A string literal type should not permit duplicate values

[NoelAbrahams]

Hi,

Version: 1.8.5

I would like to see an error when duplicate values are found in a string literal declaration:

type alphabet = 'a' | ... | 'x' | 'y' | 'a';  // Error: Duplicate value 'a'

This is most likely an unintentional inclusion.

[kitsonk]

I think this actually would be a significant change in behaviour that would have unintentional impacts in a lot of ways. Types have always collapsed duplicates as long as they don't conflict.

For example, this has always been valid in TypeScript:

type Foo = { foo: string } | { foo: string } | { foo: string } | { foo: string };

[xLama]

Union types permit duplicate values so I think it is by design.

#805

Basics

Some simple rules:
Identity: A|A is equivalent to A

[NoelAbrahams]

Yes, I see that now.

I wonder whether a distinction can be made between constructing a union type vs. initialising one with duplicates.

type a1 = 'a'; 
type a2 = 'a';

type a = a1 | a2; // Okay

type a3 = 'a' | 'a';  // Error

I don't see why anyone would want to do the latter.

The use-case we have is with regard to a string literal type that has a growing number of values over time. We do not want a new software engineer to add a duplicate value to the list and go away thinking they can use the new value when it is already in the list.

[mhegazy]

This looks like a TSlint rule candidate. but it would be out of scope for the compiler.