Strict checks fail with switch(true)

[kaaboaye]

TypeScript Version: 3.7.x-dev.201xxxxx

Code

Minimal reproduction

function foo(someString: string | undefined) {
    switch (true) {
        case someString !== undefined:
            // this line is broken
            // it requires !non null assertion even though I've checked it above
            return someString.toUpperCase()

        default:
            return 'some default value'
    }
}

Expanded reproduction

// broken function
function foo(isSomething: boolean, someString: string | undefined) {
    switch (true) { 
        case isSomething:
            return 'is something'

        case someString !== undefined:
            // this line is broken
            // it requires !non null assertion even though I've checked it above
            return someString.toUpperCase()

        default:
            return 'some default value'
    }
}

// does the same staff as `foo` but has different implementation
function bar(isSomething: boolean, someString: string | undefined) {
    if (isSomething) {
        return 'is something'
    }

    if (someString !== undefined) {
        return someString.toUpperCase()
    }


    return 'some default value'
}

// ensure that the implementations are equivalent
([[true, undefined], [false, undefined], [true, 'asd'], [false, 'asd']])
    .forEach(([isSomething, someString]: any) =>
        console.assert(
            foo(isSomething, someString) === bar(isSomething, someString),
            'test feild for', isSomething, someString
        )
    )

Expected behavior:

Compile just fine

Actual behavior:

Throws

(parameter) someString: string | undefined
Object is possibly 'undefined'.(2532)

for

        case someString !== undefined:
            return someString.toUpperCase()

Playground Link:

https://www.typescriptlang.org/play/#code/PTAECMCcHsGsFMB2oBmBXRBjALgS2ogFDpZ4GrTQAUuAzgMrQC282AFrogOYBcElAG3gBDRABpQtZvHrZInXpLkLQAH1AYAJvBSd4mgJSgA3oVDnJAd1zZMbUFTlp4R46DMXPmYbXig6jCzsCjwenuGgkKxokMgA5HSS0sHccYRh4d6+SSyy8tygAIQAvMUaiNq6iPqhEREgoMG0oAJ6-s1QcEgZdaANNpHwAI5ouFHNhYjkiGgCAqA+vpBkyPAAbkiNbNBoXPYAknEboHbwmAia-tgL4NAbPXVR2DHIUrnK3AB02NAAqgAO-3gkAAwj54FQDOlepVhLNsLVehYni9QHE3n5YfDQGthAJnGlPABfQgkwgNTTQeDNdh+WjCFhKYQoFALZoAAxQlHZEDQ1zYPlAmlwLOBSGuuCY-yELEQ2GEK2IGBw+GQ4GEkBoDGSHG4fFu0CEogkGLyISU+S4anKlT0hhMGRFDgCOoUrge5hRsTRiQxKS4hIsZM8Tqopo+VpKZS0Ojt7t6Xte0jNXx+AKBoPBkIyZIyibRGKFOjhAmuuPx8DSZIaSFoMT87AVWz8kul8Fl8pWzQ1fmGo3L4sIVAA2sOnPAJDGqvoALoSYcoPG+ScVWPVTRz0BjyDOCRxHyaOKbhdLidog9HmdQzyfLmQACiwjsVBHLqCuq4JuTEZnfFEAE8jGKAA+D0TgIKQhE+RZgWwKgwM8LlqDfVgPy-d5LSA0oIA1LVAlQhR0JkCMDDEBCLDibBqWuFB4FwARLjvOIJBQ-0iJTLgwOvCwoUIIA

[IllusionMH]

Duplicate of #8934

Recent discussion #37178