no-unbound-method: add another exception (#3610)

Reading a property with square brackets, though rare, should be allowed because it won't be effected by an unbound this.

src/rules/noUnboundMethodRule.ts

@@ -124,8 +124,9 @@ function isSafeUse(node: ts.Node): boolean {
             return (parent as ts.CallExpression).expression === node;
         case ts.SyntaxKind.TaggedTemplateExpression:
             return (parent as ts.TaggedTemplateExpression).tag === node;
-        // E.g. `obj.method.bind(obj)`.
+        // E.g. `obj.method.bind(obj) or obj.method["prop"]`.
         case ts.SyntaxKind.PropertyAccessExpression:
+        case ts.SyntaxKind.ElementAccessExpression:
             return true;
         // Allow most binary operators, but don't allow e.g. `myArray.forEach(obj.method || otherObj.otherMethod)`.
         case ts.SyntaxKind.BinaryExpression:

test/rules/no-unbound-method/default/test.ts.lint

@@ -6,6 +6,7 @@ function f(i: I) {
     i.m ? 1 : 2;
     if (i.m) {}
     !i.m;
+    i.m["length"];
     return i.m!;
            ~~~ [0]
 }