Support 'did you mean' for assignments to unions with string literals

[orta]

Suggestion

type A = "abc" | "abcd" | "abcde";
const a: A = "abb";
// Type '"abb"' is not assignable to type 'A'.

Could instead be:

type A = "abc" | "abcd" | "abcde";
const a: A = "abb";
// Type '"abb"' is not assignable to type 'A'.
//   Did you mean "abc"?

🔍 Search Terms

did you mean literals unions assignment

✅ Viability 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, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

📃 Motivating Example

If we already know all/a set of the possible entries, it can't hurt to see if any are close enough and try recommend them like we do in other places.

My actual case was this:

src/compiler/commandLineParser.ts:1088:13 - error TS2322: Type '"strong"' is not assignable to type '"string" | "number" | "boolean" | "object" | ESMap<string, string | number> | "list"'.

1088             type: "strong",
                 ~~~~

  src/compiler/types.ts:6272:9
    6272         type: "string" | "number" | "boolean";
                 ~~~~
    The expected type comes from property 'type' which is declared here on type 'CommandLineOption'

Where we know a set of literals and a non-literal are possible, and as I was very close to one I figured TS would have enough knowledge to offer the case for what the typo probably is.

💻 Use Cases

Just dev workflow stuff.

[DanielRosenwasser]

I think when I last checked, spelling suggestions operate on __Strings, which might've given me some "stop" energy when I last looked into this. @sandersn might have some insight into how difficult this would be.

[sandersn]

Completions already work, and you can cast any string to __String without a problem, so I think it should be straightforward.

[DanielRosenwasser]

Thanks @hi-ogawa!