default createRule<Options, MessageId>({
       // Since typescript array index signature types don't represent the
       //  possibility of out-of-bounds access, if we're indexing into an array
       //  just skip the check, to avoid false positives
-      if (isArrayIndexExpression(expression)) {
+      if (!isNoUncheckedIndexedAccess && isArrayIndexExpression(expression)) {
         return;
       }
 
@@ -424,12 +424,13 @@ export default createRule<Options, MessageId>({
         //  possibility of out-of-bounds access, if we're indexing into an array
         //  just skip the check, to avoid false positives
         if (
-          !isArrayIndexExpression(node) &&
-          !(
-            node.type === AST_NODE_TYPES.ChainExpression &&
-            node.expression.type !== AST_NODE_TYPES.TSNonNullExpression &&
-            optionChainContainsOptionArrayIndex(node.expression)
-          )
+          isNoUncheckedIndexedAccess ||
+          (!isArrayIndexExpression(node) &&
+            !(
+              node.type === AST_NODE_TYPES.ChainExpression &&
+              node.expression.type !== AST_NODE_TYPES.TSNonNullExpression &&
+              optionChainContainsOptionArrayIndex(node.expression)
+            ))
         ) {
           messageId = 'neverNullish';
         }
@@ -835,7 +836,10 @@ export default createRule<Options, MessageId>({
       // Since typescript array index signature types don't represent the
       //  possibility of out-of-bounds access, if we're indexing into an array
       //  just skip the check, to avoid false positives
-      if (optionChainContainsOptionArrayIndex(node)) {
+      if (
+        !isNoUncheckedIndexedAccess &&
+        optionChainContainsOptionArrayIndex(node)
+      ) {
         return;
       }
 

packages/eslint-plugin/tests/rules/no-unnecessary-condition.test.ts

@@ -399,6 +399,21 @@ const tuple = ['foo'] as const;
 declare const n: number;
 tuple[n]?.toUpperCase();
     `,
+    {
+      code: `
+declare const arr: Array<{ value: string } & (() => void)>;
+if (arr[42]?.value) {
+}
+arr[41]?.();
+      `,
+      languageOptions: {
+        parserOptions: {
+          project: './tsconfig.noUncheckedIndexedAccess.json',
+          projectService: false,
+          tsconfigRootDir: getFixturesRootDir(),
+        },
+      },
+    },
     `
 if (arr?.[42]) {
 }
@@ -417,6 +432,23 @@ declare const foo: TupleA | TupleB;
 declare const index: number;
 foo[index]?.toString();
     `,
+    {
+      code: `
+type TupleA = [string, number];
+type TupleB = [string, number];
+
+declare const foo: TupleA | TupleB;
+declare const index: number;
+foo[index]?.toString();
+      `,
+      languageOptions: {
+        parserOptions: {
+          project: './tsconfig.noUncheckedIndexedAccess.json',
+          projectService: false,
+          tsconfigRootDir: getFixturesRootDir(),
+        },
+      },
+    },
     `
 declare const returnsArr: undefined | (() => string[]);
 if (returnsArr?.()[42]) {
@@ -3272,5 +3304,119 @@ declare const t: T;
 t.a.a.a.value;
       `,
     },
+    {
+      code: `
+declare const test: Array<{ a?: string }>;
+
+if (test[0]?.a) {
+  test[0]?.a;
+}
+      `,
+      errors: [
+        {
+          column: 10,
+          endColumn: 12,
+          endLine: 5,
+          line: 5,
+          messageId: 'neverOptionalChain',
+        },
+      ],
+      languageOptions: {
+        parserOptions: {
+          project: './tsconfig.noUncheckedIndexedAccess.json',
+          projectService: false,
+          tsconfigRootDir: getFixturesRootDir(),
+        },
+      },
+      output: `
+declare const test: Array<{ a?: string }>;
+
+if (test[0]?.a) {
+  test[0].a;
+}
+      `,
+    },
+    {
+      code: `
+declare const arr2: Array<{ x: { y: { z: object } } }>;
+arr2[42]?.x?.y?.z;
+      `,
+      errors: [
+        {
+          column: 12,
+          endColumn: 14,
+          endLine: 3,
+          line: 3,
+          messageId: 'neverOptionalChain',
+        },
+        {
+          column: 15,
+          endColumn: 17,
+          endLine: 3,
+          line: 3,
+          messageId: 'neverOptionalChain',
+        },
+      ],
+      languageOptions: {
+        parserOptions: {
+          project: './tsconfig.noUncheckedIndexedAccess.json',
+          projectService: false,
+          tsconfigRootDir: getFixturesRootDir(),
+        },
+      },
+      output: `
+declare const arr2: Array<{ x: { y: { z: object } } }>;
+arr2[42]?.x.y.z;
+      `,
+    },
+    {
+      code: `
+declare const arr: string[];
+
+if (arr[0]) {
+  arr[0] ?? 'foo';
+}
+      `,
+      errors: [
+        {
+          column: 3,
+          endColumn: 9,
+          endLine: 5,
+          line: 5,
+          messageId: 'neverNullish',
+        },
+      ],
+      languageOptions: {
+        parserOptions: {
+          project: './tsconfig.noUncheckedIndexedAccess.json',
+          projectService: false,
+          tsconfigRootDir: getFixturesRootDir(),
+        },
+      },
+    },
+    {
+      code: `
+declare const arr: object[];
+
+if (arr[42] && arr[42]) {
+}
+      `,
+      errors: [
+        {
+          column: 16,
+          endColumn: 23,
+          endLine: 4,
+          line: 4,
+          messageId: 'alwaysTruthy',
+        },
+      ],
+      languageOptions: {
+        parserOptions: {
+          project: './tsconfig.noUncheckedIndexedAccess.json',
+          projectService: false,
+          tsconfigRootDir: getFixturesRootDir(),
+        },
+      },
+    },
   ],
 });