Is Tuple union type still work?

[xiaojingzhao]

On http://www.typescriptlang.org/docs/handbook/basic-types.html , It said

When accessing an element outside the set of known indices, a union type is used instead

However, when I tested this example locally, both tsc and deno threw an error:

Type '"some string here"' is not assignable to type 'undefined'

Here is the example:

let x: [string, number];
x = ["hello", 10];
console.log('x[0].substr(1)', x[0].substr(1));
x[2] = 'world'; // Error was threw here

[ApolloTang]

microsoft/TypeScript#28894 (comment)
according to that comment tuples is now fixed length

[xiaojingzhao]

Microsoft/TypeScript#28894 (comment)
according to that comment tuples is now fixed length

Copy~thanks

[hateonion]

#998

I've created this pr, hope can fix it.