Allow explicit fallthrough when noFallthroughCasesInSwitch is enabled

🔍 Search Terms

fallthrough noFallthroughCasesInSwitch

✅ Viability Checklist

• 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 isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

    switch (...) {
        case 'A':
            doSomething();
            // fallthrough - this will allow falling to next statement
        case 'B':
            doSomethingElse();
   }

Instead of usage of @ts-expect-error this is nicely readable and intention is clearly visible in generated code.

📃 Motivating Example

Until now for noFallthroughCasesInSwitch is missing good way to allow falling through if needed. The only possible way @ts-expect-error is disabling not only fallthrough check, but also other checks which is not desired.

💻 Use Cases

1. What do you want to use this for? Because eslint rule no-fallthrough does not handle TS exhaustive match, I would like to you noFallthroughCasesInSwitch config rule

2. What shortcomings exist with current approaches?

   • eslint can not be used, its no-fallthrough rule can not detect exhaustive match, so the following code is incorrectly reported by eslint:

   function transform(action: 'KEEP' | 'INVERSE', b: boolean): boolean {
       switch (action) {
           case 'KEEP':
               switch (b) {
                   case true:
                       return true;
                   case false:
                       return false;
               }
           case 'INVERSE':
               switch (b) {
                   case true:
                       return false;
                   case false:
                       return true;
               }
       }
   }
   

   • satisfying eslint is not possible e.g. by putting break, because TS will start report Unreachable code detected and this can not be disabled by @ts-expect-error
   • disabling reported case by @ts-expect-error is also not viable, because it is disabling all type checks, so e.g. in case case something: validation that something has the correct type is also disabled (and issue #19139 is still opened)

3. What workarounds are you using in the meantime? In my specific case, I had only one of these switches with exhaustive match, so I could put the statement as the last to not disable any check.

Implementation of #19139 would give me decent workaround and it is definitely more useful that this specific feature, but I believe both of them should be implemented.

Issue không nêu tên tệp hoặc test nào; hãy bắt đầu bằng cách xác định chẩn đoán noFallthroughCasesInSwitch và các kiểm tra luồng điều khiển của switch trong trình biên dịch TypeScript. So sánh marker fallthrough được đề xuất với các trường hợp @ts-expect-error và exhaustive-match hiện có, sau đó thêm coverage tập trung cho thấy fallthrough có chủ đích vẫn được kiểm tra kiểu.