Fix: no-extra-parens crash when code is "((let))" (#11444)

This issue was detected by the fuzzer in https://travis-ci.org/eslint/eslint/jobs/498031437.

Author: not-an-aardvark

lib/rules/no-extra-parens.js

@@ -471,6 +471,7 @@ module.exports = {
             const firstToken = isParenthesised(node) ? sourceCode.getTokenBefore(node) : sourceCode.getFirstToken(node);
             const secondToken = sourceCode.getTokenAfter(firstToken, astUtils.isNotOpeningParenToken);
             const thirdToken = secondToken ? sourceCode.getTokenAfter(secondToken) : null;
+            const tokenAfterClosingParens = secondToken ? sourceCode.getTokenAfter(secondToken, astUtils.isNotClosingParenToken) : null;
 
             if (
                 astUtils.isOpeningParenToken(firstToken) &&
@@ -479,7 +480,12 @@ module.exports = {
                     secondToken.type === "Keyword" && (
                         secondToken.value === "function" ||
                         secondToken.value === "class" ||
-                        secondToken.value === "let" && astUtils.isOpeningBracketToken(sourceCode.getTokenAfter(secondToken, astUtils.isNotClosingParenToken))
+                        secondToken.value === "let" &&
+                            tokenAfterClosingParens &&
+                            (
+                                astUtils.isOpeningBracketToken(tokenAfterClosingParens) ||
+                                tokenAfterClosingParens.type === "Identifier"
+                            )
                     ) ||
                     secondToken && secondToken.type === "Identifier" && secondToken.value === "async" && thirdToken && thirdToken.type === "Keyword" && thirdToken.value === "function"
                 )

tests/lib/rules/no-extra-parens.js

@@ -452,6 +452,7 @@ ruleTester.run("no-extra-parens", rule, {
         "({}) ? foo() : bar()",
         "({}) + foo",
         "(function(){}) + foo",
+        "(let)\nfoo",
         "(let[foo]) = 1", // setting the 'foo' property of the 'let' variable to 1
         {
             code: "((function(){}).foo.bar)();",
@@ -1092,6 +1093,18 @@ ruleTester.run("no-extra-parens", rule, {
             "Identifier",
             1
         ),
-        invalid("for (a in (b, c));", "for (a in b, c);", "SequenceExpression", null)
+        invalid("for (a in (b, c));", "for (a in b, c);", "SequenceExpression", null),
+        invalid(
+            "(let)",
+            "let",
+            "Identifier",
+            1
+        ),
+        invalid(
+            "((let))",
+            "(let)",
+            "Identifier",
+            1
+        )
     ]
 });