Support Intellisense for string/number literals in a widened union

Search Terms

autocomplete union wide

Suggestion

In both https://github.com/microsoft/TypeScript/issues/28743 and https://github.com/microsoft/TypeScript/issues/28743 they were correctly closed for being correctly widened to a string.

Use Cases

I'd like to pitch that this is useful only from the IDE experience, and that's purely where the value comes in from. The widening to a string makes sense, however allowing this lets you declare an API where you would like to allow any input, but there are preferred options.

Examples

// @errors: 2345 2345
type ErrorStatus = 'not_found' | 'failed' | 'parse_error'

declare function fail(status: ErrorStatus): any

fail("not")
//       ^|

declare function failString(status: ErrorStatus | string): any

failString("not")
//             ^|

Would offer "not_found", "failed", "parse_error" in the completion list.

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

The issue names no source file or test. Start by reproducing the two fail/failString examples in TypeScript's language-service completion flow and compare the suggestions for the widened union. Done means the preferred string literals appear in completions while arbitrary strings remain accepted.