Allow concatenated strings to be cast as const type

Suggestion

🔍 Search Terms

Concatenated strings const type

✅ 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

Allow as const cast to be applied to concatenated string literals like so:

// s0 would be 'ab'
const s0 = ('a' + 'b') as const;

https://www.typescriptlang.org/play?#code/FAehAIGcAZwdwPYFcA2ATcAjApuA5AIaZ7ADGCAdpAC5SwC84AFIXuANT7ECU4Bk4clWoBuYKAiQAjOACWAwsTKUaUGY0Vt+glaKA

📃 Motivating Example

It's convenient to split a long string into multiple concatenated lines to enhance code readability, so as const cast should work for it as well.

💻 Use Cases

Currently only a single string is allowed to be cast to its literal type. So as a workaround you just sacrifice code readability and make multiple strings into one:

'a' + 'b' => 'ab'

Start with the concatenated-literal example and the TypeScript design goals linked in the issue. Determine how as const currently handles a single string literal versus a concatenation, then identify the compiler tests and type-checking behavior that would define completion while preserving emitted JavaScript.