multiple cases with the same value are allowed in switch/case

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
45/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Stale
Tech stack
typescript
Domain
compilers

Research direction

Start with the TypeScript Playground example in the issue and trace how the compiler checks duplicate switch case values. The work is done when repeated case values produce an error while valid switch cases continue compiling and executing as before; add or update coverage in the relevant compiler tests.

Written by the indexing model from the issue text.

Description

Awaiting More Feedback Suggestion

Bug Report

🔎 Search Terms

switch / case
same values on multiple cases

🕗 Version & Regression Information

4.9.4

Playground:
https://www.typescriptlang.org/play?#code/FAMwrgdgxgLglgewgAhgUwM4wBTIG4CGANmGgFzJYBOcEA5sgJTIDewyHlA7nDFABa5CJNE1bAAkBKgEMogOQF5FNlOlIMCImgB0RBHVwAiGXOQBGI4wDckqQCMqaAgGtbUgL7A7phfeXialAaWrr6hsgmsqIATFbuDk6uCV52APRplPwIYEQAJsgQCDDI9qLE+lxoeT7RyIoBqlLBEJraegbGvsgAzPF2Eo7ObnZeEl6p6Fi4DUxAA

💻 Code

// we are allowed to use same value on multiple cases.
// only the first one is executed

function test( value: string ) {
    switch( value ) {
		case 'a': {
			console.log( "case 1");
			break;
		}

		case 'b': {
			console.log( "case 2");
			break;
		}

		// should not be allowed
		case 'a': {
			console.log( "case 3");
			break;
		}
	}
}

test( 'a' )
🙁 Actual behavior

the first case is executed

🙂 Expected behavior

an error should be emit to prevent such error

Dominant language
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Merged PRs (30d)
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