Exhaustiveness checking fails when target is a property in a non-union type object

[zxTheX]

🔎 Search Terms

Exhaustiveness checking

🕗 Version & Regression Information

TS 5.5, v5.6.0-dev.20240729

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/GYVwdgxgLglg9mABADwIwAoCeAuRBvAL1wHJUAmAZmIF9EAffIxYygFhoEp8BYAKEQGIAzgHcYUCAAssAOgJc8fQcsQQAhkICmzclWxKVhgEYAnTWoDWAbgOH1W5m2L7+hlafPXbKgCabgaiAANlAubiro6AD62GCaAG6aJhwAvAB8eNQcWBw2ruECHpZ5yrR81Hx8oJCwCChkWLiEJLqcPPnCYhLSmHIK3sr22qSUzgNuRV4dyn4BwaHjhpExcYnJ6ZnZmLmL7mbF3mW8FbxV4NDwSMgUjTqj-R2i4lI57eFDd3q7ypMlbrOBEJhAoCZaxBJJVIZLI5P4g36HRDlIA

💻 Code

// ok
function x1(y: {z: '123'} | {z: '234'}) {
    switch(y.z) {
        case '123':
            break;
        case '234':
            break;
        default:
            ((_:never)=>{})(y);
            break;
    } 
}

function x2(y: {z: '123'}) {
    switch(y.z) {
        case '123':
            break;
        default:
            ((_:never)=>{})(y);  // error, y is treated as { z: '123'; } here
            break;
    } 
}

// ok
function x3(y: '123') {
    switch(y) {
        case '123':
            break;
        default:
            ((_:never)=>{})(y);
            break;
    } 
}

🙁 Actual behavior

function 'x2' get an error : Argument of type '{ z: "123"; }' is not assignable to parameter of type 'never'.

🙂 Expected behavior

Compile pass, because function 'x1' shows that TS can narrow the parent type and 'x3' shows that TS can narrow non-union type

Additional information about the issue

No response

[MartinJohns]

Duplicate of #18056 / #23572.

[zxTheX]

I am experiencing the exact same situation in #23572.

However, if the non-union type is unsupported as described in issue #18056, there should be 2 separate error message for examples 'x2' and 'x3'.

[typescript-bot]

This issue has been marked as "Duplicate" and has seen no recent activity. It has been automatically closed for house-keeping purposes.